The visual cliff's forgotten menagerie: rats, goats, babies, and myth-making in the history of psychology.

Journal of the History of the Behavioral Sciences, Vol. 51(2), 113–140 Spring 2015 View this article online at Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/jhbs.21712

C 2015 Wiley Periodicals, Inc.

THE VISUAL CLIFF’S FORGOTTEN MENAGERIE: RATS, GOATS, BABIES, AND MYTH-MAKING IN THE HISTORY OF PSYCHOLOGY ELISSA N. RODKEY

Eleanor Gibson and Richard Walk’s famous visual cliff experiment is one of psychology’s classic studies, included in most introductory textbooks. Yet the famous version which centers on babies is actually a simplification, the result of disciplinary myth-making. In fact the visual cliff’s first subjects were rats, and a wide range of animals were tested on the cliff, including chicks, turtles, lambs, kid goats, pigs, kittens, dogs, and monkeys. The visual cliff experiment was more accurately a series of experiments, employing varying methods and a changing apparatus, modified to test different species. This paper focuses on the initial, nonhuman subjects of the visual cliff, resituating the study in its original experimental logic, connecting it to the history of comparative psychology, Gibson’s interest in comparative psychology, as well as gender-based discrimination. Recovering the visual cliff’s forgotten menagerie helps to counter the romanticization of experimentation by focusing on the role of extrascientific factors, chance, complexity, and uncertainty in the experimental process. C 2015 Wiley Periodicals, Inc.

“Human infants at the creeping and toddling stage are notoriously prone to falls from more or less high places” (Gibson & Walk, 1960, p. 67). Thus began Eleanor J. Gibson and Richard Walk’s (1960) article “The “Visual Cliff,” published in Scientific American, accompanied by four engaging photographs of babies on the visual cliff apparatus. The photo that Gibson most often reproduced to illustrate the study includes all the essential elements of the experiment as it became popularly known: the uncertain baby, the encouraging mother, and the patterned and clear surfaces that constitute the visual cliff (Figure 1). This image exemplifies the version of the visual cliff story told and retold in introductory psychology courses and textbooks—often accompanied by one of these Scientific American photos (Rodkey, 2010). What has been forgotten in these retellings is that the visual cliff was not a single study, nor did the first experiment involve human infants. Although Gibson and Walk begin their Scientific American article with babies, they also discuss the wide range of animals tested on the visual cliff prior to infants, including chicks, turtles, rats, lambs, kids, pigs, kittens, and dogs (Gibson & Walk, 1960). In fact, the majority of the photographs in the article—10 out of 14—were of animals, not infants. However, these other images—of a kitten, a kid goat, and a rat on the visual cliff—never achieved the iconic status of the baby photographs, and over the years the animal experiments on the visual cliff have lapsed into relative obscurity. The various animals tested prior to babies now come last in accounts of the visual cliff, if they are mentioned at all (Figure 2).1 1. For example, Baron, Earhard, and Ozier (1997), Carlson (1990), Gray (1991), Haber and Runyon (1986), Lindzey, Thompson, and Spring (1988), and Weiten (1998) all reference the visual cliff in their textbook, but do not mention any animals in their description of it, instead including only the baby experiments. See Rodkey (2010) for an analysis of the textbook depictions of the visual cliff. ELISSA N. RODKEY recently received her PhD in history of psychology at York University and teaches psychology at Crandall University. Her main research interests are the history of developmental psychology and the historical relationship between psychology and religion. Her recent publications include “The infancy of infant pain research: The experimental origins of infant pain denial,” Journal of Pain 14 (2013): 338–350. Correspondence concerning this paper should be sent to Elissa Rodkey, Crandall University, Box 6004, Moncton, NB E1G 3H9, Canada; [email protected].

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FIGURE 1. The most popular of the Scientific American photographs of the visual cliff. Image from the Richard D. Walk papers, courtesy Drs. Nicholas and Dorothy Cummings Center for the History of Psychology, The University of Akron.

This simplification of the visual cliff story has facilitated the creation of various myths about the experiment’s origins, such as the story that Gibson was inspired by her small children’s escapades on the edge of the Grand Canyon. While the Gibsons did visit the Grand Canyon in 1946, according to Gibson, “Contrary to a popular myth, this occasion was not the inspiration for my later research on the visual cliff” (Gibson, 2002, p. 47). The Grand Canyon myth only makes sense when combined with the impression that the study was primarily about babies JOURNAL OF THE HISTORY OF THE BEHAVIORAL SCIENCES DOI 10.1002/jhbs

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FIGURE 2. One of the cliff’s forgotten subjects: a goat contemplates the apparent drop off. Image from the Richard D. Walk papers, courtesy Drs. Nicholas and Dorothy Cummings Center for the History of Psychology, The University of Akron.

and that animals were an afterthought. In fact, in good comparative tradition, the cliff’s first subjects were rats. Other historians have investigated famous psychology experiments or incidents, such as Benjamin Harris (1979) in the case of Little Albert, Rachel Manning, Levine, and Collins (2007) for the Kitty Genovese Murder, and Marga Vicedo (2009) for Harry Harlow’s famous monkey attachment research. These articles have highlighted the process whereby experiments are converted into useful origin myths that foster a “false sense of continuity” (Harris, 1979, p. 157) within psychology and prepare the way for convenient pedagogical morals to be drawn. While the impact of the visual cliff myth on the history of developmental psychology is perhaps not as wide-reaching as Gordon Allport’s pronouncements on the origins of social psychology (Samelson, 1974), the simplified version recounted in numerous textbooks shapes the discipline’s identity and experimental expectations (Morawski, 1992). Seen in retrospect, the rationale for the visual cliff is assumed to be consistent with the results obtained and these results themselves seem commonsensical, obscuring the actual process of innovation (including mistakes, blind alleys, and serendipity) that contributed to its design and results. This textbook demand for simplicity and retroactive consistency also means that findings that were actually the product of a series of related studies are compressed into one dramatic discovery that usually conforms to romanticized expectations about the scientific method (see Skinner, 1956). The simplification of the visual cliff experiment is consistent with these predictions; the multiplicity of experiments and animals were dropped in favor of an appealing story: a single experiment using crawling babies. Much of the visual cliff mythology, in fact, consists in oversimplification: what we might call myth as simplification rather than myth as falsehood. This distinction implies that a myth need not be a factual error or outright falsehood; it can also be an exaggeration, an idealization, an overemphasis, or a simplification of the reality of the original experiment. JOURNAL OF THE HISTORY OF THE BEHAVIORAL SCIENCES DOI 10.1002/jhbs

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Dewsbury (1998) has observed that the simplification of research is inevitable, and happens on multiple levels as the research is summarized for various audiences and by people other than the original experimenter. There is generally reasonable simplification at each level, but successive simplifications can lead to serious distortions in the research findings. As Dewsbury (1998) demonstrates with his example of Robert Yerkes’ simplification of his dominance research in chimpanzees, in its extreme form simplification can border on scientific fraud. This analysis suggests that all famous psychology studies are simplified to some degree, but when the simplification is sufficiently distant from experimental reality, or when only the simplified version is consistently reported, that simplification can be considered a myth. In contrast to its simplified, popularized version, the visual cliff experiment was more accurately a series of experiments, employing varying methods and an evolving apparatus, which were modified in order to test a variety of animals. This paper will highlight the initial, nonhuman subjects of the visual cliff, resituating the study in its original experimental logic, connecting it to Gibson’s interest in comparative psychology, and placing it in the history of comparative psychology as well as the history of gender-based discrimination in psychology. I will suggest some reasons why the visual cliff became so strongly associated with infants, rather than animals, and I will use this reconstruction to challenge subsequent simplified retellings of the visual cliff, suggesting that they represent a convenient origin myth in the history of psychology. Highlighting the visual cliff’s forgotten menagerie contrasts the simplification and romanticization of famous experiments, represented by the mythic version of the visual cliff, with the real-life complexity and uncertainty of the experimental process, represented by the wide variety of animal tests. Recovering the forgotten subjects of the visual cliff can serve to disrupt the simplification of the experiment, since the numerous animals tested on the cliff illustrate the difficulty with designating any one of the experiments “the” visual cliff experiment. How the experiments actually came about is a tale of rats, goats, the challenges of comparative research, and pragmatism in the face of challenges, which makes for a story not nearly as simple or charming as that of the Gibson family picnicking on the edge of the Grand Canyon.

ORIGINS OF THE VISUAL CLIFF Assessing the origins of the visual cliff is complicated by the fact that it was the result of a collaboration between Eleanor Gibson and Richard Walk. In 1957, when the experiment was first published, both were members of Cornell University’s psychology department, Walk as a professor of experimental and social psychology, Gibson as a research associate. Ideally in the search for the cliff’s origins, one would trace both of these players’ life histories prior to the experiment; however, this is complicated by a lack of available evidence about one party. Despite their official status at Cornell at the time of the experiment (Walk outranked Gibson) Gibson went on to have significantly more academic success and prestige.2 This has resulted in far more material being generated by and about Gibson (i.e., her publications, publications about her, and interviews conducted with her), which has then preserved her memories and retelling of the famous experiment. Although some elements of Walk’s past may be connected to the origins of the visual cliff, such as his previous experience studying paratroopers’ fear

2. For example, she received the Distinguished Scientific Contribution Award (1968), the Gold Medal Award of the American Psychological Association (1968), the National Medal of Science (1992), as well as being elected to the National Academy of Sciences (1971) and the American Academy of Arts and Sciences (1977).

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of heights while serving in the military, the lack of evidence available on Walk makes it hard to assess the impact of such factors.4 However, while both Gibson and Walk have plausible sounding back stories, there is reason to emphasize Gibson’s role in the origin of the visual cliff, and not merely because of a lack of evidence about Walk. From what can be reconstructed about the series of National Science Foundation-funded experiments that led to the visual cliff, it appears that Gibson was more active, both in instigating the experiment and in interpreting its results. Perhaps counterintuitively, given their relative status, it was Gibson who initiated the collaboration with Walk, came up with the idea for their rat-rearing research, and wrote the National Science Foundation (NSF) grant proposal (Gibson, 1998). This can be understood in the context of Gibson’s life and the gender-based discrimination experienced by second generation women psychologists. The collaboration with Walk was but one instance of a larger pattern in which Gibson responded flexibly and proactively to career barriers. In the period in which the visual cliff took place Gibson, relegated to research associate status and deprived of laboratory access as a result of Cornell’s antinepotism rules, repeatedly sought out collaborations that allowed her the ability to conduct research. The collaboration with Walk was one such endeavor at Cornell, one of at least five separate collaborations in her time as a research associate. The result of this situation can be assessed negatively, as one Gibson biographer has put it, “One consequence of this [move to Cornell] was that she participated in research on the barest fringes of the issues that concerned her” (Caudle, 1990, p. 107). However, Gibson herself had a more optimistic take on her strategy: “I did have a theme, a sort of direction, and opportunities, even very unlikely ones, can sometimes be bent to one’s theme”(Gibson, 1977). Although Gibson was exceptionally good at finding unlikely opportunities that might suit her purpose, she was not alone in her experience on the margins of academia. Johnston and Johnson found that second generation women in psychology tended to be stuck in less prestigious or part-time academic positions, and as a result became “creative opportunists” (2008, p. 63) to flourish in the hostile academic environment. As a result, Gibson’s actions were fairly predictable: beginning with the barriers she encountered at graduate school, “she employed a strategy that she would eventually use a number of times, namely, accepting a lesser short-term goal so that she could continue, uninterrupted, towards long-range objectives” (Caudle, 1990, p. 105). Given this background, the visual cliff’s origins most clearly begin with Eleanor Gibson’s graduate school experience, and her desire to do comparative research. Gibson first demonstrated this interest in comparative psychology with her choice of doctoral program. Fresh from Kurt Koffka’s Gestalt psychology at Smith College, she was ready for “hard science” and was interested in animals (Gibson, 1980). Gibson’s motive for picking Yale was in large part due to her desire to do comparative research with Robert Yerkes: “I wanted a super-scientific, strongly experimental atmosphere where I could work with animals—Yale!” (Gibson, 1979).5 3. Walk recorded that his experience watching recruits practice jumping off a tower influenced his ideas for research prior to the visual cliff experiment: “At one point I contemplated constructing a miniature mock tower and training rats to jump from it; it was an idle thought that, fortunately, was never carried out” (Walk, 1979, p. 85). Instead, Walk says, “Fortunately I went to Cornell from Fort Benning, and got to know Eleanor and James Gibson” (Walk, 1979, p. 85). 4. In contrast to the wide range of material on Gibson, there are only two helpful sources of information about Walk and the visual cliff: Pick and Tighe (2001) and Walk (1979). However this could soon change: the Cummings Center for the History of Psychology at the University of Akron recently acquired Richard Walk’s papers. Once this collection is processed, the question of Walk’s influence on the visual cliff ought to be revisited. 5. In a retrospective on Gibson, one of Gibson’s collaborators commented on how this comparative interest influenced even her later infant development research: “The comparative developmental perspective is reflected in a functional orientation in much of Gibson’s writing” (Pick, 1992, p. 793). JOURNAL OF THE HISTORY OF THE BEHAVIORAL SCIENCES DOI 10.1002/jhbs

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In 1935, Yerkes was at the peak of his influence; his well-funded Laboratories of Primate Biology, associated with Yale’s interdisciplinary Institute of Human Relations, appeared to be the ideal training ground for the would-be comparative psychologist (for a history of the Yerkes’ Laboratories, see Dewsbury, 2006). Gibson later commented on Yerkes’ status in the Institute, “Yerkes had the most impressive empire, or at least the most noticeable one, because of the chimpanzees. They had outdoor cages atop one of the buildings and, in fair weather, could be heard from every quarter of the institute” (Gibson, 1980, p. 246). Indeed, Pettit, Serykh, and Green’s analysis (2015) of the multispecies research networks of the Committee for Research in Problems of Sex confirms that Yerkes’ empire was strategically central in comparative research. Yerkes had a virtual monopoly on chimpanzee research within psychology, and was correspondingly influential. However, upon requesting Yerkes’ advisorship, Gibson was promptly shown the door, Yerkes proclaiming “I have no women in my laboratory” (Gibson, 2002, p. 21). By approaching Yerkes rather than pursuing the less prestigious child psychology or mental testing work offered by the two psychologists who traditionally accepted women, Gibson had unwittingly violated the unspoken agreement about the proper place of women in psychology at the time (Johnston & Johnson, 2008). Foiled in her attempt to do animal psychology, but still set on experimental work, Gibson approached Clark Hull. Hull agreed to advise Gibson, provided that she use his strict behaviorist methods and theory and Gibson conducted a verbal learning experiment using human subjects (Gibson, 1991). Gibson was again stymied in pursuing comparative research of her own by her arrival in 1949 at Cornell University where her husband James had been hired as a faculty member. Due to Cornell’s strict antinepotism rules, for the next 16 years, until she was finally made a full professor in 1966, Eleanor Gibson had to create her own research opportunities, which meant that she worked with animals only when she collaborated with another Cornell professor, since comparative work required laboratory space and funding that she did not have as a mere research associate (Gibson, 1977). Gibson later summed up her situation at Cornell bluntly: “Couldn’t I just set out to work on the research of my choice? No, I couldn’t. One needs a lab, and I didn’t have one” (Gibson, 1977). The first of her attempts to do work as a research associate involved working as an assistant for two years at the Behavior Farm, the laboratory of Cornell professor Howard Liddell (for more on Liddell, see Freeman, 1985). Liddell was a staunch behaviorist engaged in the classical conditioning of goats, using shock in an attempt to induce experimental neurosis. Cornell’s Physiological Field Station, first established in 1922, had been transformed by Liddell into the Behavior Farm—a setup modeled on Pavlov’s laboratory6 designed to test Pavlov’s ideas and to establish appropriate classical conditioning methods for sheep and goats. Owing to Liddell’s academic success, in 1938 he was able to have a laboratory built to his specifications: he “abandoned the old Physiological Field Station and moved onto a newly constructed laboratory building on a 100-acre farm two miles from the Cornell Campus” (Liddell, 1958, p. 250). Initially enthusiastic about working with Liddell, Gibson was dismayed to find that by the 1950s Liddell rarely visited the farm, leaving most of the work to his brother-in-law Ulric Moore, the farm manager (Gibson, 1980).The sole purpose of the Behavior Farm research at this point was to induce experimental neuroses in the animals, but Gibson had grave doubts about whether they were doing even that. The “miles of records of heart rate, breathing, and movements” Gibson helped collect were largely ignored; the only proof of neurosis offered 6. Liddell visited Pavlov’s Leningrad laboratory in 1926 and 1934 (Block, 1963).



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was the dramatic cases produced for guests: “One was Brown Billy, a mature goat who was a real performer. He lifted his foreleg and rolled his eyes for the visitors from the Rockefeller Foundation and was rewarded with cigarettes” (Gibson, 1980, p. 254). Despite her skepticism, Gibson used the experimental setup to pursue her own interests by comparing the goats’ reactions when a shock was avoidable with when it was not (Gibson, 1952).7 Since goats were bred on the farm, Gibson also set up an observational study of development and imprinting, with twin pairs of kids divided into experimental and control animals. But this study came to a premature end when Gibson returned to the farm one weekend only to discover that her control group had been given away as Easter presents (Gibson, 2002). Furious at this development, Gibson quit.8 Despite the ultimate failure of Gibson’s observational study of maternal-infant interactions, her time at the Behavior Farm was not entirely wasted (Figure 3). The incident that has the best claim to be the inspiration of the visual cliff—Gibson called it “good preparation” (1991, p. 103)—took place at the Behavior Farm. Gibson was focusing on the role of olfactory input in maternal bonding and so would wash one twin in a detergent immediately after birth, before the mother goat could lick it. Gibson had just finished washing a kid when its twin began to emerge. As she hurriedly looked about for a place to keep the washed kid clean, Moore, who was watching, suggested placing it on a high camera stand nearby. To Gibson’s surprise, the newborn stood calmly on the stand until she could deal with it, a reminder of his cliff dwelling ancestors (Gibson, 1991). The visual cliff experiment was itself the result of another of Gibson’s attempts to do comparative research without a laboratory of her own. In 1956, Gibson began collaborating with Richard Walk, a young Cornell professor, who had access to laboratory space and animals.9 Walk had arrived at Cornell in 1953, just beginning his academic career in earnest, having served in the Army 1942–1945, and received his doctorate from Harvard in 1951 only to be called up again for the Korean War that same year (Pick & Tighe, 2001). By all accounts they had a warm, collegial relationship; however, Gibson was 10 years older than Walk, and despite her unofficial standing at Cornell, the senior faculty member in the relationship and a far more experienced researcher.10 In their 1955 NSF grant proposal, Walk lists just three publications, compared to Gibson’s 12 (Gibson & Walk, 1955). In her 1998 oral history interview, Gibson recounts the collaboration with Walk as a direct result of the aborted goat rearing study:

7. An attempt Liddell made to appropriate this research of Gibson’s is a good example of the pressure to share the credit for their work that academic women of the era often faced. Gibson had written up her results and the paper had been accepted for presentation at the Eastern Psychological Association. According to Gibson “Liddell said, when I told him about this . . . ‘You know, if you don’t mind I think I’ll just give the paper for you’ . . . I said, ‘No thanks, I’m giving that paper myself,’ because he had nothing to do with it, absolutely nothing” (Gibson, 1998, p. 6). 8. Gibson recounted this memorable incident in an oral history interview: “I said to the farm manager, ‘Where are my kids? I can’t find any of the control group.’ ‘Oh,’ he said, ‘a lot of people come in and wanted kids for Easter presents for their children, so I thought those were the best ones to give them.’ He’d given them all away, so I was absolutely furious, you know. My experiment was ruined, so I quit” (Gibson, 1998, p. 7). When Gibson bemoaned the wreck of her experiment Moore told her “I can tell you what they would have done” (Gibson, 1998, p. 7). 9. In a 1977 talk in which Gibson recounted her lengthy quest for a laboratory of her own (a la Virginia Woolf’s A Room of One’s Own) Gibson noted the gendered inequality that resulted in her collaboration with Walk: “ . . . I went back to animal research with a male colleague, Richard Walk, who of course, did have a lab” (Gibson, 1977). 10. In an oral history interview Gibson indicated that despite her unofficial status at Cornell, her colleagues treated her as a fellow faculty member long before she became a full professor: “They had accepted me as a colleague before that. I had much more prominence than most of them” (Gibson, 1976, p. 8). In addition, in their 1961 paper on the visual cliff Walk and Gibson acknowledge James Gibson’s help “at every stage of the research,” a factor which likely reinforced Eleanor’s authority (Walk & Gibson, 1961, p. 1).


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FIGURE 3. Eleanor Gibson tests a reluctant kid goat on the visual cliff. Image from the Richard D. Walk papers, courtesy Drs. Nicholas and Dorothy Cummings Center for the History of Psychology, The University of Akron.

By then I was very interested in rearing experiments, and I approached Richard Walk who was a young man in the department that had rat labs. He did kind of ordinary rat research. And I said, “What if we write a grant, you and I together, to do some rearing experiments with rats?” “Okay,” he said, “Why not?” (Gibson, 1998, p. 7). Gibson and Walk decided on a study that would look at the role of environment in development, based on Donald Hebb’s popular experiments (i.e., Hebb, 1937). Gibson later noted that “rearing experiments with either impoverished or enriched environments were very fashionable in the fifties” (Gibson, 1991, p. 125), mostly due to Hebb’s recently published book The Organization of Behavior (1949). In this work, Hebb provided a neurophysiological theory of learning that tied the complexity of the learning environment to the growth of brain tissue and the proliferation of cerebral connections. In Gibson and Walk’s first experiment, hooded rats were exposed from birth to metal shapes on their cage walls. At three months they were tested against control rats in a discrimination task using the shapes, and it appeared that prior exposure had given them an advantage (Gibson & Walk, 1956). But a series of eight follow-up studies funded by the NSF failed to replicate this result (Gibson, 1980). Some of the later experiments compared light- and dark-reared rats, with the result that the 90-day-old dark-reared rats performed just as well on JOURNAL OF THE HISTORY OF THE BEHAVIORAL SCIENCES DOI 10.1002/jhbs


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the discrimination task as the light-reared rats that had spent the time being exposed to the shapes (Gibson, Walk, & Tighe, 1959). THE BIRTH OF THE VISUAL CLIFF It was the “serendipitous” (Gibson, 1991, p. 141) inclusion of dark-reared rats in the study that resulted in the creation of the visual cliff. Raising large groups of rats in the dark for three months was very labor intensive, so Walk and Gibson decided to put the last group of 19 dark-reared rats to good use by testing them on a second task. The idea of studying depth perception came from Lashley and Russell’s classic study that measured the force of rats’ jumps (Lashley & Russell, 1934). The experiment had shown that dark-reared rats adjusted the force of their jumps between a jumping stand and a platform based on distance, which led Lashley and Russell to conclude that depth perception was innate in rats. Gibson and Walk settled on the idea of constructing an artificial cliff; if the dark-reared rats had failed to develop depth perception, they would presumably walk indiscriminately on both the “deep” and “shallow” sides of the cliff (Gibson, 1991). This setup had the advantage of correcting a flaw in Lashley’s study—it would eliminate the training period in light, which was necessary for the jumping task.11 Gibson and Thomas Tighe, a research assistant, quickly constructed a cliff apparatus with items found around the laboratory.12 According to Gibson’s later reminiscences, they came up with the idea of the cliff and constructed the apparatus in a single day (Gibson, 1998). In contrast to later more sophisticated models, the first visual cliff was simply a sheet of glass backed by green, gray, and white checked wallpaper and held above the ground with some clamps and rods (Gibson, 1991). The deep side was separated from the shallow side by a center board three inches high, where the rats were to be placed at the start of the experiment, and then observed for five minutes to see which side they descended to (Walk, Gibson, & Tighe, 1957). To the researchers’ surprise, the dark-reared rats acted the same as the light-reared controls and consistently avoided the deep side, descending only on the shallow side. At this point the experimenters began to be worried, wondering if there was something that was biasing the rats toward the shallow side, a draft or odor, perhaps (Gibson, 1991). They checked this by adding wallpaper just beneath the glass of the deep side, so that the optical depths of the two sides were identical. After that change the rats “descended willy-nilly to one side or the other, and then wandered back and forth” (Gibson, 2002, p. 80). The consistency of the effect was completely unexpected; Gibson sums up the collective sentiment by quoting Tighe as saying “I wouldn’t have believed it if I hadn’t seen it” (Gibson, 1991, p. 141). They published their surprising results in Science (Walk, Gibson, & Tighe, 1957). After this “accidental beginning” (Gibson, ca. 1961), Gibson and Walk built a more permanent apparatus and tested less than one-day-old chicks, finding that they too consistently avoided the deep side of the visual cliff. The same was true of newborn kids and lambs, tested 11. Greenhut and Young (1953) had criticized Lashley and Russell for this flaw, as well as for using increasing, rather than randomized, distances. Walk and Gibson dismissed Greenhut and Young’s replication as inconclusive, but wrote “Their criticisms, however, make confirmation by another technique important” (Walk & Gibson, 1961, p. 4). 12. Based on the funding, they requested from the NSF, it appears that Gibson was more involved in the research that would result in the visual cliff experiment than Walk. Compared to Gibson’s funding for 10 months half time, Walk’s involvement was to be only three months, at one-third time (Gibson & Walk, 1955). Although from Gibson’s retellings it appears that the idea of the visual cliff arose out of mutual discussions (Gibson, 2002), Walk seems not to have helped to construct the original apparatus (Gibson, 1991). Given that Walk was presumably also teaching a full course load, it is likely that he was less involved than Gibson in the everyday details of the cliff.


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on a larger apparatus (Gibson & Walk, 1960; Walk & Gibson, 1961). Further experiments at the Behavior Farm resulted in a modification of the apparatus so that the wallpaper’s distance from the glass was easily adjustable. The researchers found that the goats would freeze if the wallpaper dropped more than a foot from the glass (Gibson & Walk, 1960) and that, despite their experience of the glass’s support, the animals never became comfortable on the deep side of the visual cliff. Experiments with pigs, adult chickens, kittens, dogs, and monkeys confirmed their finding that animals tend to choose the shallow side, avoiding the deep side (Walk & Gibson, 1961). Since these animals avoided the deep side of the cliff, the experimenters also tried setting them in the center of the glass on the deep side, which resulted in stereotyped defensive behavior (freezing, refusing to put feet down, etc.), as well as some humorous species-specific responses, such as goats leaping away from the deep side when released, and adult chickens opting to fly over the apparent drop off (Walk & Gibson, 1961). Perception of depth was not innate in all species, however. Experiments with dark-reared kittens resulted in kittens that walked indiscriminately on both sides of the visual cliff. In the Scientific American article, Gibson and Walk paid particular attention to the performance of dark-raised kittens, since they offered the first evidence that the ability to perceive depth was not innate, but dependent on the experience of locomotion. The dark-reared kittens wandered around the visual cliff, apparently blindly, since some walked into the sides of the apparatus. But after six days in the light all of the kittens avoided the deep side of the visual cliff, despite never having experienced falling while on the deep side (Gibson & Walk, 1960). It was not until after quite a number of comparative experiments that Walk and Gibson finally pursued the idea of recruiting human subjects (Szokolszky, 2003; Figure 4). This appears to have been because they did not have easy access to infants—Gibson had to recruit crawling babies by placing an advertisement in the newspaper that ran along the lines of “Have you got a crawling baby? If so, your baby can earn three dollars in an experiment” (Szokolszky, 2003, p. 278). Despite her husband’s concern that mothers would be too suspicious of shock-happy psychologists to volunteer, Gibson had no problem recruiting a sufficient number of babies for their first experiment: “the telephone in the lab rang and rang” (Szokolszky, 2003, p. 278). Thirty-six babies ranging from 6 to 14 months were tested on the visual cliff, with their mothers alternating standing just beyond the shallow and the deep side of the cliff, where they spun a pinwheel and encouraged the baby to crawl to them. Of the 27 babies who left the board, all crawled on the shallow side; only three crawled onto the glass of the deep side (Gibson & Walk, 1960). The conclusions Gibson and Walk reached based on their many visual cliff studies was far more nuanced than the nativist moral later reported by many textbooks: the development of depth perception depended on the species, particularly whether the species needed to navigate from birth. Gibson and Walk concluded their Scientific American article as follows: “From our first few years of work with the visual cliff we are ready to venture the rather broad conclusion that a seeing animal will be able to discriminate depth when its locomotion is adequate, even when locomotion begins at birth” (Gibson & Walk, 1960, p. 71). THE VISUAL CLIFF MYTHOLOGY The rather mundane origins of the cliff can now be contrasted with the more colorful myth that has grown up around them. Although the myth of the visual cliff is best described as a simplification myth rather than a falsehood myth, the simplification of the visual cliff has also fostered a host of persistent errors. The two most common and persistent erroneous “facts” about the visual cliff are that it was inspired by the Gibsons’ visit to the Grand Canyon, JOURNAL OF THE HISTORY OF THE BEHAVIORAL SCIENCES DOI 10.1002/jhbs


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FIGURE 4. Richard Walk tests a monocular infant on a version of the visual cliff. Image from the Richard D. Walk papers, courtesy Drs. Nicholas and Dorothy Cummings Center for the History of Psychology, The University of Akron.

and that it was conducted to address the nativist-empiricist controversy (Rodkey, 2010). David Myer’s popular textbook provides a typical account of the Grand Canyon genesis: “Gibson’s inspiration for these experiments occurred while she was picnicking on the rim of the Grand Canyon. She wondered: would a toddler peering over the rim perceive the dangerous drop off and draw back?” (Myer, 2001, p. 215). This anecdote, which generally includes the Gibson children frolicking on the edge the cliff, has also been used to cement the importance of the nativist-empiricist debate in her thinking—one retelling has James Gibson the nativist JOURNAL OF THE HISTORY OF THE BEHAVIORAL SCIENCES DOI 10.1002/jhbs

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proposing “a simple but ruthless experiment” with their infant to his horrified empiricist wife (Sutherland, 1992, p. 45). The Grand Canyon story is loosely grounded in truth; the Gibsons stopped there in 1946, on their way from California to Massachusetts after the war, but at this point the Gibson children were not crawling infants but ages two13 and six (Caudle, 1990). What really happened, according to Gibson herself, was that she was nervous about the children’s proximity to the edge, but “My husband reminded me that they could see the depth as well as I could, and I believed him” (Gibson, 2002, p. 47). The Grand Canyon is not a falsehood myth in the fullest sense since Gibson did sometimes invoke it when discussing the visual cliff. According to Gibson’s An Odyssey in Learning and Perception retelling, this Grand Canyon visit was remembered, not by her, but by Walk, when they were trying to come up with additional discrimination tasks for the rats in the research project that immediately preceded the visual cliff (Gibson, 1991). Gibson also lightheartedly mentions the experience in her autobiography “Walk had worked in the army with trainees learning to do parachute jumping from a high platform, and I had a long-standing aversion to cliffs dating from a visit to the Grand Canyon with a small child” (Gibson, 1980, p. 258). Students of the Gibsons confirm that Gibson did tell the Grand Canyon inspiration story (see Adolph & Kretch, 2012). However, even if Gibson sometimes used the anecdote, she clearly objected to its use as the definitive inspiration of the visual cliff. Her negative framing of the story in her 2002 autobiography, “Contrary to a popular myth . . . ,” demonstrates that she regarded this use of the Grand Canyon story as beyond the bounds of reasonable simplification, even if the myth was partially of her own making. The Grand Canyon story is generally paired with the second persistent error about the cliff—that the study was designed to address the nature-nurture debate. The visual cliff often makes its appearance in textbooks as an illustration of the nature-nurture controversy, and in these accounts it appears that Gibson and Walk set out to study the depth perception of babies in order to determine whether the ability was innate. McConnell’s introductory textbook provides an example of this framing of the experiment: Gibson was really asking two very important questions about perception. First, are babies born with an innate ability to perceive depth? And second, do infants have a built-in fear mechanism that would make them retreat from sharp drop-offs even without having been trained to do so? (McConnell, 1986, p. 217, emphasis in the original) In some texts, this is taken a step further and Walk and Gibson are said to be nativists, who designed the study hoping to find evidence to support their position (see Hock, 2005). This version of events is not only an inaccurate depiction of Walk and Gibson’s allegiances and involvement in the nature-nurture debate, but, as we have seen, implies a greater degree of intentionality in the initial experiment than was actually the case. Although Gibson and Walk had some interest in whether depth perception was innate, they were not particularly loyal to either faction. Indeed, in their 1961 summary of the visual cliff research, they explicitly distanced themselves from the debate: It was not the purpose of this research to solve the “nature-nurture” problem which has, rightly or wrongly, caused psychologists to do battle with one another for centuries . . . Our purpose, as stated, was a comparative investigation of depth perception with a technique which could be used identically for many species, at an early age and without 13. When they visited the Grand Canyon, Jean Gibson was almost three years old. She was born June 29, 1943, and the Gibsons left California in April 1946 (Gibson, 1980).



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special training. But the question was asked us over and over again, is it innate or is it learned? (Walk & Gibson, 1961, p. 39) Far from hoping to contribute new evidence to one side or another, the nature-nurture battle was not one Gibson and Walk considered worth fighting, as they made clear: . . . we suggest that the nativism-empriricism controversy be abandoned as such, with the aim of restating the problems of development more specifically. They should be stated in terms of the species under consideration, its environment and means of adjusting to it, and especially in terms of the information provided by the environment for this adjustment. (Walk & Gibson, 1961, p. 42) The persistence of the association of the visual cliff with the nature-nurture debate despite these clear denials can most clearly be attributed to introductory psychology textbooks, which have previously been implicated in the introduction of errors and the creation of myths in psychology (Thomas, 2007).14 These inaccuracies are transmitted to the next generation, as a part of the “psychological folklore” (Todd & Morris, 1992, p. 1441) that students accept as a natural part of their socialization into the discipline. Since these errors have the weight of published authority behind them, and generally involve appealing, intuitive stories, they are often difficult to extinguish. Given the pedagogical usefulness of memorable stories and nature-nurture illustrations, it is no surprise that the visual cliff has become known in this simplified, distorted form. THE CLIFF IN THE CONTEXT OF COMPARATIVE RESEARCH Rather than as a contribution to the nature-nurture debate, Gibson and Walk regarded the cliff experiments as part of a comparative research program, as a development in research on depth perception in various species. Since the first visual cliff experiment was relatively spontaneous, it is unlikely that Gibson and Walk conducted a thorough review of depth perception research prior to the experiment. However, in an undated lecture in her archive, Gibson put her experiment in the context of earlier comparative studies that involved depth perception (Gibson, ca. 1961), which probably influenced the later visual cliff experiments. The earliest research Gibson cited was from 1875, in which, “anticipating blind rearing studies” (Gibson, ca. 1961), Douglas Spalding had blindfolded baby pigs at birth, and later set them on a chair and removed the blindfold (Spalding, 1875). According to Spalding, each piglet “knew the height to require considering, went down on its knees, and leapt down” (Spalding, 1875, p. 502). The next instance was E. L. Thorndike’s 1899 study The Instinctive Reactions of Young Chicks, in which he recorded the latency of chicks’ jumping off a pedestal at various heights to join cheeping peers below (Thorndike, 1899). According to Thorndike, the chick “meets the situation ‘loneliness on a small eminence’ by walking around the edge and peering down; 14. The pattern of simplification can be seen in a survey of 20 introductory psychology textbooks from 1960–2000 (Rodkey, 2010). Nineteen of 20 textbooks surveyed mentioned the visual cliff, confirming its status as a famous experiment within psychology. There was a trend away from mentioning the animals of the visual cliff; in textbooks from the first two decades, animals were always mentioned, whereas there were three (out of five) mentions of animals in textbooks from the 1980s, and only one (out of five) in textbooks from the 1990s. In most textbooks, the description of the visual cliff was paired with a photograph or drawing—these were of a baby in 15 cases and of an animal in three (two photographs of a goat and one of a kitten). Each of the photographs of animals were in textbooks published within the first two decades of the visual cliff experiment—there were no photographs of animals in the 1980s or 1990s. A majority of textbooks (12 out of 19) mentioned the visual cliff in the context of the nature-nurture controversy.


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he meets the situation ‘sight of fellow chicks below’ by (after an amount of hesitation varying roughly with the height) jumping off, holding his stubby wings out and keeping right side up” (Thorndike, 1899, p. 285). Thorndike came to a conclusion consistent with the visual cliff findings: “at any given age the chick without experience of height regulates his conduct rather accurately in accord with the space-fact of distance which surrounds him” (Thorndike, 1899, p. 284). Walk and Gibson also cited Yerkes’ (1904) study of turtles at the New York Zoological Gardens, in which he tested aquatic, terrestrial, and semiaquatic tortoises to see whether they would leave a board suspended varying distances above a net (Yerkes, 1904).Yerkes found that the aquatic turtles were far more willing to descend from each height, apparently demonstrating the role of a species’ habitat in depth discrimination. As Yerkes put it, “A land tortoise has cause to notice heights and to react to them in a manner different from that of a water form. The former plunges over a precipice and is dashed to pieces, the latter plunges into the water from an equal height without injury” (Yerkes, 1904, p. 22). Yerkes also tested the turtles on the board while blindfolded by tinfoil caps, which resulted in the aquatic turtles rushing unhesitatingly off the board, whereas the terrestrial and semiaquatic animals tried to remove the blindfolds and, failing that, tended to remain stationary. Finally, Gibson cited Waugh’s (1910) paper, The Role of Vision in the Mental Life of the Mouse, which had found that for mice placed on pedestals, including a dark-reared group, time to descend was related to the degree of depth (Waugh, 1910). Waugh’s apparatus for this test was a wooden disk suspended above a bench by a column of adjustable height. In some of the later trials Waugh added electric shock to the disk in order to encourage quicker descents. In summarizing his research, Waugh admitted “this is not an entirely satisfactory method of studying distance perception in the mouse” (1910, p. 576). Based on this literature review, Gibson answered her own question—“Did no-one use a cliff before?”—with the reply, “not a two sided one, but the idea is an old one” (Gibson, ca. 1961). According to Gibson, the development of the cliff was “inevitable” (Gibson, ca. 1961) since it possessed so many advantages over prior methods: “A technique which requires no training and is applicable to many species was needed” (Gibson, ca. 1961). In addition to these requirements, Walk and Gibson said, the apparatus needed to “permit control of all cues other than optical ones” (1961, p. 5). According to Walk and Gibson, of the various apparatus used in prior depth perception research, none satisfied all three requirements for proper testing of depth discrimination: no pretraining required, control of all nonvisual cues, and adaptability for various species. The visual cliff apparatus possessed an additional advantage, one noted only briefly by Walk and Gibson. All prior studies had depended on the animals jumping or leaping from a height in order to demonstrate their depth perception. In some cases, as with Yerkes’ tortoises, it was difficult to determine whether a failure to jump was the result of deficient depth perception, or a result of a disinclination to exert itself on the part of the animal. Waugh acknowledged this problem in his discussion of his apparatus: mice’s failure to jump might be due “not to any visual short-sightedness, but to the fear of jumping in general” (Waugh, 1910, p. 577). Based on a review of these prior studies, the chief innovation of the visual cliff was that it was two sided; animals were not required to jump in order to demonstrate their depth discrimination, they simply had to show a preference for the shallow side. Further, not only did the addition of a glass surface isolate visual cues as the factor being tested, but it also improved the apparatus’ safety, a development that made testing babies feasible. Restoring the visual cliff to its original context within the comparative research of depth perception puts the neglect of its animal subjects in a new light. The status of comparative JOURNAL OF THE HISTORY OF THE BEHAVIORAL SCIENCES DOI 10.1002/jhbs


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research within psychology has been called “ambiguous” (Dewsbury, 1992, p. 14) and “peripheral” (Dewsbury, 1992, p. 4), due to its several unique challenges. The factors that helped to marginalize comparative psychology likely also influenced the way the visual cliff experiment was perceived and remembered. For example, it seems relevant that Eleanor Gibson’s early inclination toward comparative psychology and subsequent shift toward more human-centered research was a common career pattern in the early part of the twentieth century. Dewsbury (1992) has documented how during this period young comparative psychologists tended to abandon their comparative research after graduate school because they were unable to secure jobs or research funding in what was deemed a peripheral area of psychology. While Gibson was initially prevented from pursuing her comparative interests by her gender, not by comparative research per se, the problems associated with comparative research show up in her research limitations at Cornell. Animal research is resource-heavy, labor intensive (e.g., dark-rearing rats), and often smelly and noisy (e.g., the chimpanzees on the Yale rooftop; Dewsbury, 1992). It is an endeavor not lightly undertaken, and rarely without institutional support. Yerkes himself found Yale’s funding insufficient for his primate research ambitions, and so sought outside philanthropic patronage. In fact, at the same time that Gibson was unsuccessfully petitioning for her admission to the world of primatology through Yerkes, Yerkes was himself desperately lobbying for the continued support of the Rockefeller Foundation for his laboratory (Dewsbury, 2005; Biehn, 2008). For years, Yerkes had enjoyed the generous patronage of the Rockefeller Foundation for his research on primate sexuality, which had appealed to the Foundation because of its purported human application, a good fit with their interest in promoting social hygiene. But an increasing emphasis on biology, and political changes at the Rockefeller Foundation, combined with Yerkes’ failure to deliver on his lofty promises of findings indispensable to human engineering, made the Rockefeller Foundation wary of giving the Institute further money (Biehn, 2008). As Dewsbury (1992) points out, the primary reason for comparative research’s peripheral status was that psychology was considered a human science, and any comparative work had to be justified in terms of its relevance for human concerns. The dilemma or paradox of animal psychology was that it had “developed as the study of non-human animals in a science whose emphasis was on the mind and behavior of humans” (Dewsbury, 1992, p. 4). This meant that comparative psychologists constantly had to justify their work in human terms—both Yerkes’ success and failure in receiving patronage stemmed from his ability to connect his primate research to the “welfare of mankind” (Biehn, 2008). It seems likely that this anthropocentric bias in psychology contributed to the visual cliff’s initial comparative context being forgotten. CHANGES TO THE VISUAL CLIFF Gibson and Walk published several articles on the initial visual cliff experiments (Walk, Gibson, & Tighe, 1957; Gibson & Walk, 1960, 1961; Walk & Gibson, 1961), of which the version published in the Scientific American was the most influential. Less well known but more extensive is Walk and Gibson’s monograph A Comparative and Analytical Study of Visual Depth Perception (Walk & Gibson, 1961). This 44 page long piece elaborates on the details of the apparatus, the testing procedure, and the behavior of the various species tested on the visual cliff, which allows for a reconstruction of some of the forgotten details of the experiments. Interestingly, the version of the visual cliff presented in the Comparative monograph is far from the neatly finished product of legend. The details of the experiment appear to be in flux, as the experimenters respond to the requirements of each animal, adjusting the cliff JOURNAL OF THE HISTORY OF THE BEHAVIORAL SCIENCES DOI 10.1002/jhbs

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in order to overcome the latest species-specific problem. Walk and Gibson note the “slight alterations” that had to be made to the original apparatus in order to make it appropriate for additional species, although they argue that “a common visual stimulus was present, and the same procedure was used for all” (Walk & Gibson, 1961, p. 13). While Walk and Gibson are probably right about the common visual stimulus, the testing procedure was not identical or static. In discussing the changes to the apparatus, Gibson and Walk note “The slight alterations themselves are informative” (Walk & Gibson, 1961, p. 13). This statement holds true for the changes in testing procedure as well. Changes to the Visual Cliff Apparatus The first alteration to the original plan for the visual cliff was the introduction of the center board, which occurred before the first original experiment with rats. Before using their valuable dark-reared rats, pilot studies with light-reared rats had revealed the need for a center board. In the Comparative monograph, Gibson and Walk recount that they previously tested nine young rats using a center board only 1.5 inches high and discovered that six descended to the shallow side, and three to the deep side. While this preference for the shallow side was still statistically significant, it was not a dramatic enough for the experimenters’ tastes, and they subsequently adjusted the height of the board to 2.5 inches, since “for the rat, the board must be high enough to induce some disinclination to jump down so that predominately visual cues will be used for descent; otherwise this animal will tend to respond on the basis of tactual and kinesthetic cues” (Walk & Gibson, 1961, p. 14). The innovation of the center board demonstrates how a problem presented by a particular species could leave its mark on the experiment. This alteration to the apparatus was not necessary for larger, more sight-dependent species, such as human infants, yet the center board persisted, an artifact of the cliff’s original subjects. However the depth of the center board also changed, according to the needs of the animals being tested on it. For newborn chicks, a 1.5 inch center board was sufficient, but this was not surprising given that chicks behaved in ways that indicated a far more visual approach than rats: the chick “remains motionless for some period of time, then begins to cheep and move its head from side to side and finally jumps off the board with little of the locomotion up and down the center board so characteristic of the rat” (Walk & Gibson, 1961, p. 16). For larger animals, it was the width of the center board, which sometimes had to be adjusted, as with young kittens that would fall off the narrow board. In this case Gibson and Walk admitted “Our first attempt to study behavior of the kittens on the visual cliff was a failure, since the kittens were tried immediately after the eyes had opened (about 10 days) and proved unable to locomote with any control at all of motor coordination. They fell backward or lay on the center board and mewed” (Walk & Gibson, 1961, p. 19). Both the width and the height of the center board were adjusted “as the size of the kitten required” (Walk & Gibson, 1961, p. 20) in future experiments. The center board was not the only part of the apparatus which varied across experiments. The crude first cliff was soon abandoned in favor of a model that allowed for better control of illumination. In fact, there were three versions of the visual cliff used in the experiments, which Walk and Gibson designated Cliff Model I, II, and III (Walk & Gibson, 1961). Cliff Model I was that first basic model: a glass sheet, with no protective edges, held up by rods and clamps, and surrounded by a cheesecloth enclosure to shield the experimenters from view. This version depended on the room’s florescent light, with the addition of some lighting from below, onto the deep side, in order to equalize the brightness readings of the two sides. However, the readings were not exactly equal, so Cliff Model II was designed to address this problem. JOURNAL OF THE HISTORY OF THE BEHAVIORAL SCIENCES DOI 10.1002/jhbs


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Rather than being in table form, this version was a wooden box suspended from the ceiling beams by two upright wooden two-by-four, which also supported a light-diffusing canopy made of white cotton cloth. This version of the visual cliff allowed for precise control of the visual stimulus, allowing Gibson and Walk to validate the cliff as an apparatus, reassuring themselves that variations in the apparatus’ lighting, reflections on the glass, and texture size did not significantly change the visual cliff effect. The third version, Cliff Model III, was primarily designed to test larger animals and human infants. This version of the cliff was considerably heavier, so supporting legs were added to the middle of each long side, as well as at each corner. An 8-inch high board around the cliff provided protection for the subjects, and irregular green and white pattern linoleum that matched the flooring in the room provided the texture for the shallow side and center board. Although Walk and Gibson do not record this in their monograph, it seems clear there were even further, at least superficial, modifications of this cliff, as some of the photographs in the Scientific American article show a Cliff Model III with a checked surface, which a color photograph in another publication (Gibson & Walk, 1961) reveals to be a cheery red and white. In 1959, Walk left Cornell and he and Gibson agreed that only one of them should work on the cliff (Szokolszky, 2003), and they agreed that it should be Walk.15 Even after this the cliff continued to change, with Walk making further modifications over his years of experimentation, which allowed for more precise control of what the subject saw on the deep side (see Walk, 1966; Walk & Walters, 1974). The visual cliff later became commercially available as Lafayette Instrument Company Model 86030. Changes to the Visual Cliff Test Procedure The most significant innovation in testing procedure from the original depth perception research was necessitated by most animals’ refusal to leave the shallow side of the cliff— Gibson and Walk began setting animals on the glass over the deep side and recorded their reactions. This approach resulted in vivid descriptions, most often of animals’ disturbed behavior when placed above the deep side, contrasted with their normal behavior. For example, Gibson and Walk reported that whereas kittens had been quite active on the shallow side of the cliff, when placed on the deep side they backed up, looking constantly downward . . . . Sometimes it succeeded in backing to the wall; then it proceeded to creep around the edge, hugging the wall, and walked on what appeared to be a small path below the glass made by the supporting board. One kitten backed to the wall, climbed the edge and clung to it . . . This behavior was sometimes accompanied by shivering and mewing. (Walk & Gibson, 1961, p. 21) Similarly, the puppies tested on the deep side tended to stand still and then back up, some showing signs of distress, “Two of the puppies visibly trembled when placed on the deep side and another one squealed, behavior not observed on the shallow side” (Walk & Gibson, 1961, p. 21). While Albert, one of the two infant rhesus monkeys tested on the visual cliff, did not at first show anxiety on the visual cliff, and at 10 days old, could be induced to cross onto the deep side, “The manner of locomotion was different from that on the shallow side, for he crawled with stomach close to the glass and looked down constantly. When he reached the wall, he clung to it” (Walk & Gibson, 1961, p. 22). 15. This was likely because of Gibson’s continuing lack of laboratory space. She would not receive her own laboratory space until 1965, when Cornell promoted her from research associate to professor.


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Regarding their use of this more descriptive methodology, Gibson and Walk commented that their observations of animals’ reaction to lack of visual support were not quantified because “the observations were carried out incidentally to the main line of investigation” (Walk & Gibson, 1961, p. 41). The inclusion of descriptions of animals’ disturbed response to the deep side of the cliff, contrasted with their normal behavior, appear calculated to reinforce the quantitative evidence of preference for the shallow side. The backing, freezing, trembling, and mewing all powerfully testified to the animals’ ability to discriminate depth. Problems of Motivation The most significant obstacle Gibson and Walk encountered in their use of animals other than rats on the visual cliff was the problem of motivation. Although rats possessed a “natural exploratory drive” (Walk & Gibson, 1961, p. 8) that motivated them to explore their environment, this was not true of all other animals, which would sometimes remain immobile on the center board, frustrating testing. This consideration resulted in changes to the testing protocol, generally with the addition of some sort of reward. For example, while chicks had proven themselves willing subjects, adult chickens turned out to be frustratingly stationary: “these subjects all remained on the board for the entire 5-minute period. To motivate the hungry subjects, a very small quantity of cracked corn was scattered along the center board in front of it” (Walk & Gibson, 1961, p. 16). In order to motivate the infant rhesus monkeys, experimenters used a blanket placed on the glass of the edge of the cliff as a lure in the first trial, and later used their feeding bottles on alternating sides of the cliff. However, this approach was not always effective: “At 1½ months, Albert became very emotional when placed on the cliff. He hugged his legs, head down on the board and refused at length to move. He finally leapt to his blanket when it was waved at him on the shallow side, but refused to reach for it on the other side” (Walk & Gibson, 1961, p. 22). By far the most troublesome subjects were turtles. Gibson and Walk note that some of the turtle trials had to be conducted on multiple days “because the animals often did not move in 15 to 20 minutes” (Walk & Gibson, 1961, p. 19). Indeed, one turtle had only one successful trial out of six initially planned, because after one descent to the shallow side (which took 15 minutes) it simply failed to move off the board. According to Walk and Gibson, “This turtle spent several hours on the visual cliff without moving” (Walk & Gibson, 1961, p. 19). The most unusual choice of motivator was in the case of a single dachshund puppy, in which the presence of its owner standing at the far end of the cliff served to motivate it to leave the center board. This set up has some obvious parallels to the infant and mother testing, and if it preceded it, may have had some role in the development of the use of mothers as motivators for their babies. Unfortunately the exact chronology of the animal testing is not clear from published and archival sources. It seems that at least rats, chicks, lambs, and kids were tested prior to infants (Szokolszky, 2003); the evidence about pigs, kittens, turtles, puppies, and monkeys is ambiguous.16 The use of a relational motivator is an unusual alteration of the typical animal subject script, particularly given that puppies are fairly curious subjects and would generally explore the cliff independent of any extrinsic motivation. Noting that “The puppy had been petted a good deal,” Gibson and Walk record that it “rushed to its owner when he stood at the shallow 16. The testing of a litter of cocker spaniels took place at George Washington University after Walk’s departure from Cornell in 1959, and thus after the first study of infants, however, the dachshund puppy testing could have taken place at any time, making it impossible to tell which experiment influenced which.



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side, and cried and whimpered when he stood at the other side” (Walk & Gibson, 1961, p. 21). The pilot puppy study thus demonstrated the strength of the visual cliff effect, since it was not overcome by the evidently powerful puppy-owner bond. THE CHOICE OF ANIMAL SUBJECTS A question that presents itself given the diversity of animals tested on the cliff is why Gibson and Walk chose to test the animals they did. Historians of science have observed that the animals used in a particular experiment are rarely determined purely on the basis of objective criteria, therefore the choices of the experimenter have meaning (Logan, 1999; Pettit, 2010). An animal’s cultural meaning, for example, whether it is a species designated an appropriate laboratory subject (as opposed to those regarded as wild animals or “companion species”) guide these choices, in addition to issues of practicality and the constraints of a particular experiment (Ritvo 1987; Haraway, 2008; Kirk, 2014). The learning experiments that led up the visual cliff had used albino rats, a species seen as a representative organism and adopted as “the right animal for the job” (Clause, 1993) by the psychological community, both because of historical accident and for its many experimental advantages (on the history of the rat as a model organism, see also Hendrickson, 1988; Logan, 1999; Wertz, 1994). By the time that Gibson and Walk came to design their NSF-funded learning research, the rat’s transformation from pest to ideal research subject had long been completed. Norman Munn’s declaration that one could “write an essentially complete outline of the science of animal behavior, without going beyond the rat” (Munn, 1933, p. v) was almost universally, albeit implicitly, accepted. For learning tasks in particular, the albino rat was taken to be an obviously representative organism. In fact, the only reason the first subject of the visual cliff was not the albino rat is that the experiments dealt with visual perception, and since albino rats have very poor eyesight, when it came to the dark-rearing experiments, Gibson and Walk switched to using hooded rats that have better sight.17 Unfortunately, both the chronology of animal testing and Gibson and Walk’s rationale for choosing these animals has largely been lost. At some points in the Comparative monograph, Gibson and Walk comment on why a particular species was chosen, such as “The baby chick is an interesting animal to use on the visual cliff because it can be tested within a few hours of birth” (Walk & Gibson, 1961, p. 15) or “The goat and the sheep are interesting animals for studying depth discrimination, since they are descended (the goat in particular) from mountain climbing ancestors” (Walk & Gibson, 1961, p. 17). Rats and chicks were the animals of choice for experiments testing texture density and motion parallax “since the smaller apparatus for them made total control of stimuli easier. The large room required for testing the human infants, for instance, made isolation of the sources of differential stimulation impractical. The rats and chicks were plentiful and their entire field of view could be manipulated experimentally” (Walk & Gibson, 1961, p. 26). While these advantages may have played some role in subject selection, given the order in which they tested the animals Walk and Gibson’s overriding consideration seems to have been pragmatic—depending on the animals’ availability. Chicks, like rats, were commercially available and inexpensive, and lambs and kids were plentiful at the Behavior Farm, where Gibson still had connections. After transporting five 6- to 7-day-old goats to Cornell’s Morrill 17. Interestingly Gibson and Walk return to albino rats in the Comparative monograph, testing albino rats’ avoidance of the visual cliff as compared to hooded rats. The albino rats showed slightly poorer avoidance of the deep side, but not significantly so.


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FIGURE 5. An example of the unpredictability of animal subjects: a chicken flies over the deep side of the visual cliff. Image from the Richard D. Walk papers, courtesy Drs. Nicholas and Dorothy Cummings Center for the History of Psychology, The University of Akron.

Hall for testing, the visual cliff was moved to the Behavior Farm and subsequent experiments took place there. Pigs, such as the two 6-week-old pigs tested on the cliff, were also readily available at local farms, if not at the Behavior Farm itself. The fact that there was only one dachshund puppy tested illustrates the extent of Gibson and Walk’s opportunistic approach to subject recruitment; the puppy likely belonged to a friend of theirs. In one case at least, the dangers of this opportunistic strategy became clear. The first litter of kittens successfully tested on the cliff were farm kittens a month old “rather wild, since they had lived in a barn and had never been petted” (Walk & Gibson, 1961, p. 20). What is meant by “rather wild” soon becomes apparent: “The kitten which went to the deep side appeared to be attempting to escape from the experimenters” (Walk & Gibson, 1961, p. 20) and “One cat, the wildest, went an equal number of times to the two sides. It also jumped from the table side to the floor when the experimenters tried to catch it” (Walk & Gibson, 1961, p. 21). The wild kittens’ aversion to being handled by the experimenters made them bad subjects, resulting in noisy data. A completely wild animal was not the ideal subject for the visual cliff, preferable were domesticated species and animals frequently handled by humans. After this litter of kittens, Gibson and Walk turned to less convenient but more manageable tame kittens (Figure 5). However, when there was a particular reason for needing a specific species, Gibson and Walk did make the effort to obtain them, as in the case of the aquatic turtles that were needed to check Yerkes’ earlier (1904) experiments. The rhesus monkeys were also harder to obtain, loaned by Robert R. Zimmermann, a psychology professor who arrived at Cornell in 1959 fresh from studying monkey mother surrogates with Harry Harlow at the University of Wisconsin. The infant rhesus monkeys tested on the cliff were very likely descendents of Harlow’s colony, since Harlow was in the habit of sending his former graduate students off to their new jobs with monkeys and apparatus that would help them to establish new laboratory facilities in their new JOURNAL OF THE HISTORY OF THE BEHAVIORAL SCIENCES DOI 10.1002/jhbs


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institutions (H. LeRoy, personal communication, October 1, 2009). In his recommendation of Zimmermann, Harlow wrote to the chair of psychology at Cornell: I had hoped that some position might develop where he could continue working with the baby monkeys. I have told him that if it did . . . that I would be happy to ship him, say 10 of our pregnant stock animals each spring, let him take the babies, and then send the mother back. Such a program could be carried out without imposing any vast facility burden upon an institution. (Harlow, 1958; H. F. Harlow to T. A. Ryan, December 12, 1958). This ambition for a Cornell monkey colony was unrealized, since Zimmermann left Cornell for University of Montana in 1966. However, Gibson and Walk benefited from Zimmermann’s time in Ithaca; he was able to provide them with monkey subjects, presumably of interest because of their similarities to humans (Noon, 2005). THE VISUAL CLIFF’S HUMAN SUBJECTS Perhaps one reason the visual cliff became remembered for the infant studies is because whereas a number of studies of animals’ depth perception had previously been published, there had been no similar studies of human infants. As Walk and Gibson noted (1961), there was a complete lack of knowledge about the depth perception abilities of human infants. The only prior work in the area that Gibson and Walk could find were a handful of studies dealing with infant’s discrimination of distance ahead (Updegraff, 1930; Johnson & Beck, 1941; DenisPrinzhorn, 1960), and one study of adult paratrooper trainees, which showed that hesitancy and poor technique increased as the training tower height increased (Windle et al., 1956).18 The primary reason for this neglect was the difficulty of ensuring the safety of the subjects using prior methods. Although experimenters were generally somewhat concerned with the safety of their animal subjects—in his turtle studies Yerkes noted that “the fall was thus rendered harmless to the animal” (1904, p. 19) by the use of a net below—the safety procedures available for jumping methods did not generally meet standards acceptable for human infants. Even using the visual cliff there were sometimes mishaps. Walk and Gibson recorded that “Two of the lambs went over the side of the apparatus, one falling, but the other apparently jumping” (1961, p. 30). So, while the cliff’s improved safety provided the opportunity to study infant depth perception, the testing procedure again had to be altered in order to sufficiently protect the new human subjects. Before testing infants on the cliff, Gibson and Walk placed a 50 pound weight on the glass to ensure that it could support the infants’ weight. In addition, two experimenters stood on either end of the center board and moved toward the baby if he crawled too close to the edge “to safeguard him” (Walk & Gibson, 1961, p. 23). The first human tested on the visual cliff was an 18-month-old male. After crawling off the centerboard onto the shallow side, he stood up and walked around on the shallow side. He refused to cross to the deep side, but when placed on the deep side, “He firmly clutched the wooden support with one hand, curled his toes and hitched himself cautiously toward the experimenter” (Walk & Gibson, 1961, p. 23). This unnamed child served as pilot study, but it was difficult to know how far his behavior could be generalized, since his age meant that he had previously fallen “from cribs, beds, sofas, chairs, etc. on to the hard floor or the bare ground” (Walk & Gibson, 1961, p. 23). 18. This reference was likely Walk’s contribution, given his military research background.


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In the first official experiment with infants, the subjects were simply placed on the centerboard and observed for several minutes—“a procedure that had worked well with animals” (Walk & Gibson, 1961, p. 23)—while the mother stood out of sight behind a screen. The only inducement to leave the center board was a pinwheel placed at each end, which emitted a tinkling sound as it rotated. While this type of procedure had worked for animals, it was not successful for infants. The first two subjects tested refused to venture far from the board in the seven- and eight-minute testing periods. As a result “The third subject marked a change in procedure. After the infant would not move the mother was put at the shallow side. She turned the pinwheel and talked to the child” (Walk & Gibson, 1961, p. 23). The success of this approach resulted in a new standardized procedure: The mother stood twice at each side, alternating, some mothers starting at the shallow side, some at the deep. The mother stood for 2 minutes at each side unless the child got off the board and reached for a lure. If this happened, the child was put back on the board and the mother switched sides. (Walk & Gibson, 1961, p. 23) This procedure was successful: babies generally crawled to their mothers on the shallow side, but few crawled to her over the deep side, and some cried or crawled on the shallow side when faced with this situation. The addition of the maternal figure as a motivator subtly changed the dynamics of the experiment. The introduction of a mother twirling the pinwheel and encouraging her baby to come to her added a social aspect to the experiment that was absent in the animal studies. Although the mothers were not the official subjects of the experiment, and therefore their behavior was not closely scrutinized, in effect they comprised an additional participant in the experiment, one with complex attitudes and motivations. Gibson and Walk found the psychology of the mothers worth recording: The predominant impression among mothers seemed to be that the child had failed the “test” because he did not have enough sense to realize the glass was safe to crawl over. The glass on the deep side was banged with hands and fists; cigarette boxes, lipsticks, purses, crumpled bits of paper, and other releasers of infant approach behavior were proffered . . . (Walk & Gibson, 1961, p. 25) The addition of the mothers added an extra layer of interpretation to the results. Did the baby’s failure to cross the deep side indicate a lack of faith in her mother? Rather than the fairly clear meaning attached to crossing to the deep side—that an animal had failed to discriminate depth—a successful crossing could now mean that a baby had nobly set aside his fears based on his attachment and trusted in his mother. The moral valences of the visual cliff, which had previously been clear (avoiding the deep side: good; crossing it: bad) were now ambiguous. This social component of the experiment appears to have also increased the appeal of the infant research—whereas the animal subjects were more or less inscrutable, the presence of mothers allowed infants’ motives and emotions to be detected, or at least imposed on the situation.19

19. Later research on the visual cliff has investigated this social relationship, finding that the mother’s facial expressions can influence their baby’s behavior on the cliff (see Sorce et al., 1985).



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CONCLUSION This reconstruction of the visual cliff research began with the classic Scientific American photograph that shows a single baby poised on the edge of the visual cliff, attended by his encouraging mother, confronting the visual cliff dilemma. While this depiction is not inaccurate, this paper has contested this simplified, mythic version of the experiment and offered an alternative, more complex, and less “airbrushed” account of the research that highlights the extrascientific factors that influenced its development and execution. Contrary to popular myths about its conception, the visual cliff was not the result of extensive planning or theorizing, but the serendipitous result of circumstance: a research project calling for dark-reared rats, and the researchers’ desire to maximize the productivity of such a labor-intensive endeavor. Going back a level further, the reasons that Eleanor Gibson came to conduct the experiment in the first place was also a result of circumstance, but not entirely random ones. These were circumstances that were heavily circumscribed by Gibson’s gender and the cultural attitudes and rules regulating women’s participation in academia at the time (Johnston & Johnson, 2008). The institutional sexism of antinepotism rules does not fit comfortably within the romanticized visual cliff accounts, yet Gibson’s efforts to create a viable experimental research program despite her chronic lack of laboratory space were clearly an important contributor to the experiment’s origins. As this paper has shown, the visual cliff was about far more than the babies—a wide variety of animals were tested, and tested prior to infants. In so far as Gibson and Walk conducted a literature review or considered their research as arising out of a historical tradition, they drew on the comparative literature. In the context of the body of prior depth perception research, it is clear that the visual cliff possessed several advantages over prior methods, most notably that it did not require subjects to jump, and offered them a new degree of safety. Rather than a definitive contribution to the nature-nurture controversy, Gibson and Walk offered a more modest conclusion about importance of locomotion in the development of perception. However, the visual cliff apparatus and testing procedure did not emerge as completed products after the first experiment; Gibson and Walk had to adjust both for various species, given the subjects’ differing physical characteristics and motivations, and these changes permanently altered both the cliff and protocol. What species were tested on the visual cliff was influenced largely by issues of availability and convenience. And as the case of the “wild” kittens illustrates, not all animals reacted neatly and predictably to the deep side of the cliff. Some had to be motivated to move with blankets, food, or owners. Even the most famous form of experiment was the result of a failed pilot test—mothers had to be added to motivate infants’ movement off the center board. In the popular retelling all these complicating details have been lost. In terms of explaining how this simplification happened, more popular visual cliff publications, with their reliance on images, are one obvious culprit. The photographs from the Scientific American article not only shaped their readership’s perception of the experiment, but, given the images’ widespread adoption by introductory textbooks, also influenced psychologists’ understanding of the research. Ultimately it was this simplified, human-centric version of the cliff, a version stripped of historical detail, which proved most useful for psychology, since it could easily be adapted for a variety of functions (illustration of the nature-nurture debate, etc.). This accords with the scholarship about scientific popularization: scientific findings appear more authoritative if they are decontextualized, separated from the conditions in which they were produced, and if the contingencies and limits of generalizability of the findings are


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hidden (Whitley, 1985). Given these pressures, it is not surprising that the visual cliff lost much of its complexity in the retelling. While Dewsbury (1998) has observed that the simplification of research in the process of popularization is inevitable, the form the simplification takes is not. Therefore the question remains, why this particular simplification? Why babies, and not animals? Of course there is the simple fact that people find babies interesting and appealing, generally more so than rats or goats. But a 1959 Life magazine feature (Fenn, 1959) included photographs of a lamb and a pig on the visual cliff in addition to photographs of babies, and a 1961 publication in Natural History (Gibson & Walk, 1961) had only images of animals on the cliff: one of a kitten and four of goats (see Figure 2 for one of these images). The use of photographs of animals in these popularization attempts demonstrates that a simplification of the cliff relying on the animals was possible; they too could be appealing and sympathetic subjects. One reason for the visual cliff’s strong association with infants is likely the identification of the visual cliff with Gibson’s later infant perceptual development work. After Cornell finally hired Eleanor Gibson as a professor in 1966, she went on to research (in her laboratory dedicated to infant research) a topic closely connected to the visual cliff—perceptual learning. Indeed, with the publication of her book Principles of Perceptual Learning and Development (1969), Gibson helped to define perceptual learning as a distinct research focus and to pioneer methods for studying perceptual development in infants. One of her later perceptual learning experiments even tested infants’ perception of the traversability of surfaces using a version of the visual cliff, which replaced the visual drop off with an unstable water-bed surface (Gibson et al., 1987). This clear connection to an innovative research program may be one reason why the cliff became more associated with Gibson, rather than Walk. Although Walk worked with the cliff apparatus longer, he conducted less glamorous systematic research, such as varying the depth of the cliff (Walk & Walters, 1973), the color (Walk, 1966) and texture of its checked pattern (Walk, 1968a; Walk & Walters, 1974), testing monocular subjects (Shiffman & Walk, 1963; Walk, 1968b) and other species, such as rabbits, ring doves, and ducklings (Walk, 1966, 1972). Yet another reason for the linkage between the cliff and infants is Gibson’s own presentation of the study in her subsequent writings. Although her earlier publications highlighted the animal research, as time progressed, she separated her discussion of animal and infant visual cliff experiments (in some cases into different chapters), then dropped the discussion of the animals altogether, choosing to highlight only the infant study (Rodkey, 2010). In her later writings, Gibson betrayed an awareness of the mythic version of the cliff and a willingness to co-opt the experiment to serve her own ends, using it, among other things, as an example of the perception concept of an affordance (Gibson, 1983; E. J. Gibson to J. Sims-Knight, n.d.; Gibson & Pick, 2000), an idea that postdated the experiment (Szokolszky, 2003). However, this use of the cliff by Gibson may be merely another instance of her strategic use of the very gender stereotypes that entangled her. Despite Gibson’s self-conscious pursuit of hard science and early rejection of women’s work, after the visual cliff experiment she found herself associated with a highly feminized, domestically themed experiment. The checkered wallpaper of the cliff, when paired with an infant and mother instead of rats, invoked the image of a picnic tablecloth-covered table, rather than a scientific apparatus—just the sort of experiment in which a female scientist seemed the natural protagonist. While it may not be surprising that the version of cliff that has been remembered is one that was perfectly palatable to mid-century gender sensibilities, the cliff’s more complex comparative origins demonstrate how shockingly different this myth was from reality.



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To put it clearly, the myth of the visual cliff remakes the most eminent experimentalist in the second generation of female psychologists into someone focused on infant development (a feminized research area) whose own research was inspired by her own domestic relations (i.e., her experience with her children during a family vacation). Instead, as the evidence shows, Gibson was a serious research scientist seeking to contribute to the manly world of comparative and physiological psychology, but who was repeatedly blocked, because of her gender, from joining the existing multispecies research network (Pettit, Serykh, & Green, 2015). In this light Gibson’s exclusive identification with the visual cliff’s human subjects is deeply problematic. Rather than Gibson’s frequent inclusion in introductory textbooks representing a victory for gender equality, her representation in connection with the mythic version of the experiment actually helps to maintain the historical marginalization of female psychologists. Finally, perhaps the most comprehensive answer to the question of why the visual cliff was remembered as it was is to be found in the history of comparative psychology: the reasons for the neglect visual cliff’s menagerie likely have to do with the peripheral status of comparative research within psychology. The reality that animal research in psychology always had to be justified in terms its relevance for human concerns offers a compelling reason why the baby version was best remembered. In fact, this factor helps to explain both the popularity of the cliff’s anthropocentric origin myths and the neglect of its animal subjects. Although the animal studies had to be modified in order to suit human interests, the baby study had instant and obvious application—the “infants at the creeping and toddling stage”(Gibson & Walk, 1960, p. 67) who were so notoriously prone to tumble. ACKNOWLEDGMENTS The author would like to thank Helen A. LeRoy for generously sharing her Harry Harlow correspondence and Alexandra Rutherford, Michael Pettit, Jennifer Bazar, Kelli VaughnJohnson, and the anonymous reviewers for their very helpful comments. REFERENCES Adolph, K. E., & Kretch, K. S. (2012). Infants on the edge: Beyond the visual cliff. In A. Slater & P. Quinn (Eds.), Developmental psychology: Revisiting the classic studies (pp. 36–55). London: Sage Publications. Baron, E. A., Earhard, B., & Ozier, M. (1997). Psychology (2nd Canadian ed.). Scarborough, ON: Allyn and Bacon Canada. Biehn, K. J. (2008). Psychobiology, sex research and chimpanzees: Philanthropic foundation support for the behavioral sciences at Yale University, 1923–42. History of the Human Sciences, 21, 21–43. doi:10.1177/0952695108091411 Block, J. D. (1963). In Memoriam: Howard S. Liddell, PhD 1895–1962. Psychosomatic Medicine, 25, 1–2. doi:10.1097/00006842-196301000-00001 Carlson, N. R. (1990). Psychology: The science of behavior (3rd ed.). Boston, MA: Allyn and Bacon. Caudle, F. M. (1990). Eleanor Jack Gibson (1910-). In A. N. O’Connell & N. F. Russo (Eds.), Women in psychology: A bio-bibliographic sourcebook (pp. 104–116). New York: Greenwood. Clause, B. T. (1993). The Wistar rat as a right choice: Establishing mammalian standards and the ideal of a standardized mammal. Journal of the History of Biology, 26, 329–349. doi:10.1007/BF01061973 Denis-Prinzhorn, M. (1960). Perceptions des distances et constance des grandeurs: Etude genetique. Archiv fur Psychologie, Geneve, 37, 181–309. Dewsbury, D. A. (1992). Triumph and tribulation in the history of American comparative psychology. Journal of Comparative Psychology, 106, 3–19. doi:10.1037/0735-7036.106.1.3 Dewsbury, D. A. (1998). Robert Yerkes, sex research, and the problem of data simplification. History of Psychology, 1, 116–129. doi:10.1037/1093-4510.1.2.116 Dewsbury, D. A. (2005). Comparative psychology: A case study of development of support for basic research by a federal agency with an applied mission, 1948–1963. In W. E. Pickren & S. F. Schneider (Eds.), Psychology and the National Institute of Mental Health: A historical analysis of science, practice, and policy (pp. 31–60). Washington, DC: American Psychological Association.


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