SPECIAL SECTION SPECIAL SECTIONS EDITOR: JOHN E. SCHOWALTER, M.D. GUEST EDITOR: FRANK C. VERHULST, M.D.
Longitudinal Research in Child and Adolescent Psychiatry FRANK C. VERHULST , M.D .,
AN D
HANS M. KOOT, M.S.
Abstract. An indispens able approach to the study of variations in individual development and of causal mechanisms and processes underlying the course of psychopatholog y is the longitudinal method . In this introductory review , the strengths and weaknesses of longitudinal research are discussed, and factors hampering progress in this field are outlined. The many advantage s of this approach warrant continuing efforts to develop strategies that minimize its drawback s. J . Am . Acad. Child Adolesc. Psychiatry, 1991, 30, 3:361-368 . Key Words: longitudinal research, child/adole scent psychopathology. The study of continuities and discontinuities in individual development and of factors that influence developmental processes is crucial to our understanding of normal as well as abnormal functioning of children . (For brevity, "child" will be used to include adolescents.) Compared with adults, children undergo more rapid and conspicuous changes in biological, cognitive, emotional, and social functioning . Children's adaptive and maladaptive behaviors, therefore, need to be viewed in relation to these developmental changes. Research concerning the individual course of child psychiatric conditions and knowledge of the processes underlying changes in individual development is necessary for the following reasons: (1) to assess which problems in children tend to persist and which do not; (2) to assess which early factors predict adult psychopathology; (3) to evaluate the necessity and efficacy of treatment and prevention; (4) to reveal causative mechanisms ; and (5) to assess the validity of diagnostic constructs in terms of outcome. In child psychiatry , clinicians are regularly confronted with parents' questions concerning the prognosis of their child's maladaptive behavior as well as the necessity of a certain kind of treatment and the prediction of the effect of treatment. Information is therefore needed on the natural course of problem behaviors in children and on factors that
Accepted November 8. 1990. Dr. Verhulst is Professor and Director of Child and Adolescent Psychiatry and Mr. Koot is research psychologist. Sophia Children's Hospital. Erasmus University, Rotterdam, The Netherlands . This research was supported by a grant from the Netherlands' Health Research Promotion Program (SGO). The authors are grateful to Drs . Thomas M . Achenbach and Guy F .M.G. Berden for their helpful comments . Reprint requests to Dr. Verhulst. Department of Child Psychiatry . Academic Hospital Rotterdam , Sophia Children' s Hospital . Gordelweg 160, 3038 GE Rotterdam, The Netherlands. 0890-8567/91 /3003-0361 $03. OO/O© 1991 by the American Academy of Child and Adolescent Psychiatry . l.Am.Acad. Child Adolesc.Psychiatry. 30:3, May 1991
influence the development of these conditions . A necessary approach to the study of developmental variations and of causative mechanisms and processes underlying the developmental course of psychopathology is the longitudinal method, which consists of assessments of the same individuals at certain intervals over a period of time (Janson, 1981; Robins, 1980). Although the longitudinal approach to the study of psychological and somatic development has been employed since the 1920s (Rutter , 1981), its impact on our understanding of the development and causative mechanisms of childhood psychopathological conditions is still rather rudimentary. One of the major obstacles to longitudinal research has been the choice of the variables studied . A lack of standardized assessment procedures and a lack of operational definitions of syndromes and diagnostic categories have severely limited the comparability of results across different studies and across time periods (Rutter, 1981; Zeitlin, 1986). Unfortunately, this has hampered the accumulation of knowledge of children's psychopathology . Robins (1974), for example , used preexisting records in her classical follow-up study of children who attended a child guidance clinic but acknowledged that the case records were too inconsistent to provide comparisons between psychiatric diagnoses at earlier and later ages . Longitudinal research is not possible without standardized assessment procedures and valid diagnostic constructs . It is recognized, however , that longitudinal research may be an important step in the validation of diagnostic constructs . One way of testing the validity of measures of psychopathology is the power to predict over time. Stability across time provides some support for the validity of measures or diagnostic constructs. Knowing which problem behaviors are stable or predicting other problems across time may guide priorities for prevention and interventio n strategies. Low stability does not necessarily indicate poor validity. Certain problem behaviors may be transient but may never361
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theless cause considerable concern during the time they are present. This special section highlights current longitudinal research activities rele vant to the field of child psychopathology . Because longitudinal research strategies are employed in such a wide variety of studie s concerned with disorders of psychosocial development in children and adults, a comprehensive review of these studies will not be attempted. Instead, the focus is on methodological issues concerned with the linking of measures obtained at different points in time as well as on the longitudinal course of major conditions, such as antisocial behavior, pervasive developmental disorders, hyperactivity , and depression. In this introductory review , the advantages of longitudinal over cross-sectional research are discussed. Next, problems of assessment and diagnostic definitions in longitudinal research are elaborated, and, finally, critical features of longitudinal studies using different strategies and different subject samples are discussed.
Longitudinal versus Cross-sectional Research An advantage of the cross-sectional design over the longitudinal design is that the former is relatively inexpensive and that it takes a much shorter time before result s can be obtained. In addition , it is usually possible to obtain more representative samples for cross-sectional studies than for longitudinal studies, which require the continued cooperation of subjects over long periods. However, the longitudinal method has many advantages that make it an indispensable approach for some purposes .
Assessment of Developmental Changes In cross-sectional research , individuals of different ages are compared at the same point in time . The prevalence of psychiatric disorders or measures of certain psychological characteristics can be determined for different age groups . Inferences about developmental change can be made if we assume that the behavior of the older group indicates how the younger group will eventually behave (Achenbach, 1978). However, this is seldom the case. For example, findings of lower IQ scores for older adults than for young adults were initially interpreted as indicating a developmental decline in adult ability (Achenbach, 1982; Baltes , 1968) . Yet, when subsequent longitudinal studies failed to show declining ability, it became evident that the cross-sectional differences in IQ scores were due to cohort differences . Because the older cohorts had received less education, they obtained lower scores than younger people. Research on child psychopathology also shows that cross-sectional designs cannot simply be used to estimate longitudinal changes . In a study by Gersten et al. (1976), for example, parent-reported problem behaviors were assessed in a representative sample of 6- to 16-year-olds from Manhattan, New York City , at an interval of 5 years. When cros s-sectional and longitudinal analyses were compared on 12 problem scales, the conclusions concerning the developmental course of problem behaviors disagreed for nearly half the comparisons. 362
Identification of Causal Mechanisms The cross-sectional appro ach can be used to identify variables associated with the conditions being studied . In a numbe r of cross-sectional studies , poor marital relations , parental hostility, and lack of parental empathy were found to be associated with psychi atric problems in children (Richman et al., 1982; Verhulst et al., 1985). However, from these data, the direction of the causation, if any, cannot be determined. Having a difficult child may evoke marital discord or negative reactions in parents. Yet , the reverse may also be true, namel y, that marital discord , parental hostility, or lack of empathy may evoke maladaptive behaviors in the child . In order to move from simple statistical associations between two variables to the testing of psychological and biological processes that are involved in causal mechanisms, we need longitudinal assessments. For instan ce , the con clusion from research by Farrington and West (1981) that family adversity was present before the occurrence of delinquen cy could only be drawn from longitudinal design. The time dimension inherent in the longitudinal design allows the assessment of change in behavior as a result of exposure to some hypothesized etiological variable. Kovacs and Goldston (1991), in this Special Section, discuss the relation between childhood depression and deficits in school performance and social behavior. The authors suggest that depressive episodes in children may have detrimental effects on their cognitive and social development. However, they acknowledge that the available evidence is not sufficient to disentangle temporal-causal relationships. Behavioral sciences are handicapped by obstacles to using the experimental-manipulative approach for testing causal mechanisms. However, longitudinal methods are often the best available means for investigating "natural experiments" that may reveal causal mechanisms. One approach is to look for situations where children's environments change markedly. In order to test the effect of an environmental change, assessments of the child's functioning before and after the change are needed. Berden et al. (1990), for example, investigated the effect of negative life events on the behavioral functioning of an epidemiological sample of children assessed twice over a 2-year interval. To assess life events, a modified version of the Coddington (1972) ques tionna ire was used . Cont rolling for the level of problem behavior at the initial assessment, negative life events showed a significant, though small, relation with an increase in problems across the 2-year period. However, to understand the causal mechanisms, more refined techniques are needed to pinpoint the temporal relationship between life events and changes in problem behavior as well as possible "moderator" variables , such as parental attachment, famil y routines , social support, and IQ. Another test of the effects of change in children's circumstances has been provided by Hodges and Tizard (1989a, b) who conducted a longitudinal study of institutionalized children. Before the age of 4 months, the children were admitted to an institution where they remained without interruption until at least 2 years of age . One group was then adopted , a second group was restored to their biological f.Am.Acad. Child Adolesc .Psychiatry, 30:3, May 1991
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parents, and a third group was in residential care at the time of assessment. A weakness of the study is that the children were not assessed before their placement. Although the authors found little support that the three groups differed much initially, the possibility of selective placement or other uncontrolled variables cannot be ruled out. Therefore, strictly speaking, this study of children's problem behavior in natural settings does not meet the criteria for a truly controlled experiment . However, the study provides some interesting results. The children were assessed at ages 4, 8, and 16 (Hodges and Tizard, 1989a, b; Tizard and Hodges, 1978; Tizard and Rees, 1974, 1975). The restored group was exposed to the influences of their disturbed and disadvantaged families. Most children in this group were reclaimed by their mothers after they were married, usually not to the putative father. These children showed more antisocial behavior and apathy compared with the adopted children . The adopted children, most of whom were living in stable homes, were able to form strong attachments to their adoptive parents. The adoptive families differed little from a normal comparison group; whereas , they differed greatly from the families of the restored group . It could be concluded that early institutional care did not prevent children from making close attachments to parents . However, adopted children resembled the restored children in their being more oriented toward adult attention and in their having more difficulties with peers and fewer close relationships than comparison children. These difficulties appear to have their origins in the children's early experience in institutional care and seem somewhat resistant to later positive influences. This study demonstrates the advantage of the longitudinal approach that made it possible to determine the heterogeneity of outcome in children with similar adverse backgrounds. The relatively favorable outcome of adopted children would not have been apparent if retrospective data were used, for instance , in a study of identified psychiatric patients. This study also shows nicely that the beneficial effect of adoption could only be demonstrated by comparing the adopted children's functioning with that of children in a comparison group of normal children and in the group of children restored to their biological parents. Determination of Timing
Longitudinal data are essential for questions concerning the timing of associations. An example of the importance of a developmental approach to the investigation of intellectual changes is given by Scarr and Weinberg (1983) who assessed similarities in parent-child and sibling intellectual functioning for two groups of adopted children. The first group consisted of transracially adopted children, aged 4 to 18 years, and the second was comprised of adopted adolescents, aged 16 to 22 years. It was found that younger siblings were intellectually quite similar to each other whether they were genetically related or not. The IQ correlations of adopted siblings (average age, 7 years) were as high as those of biological siblings (average age, 10 years) reared together. However, the IQ scores of adolescents with an average age of 18 years were similar to those of their parents and siblings only if they were biologically related. The l.Am.Acad. Child Ado/esc. Psychiatry, 30:3, May 1991
adopted adolescents' and their siblings' IQ correlations indicated that they were not more similar than biologically unrelated children from different families, notwithstanding the fact that they were reared together for 18 years in the same family. The authors concluded that younger children are more influenced by differences among their family environments than older adolescents who are freer to seek their own environments. However, the authors acknowledged that their hypothesis cannot be tested fully because they used cross-sectional data. The samples they studied may have been biased in some respects. There may have been factors other than age in the adolescents themselves or in the family environment that accounted for the lower correlations in the older age group . In a cross-sectional design , it is not possible to study groups of children from different ages with all relevant factors controlled for simply because it is not really known what factors are relevant. More definitive answers on the relative contribution of family environments to intellectual development may be obtained if the groups of adopted and biologically related siblings are followed longitudinally from early childhood into adolescence. Escape from Adverse Events
Although it is important to investigate the mechanisms involved in the detrimental outcome of adverse circumstances or events, it is equally important to study children who are resilient to these circumstances and who escape their influences. It is known that perinatal hazards have the greatest negative impact when they interact with psychosocial disadvantages (Sameroff and Chandler, 1975). In a longitudinal study of the perinatal complications and adverse rearing conditions in a birth cohort from the island of Kauai, Hawaii, Werner (1989) stressed the importance of regarding both vulnerability and resiliency as changeable over time. Thus, what may be a risk factor at one developmental phase may not be at another. For instance, males were more vulnerable than females when exposed to biological insults and adverse caregiving in the first 10 years of life, whereas the reverse was true in the second decade. This study demonstrates nicely that changes in the relative impact of risk factors within individuals over time can only be studied longitudinally . Another study demonstrating the resiliency to psychosocial adversities in some individuals is the one by Quinton and Rutter (1985). In this study, women institutionalized in childhood were followed up. The adult outcome of the institution-reared women, as a whole, was substantially worse than that for a comparison group of noninstitutionalized women. However, the developmental course in some exinstitutionalized women had been positively modified by favorable school experiences in childhood and by the characteristics of their spouse and marriage in adult life. Although the authors had access to questionnaire data on teacher-reported problems, most of the conclusions on school experiences were drawn from retrospective recollections and not from true longitudinal data . It is possible that recollection biases or other factors, such as the individuals' social 363
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and communication skills or feelings of self-esteem and self-efficacy before their school experiences, are partly responsible for these findings . Thi s study clearly generated valuable hypotheses, which however, can be tested more rigorousl y through prospective longitudinal research . Identificati on of Predictive Factors Longitudinal strategies may reveal relevant factors that predict later dysfunctioning. From an extensive review of prospective and retrospective studies , Loeber and Dishion (1983) found that the child' s conduct problems and poor academic performance as well as the parents' family management and parental criminality were the strongest predictors of delinquency in males. However , Farrington (1987) concluded that more prospective longitud inal studies are needed to investigate the early predictors of repeated delinquent acts . Especially for determining the sequential patterns of a delinquent developm ent, the longitudinal method is indispensable. Using the Child Behavior Checklist (Achenbach and Edelbrock , 1983) , a 4-year follow-up of a Dutch general population sample , originally aged 4 to 12 years, identified the problem behavio rs that best predicted the follow ing outcome variables: referral to mental health services, referral to special education services, and trouble with the police (Verhulst and Koot , 1991). Logistic regression analyses showed that aggressive and other extern alizing behaviors predicted referral to mental health services and cont act with the police . Attention problems/hyperactivity and internalizing problems predicted referral to mental health services as well as referral to special education services. It was concluded that parental ratings of children 's behavioral! emotional problem s were good predictors of outcomes over a 4-year period . Advantages and Disadvantag es of Longitudinal Research Designs In the discussion of the qualit ies of longitudinal and crosssectional research design s , the strengths of the longitudinal approach have been illustrated. One of the most crucial advantages of longitudinal data is that they provide the opportunity of investigat ing variations over time within individuals as well as investigating variations between individuals; whereas, cross-sectional data allow only the study of variations between individuals. Summing up, the advantages of longitudinal data are that they provide the following opportunities: I . To assess changes in individual development by comparing the same subjects at differ ent times; only in this way can the age of onset and the term ination of problem behavior as well as change s in its manifestation be determined. 2. To identify causal mechani sms by demonstrating that changes in one factor are followed by changes in another ; without the time dimension , it is not possible to determine the direction of the causal chain. 3. To study " escape" from adverse environmental circumstances or events and to evalu ate the factors that 364
make children resilient to adversity . 4. To investigate how far later functioning can be predicted by earlier functioning or events . Longitudinal studies also have disadvantage s. Farrington (199 1), in this Special Section, discusses the major problems of longitudinal surveys: confounding aging and period effects; the long delay between the start of the study and the availability of the first result s; the attrition of the sample during the study; and the effects of the repeated testing of subjects. Also , there is the problem of obtaining funding and keeping personnel across such a long time span. Farrington (199 1) offers an alternative approach to circumvent these problems by empl oying a strategy, which is intermediate between a single longitudinal and a cross-sectional survey, that is called the' 'accelerated longitudinal " design . This approach has the advantage that the follow-up period can be greatly reduced ; whereas, many of the adva ntages of the longitudin al approach can be kept.
Assessment and Definition of Disorders A major problem in the longitudinal study of childhood psychopathology is the lack of standardized assessment procedures and the uncertainty concerning operational definitions for child psychiatric syndromes. In DSM-III-R (American Psychiatric Association, 1987), for instance , major shifts have been introdu ced in many of the definitions of childhood disorders from the third to the third-revised edition . This diagnostic confusion is an especially serious handicap to the longitudinal study of childh ood disord ers, since diagno stic criteria used at the time of the initial assessment may be obsolete by the time of the next measurement. In the prospective longitudinal study of hyperactive boys by Gittelman et al. (1985) for instance , the inves tigators initially used DSM-ll criteria and were not able to assess all the beha vioral criteria listed in DSM-Ill-R . Instead of treating diagn ostic terms , such as hyperactivity, depression, or conduct disorder, as fixed, tangible, and diagnosable entities, it may be better to approach the problem of how to identify a comm on set of pathological phenomena empiri cally and to use longitudinal data for validation. Many researchers investigating childhood psychopath ological conditions longitudinally emphasize the problem of the lack of comp arability of the results betwe en studies or stress the import ance of a proper classification (Kovacs et al., 1984a, b; Thorley, 1984). It is now clearly recognized that childhood disorders cannot be merely regarded as miniature versions of adult disorders. To advance assessment and diagnosis of psychopathological conditions in children, it is important to take account of the following features that are specifi c to child psychopathology: ( I) with the possible exception of very deviant conditions, most childhood disorders involve quantitative variation s on normal development; (2) developmental changes necessitate age-specific norm s; (3) data must be obtained from multiple informants interacting with children in different situations. In DSM-III-R, it is acknowledged that a dimensional , " polythetic" approach is preferable to describe a number of childhood categories . The l .Am.Acad . Child Adolesc .Psychiatry ,30 :3 , May 1991
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DSM has greatly influenced research on psychopathology, but most of its childhood categories are not based on empirical data, show poor reliability (Mezzich et aI., 1985; Werry et al., 1983), and do not provide for developmental changes in the manifestation of symptoms. Another general approach is the empirical-statistical approach by which syndromes are derived from multivariate analyses of data on large samples of children referred to mental health services. This approach may be less necessary for disorders involving very conspicuous but rare symptoms, such as infantile autism and Tourette's syndrome. Achenbach and Edelbrock (1983 , 1986, 1987) constructed syndromes derived empirically from parent , teacher, and selfreport ratings. Although primarily dimensional, this approach can be used to categorize individuals as normal versus deviant by using cutoff points on quantitative scales. In this way, an individual child's scores can be calibrated developmentally by comparing the scores with those of normative samples of peers . To circumvent the rapid obsolescence of diagnostic constructs in longitudinal research, it may be helpful to use standardized assessment methods to obtain reliable information on basic critical features of children's maladaptive functioning irrespective of current nosological systems. Of course, it is also helpful to include current diagnoses in the assessment procedure as well. It is extremely important, however, to avoid premature closure on empirical processes that may advance assessment methodologies and classification of child psychopathology and in which longitudinal research may play an important role. There is a dual relationship between assessment and classification, on the one hand, and longitudinal research, on the other . Not only are reliable assessment procedures and valid classification systems required for longitudinal research, but the latter is also important for testing the validity of measures. For instance, in a longitudinal study of parentreported problems in children, aged 4 to 12, from the general population, the 4-year stability of syndrome scores derived from the Child Behavior Checklist (Achenbach and Edelbrock, 1983; Achenbach et al., 1989) was tested (Verhulst et aI., 1990). High stability over the 4-year period was found for syndromes designated as aggressive and attention problems; whereas, lower but still highly significant 4-year stability was found for six of the seven other syndromes. Kovacs et al. (l984a ,b) used a clinician-administered interview schedule in their longitudinal study of depressed children. They found a differential risk for major depression at follow-up between children with DSM-III diagnoses of major depression and dysthymic disorder, on the one hand,
and adjustment disorder with depressed mood on the other. The latter diagnostic group was associated with only a minimal risk for later major depressive episodes; whereas the former two groups were strongly related to later major depression. This difference in predictive power supports the validity of the distinction between the two groups of tested diagnoses. A similar distinction based on different outcomes can be made for the broad categories of externalizing or conduct disorders and internalizing or emotional disorders, with the latter being somewhat less persistent (Verhulst et J. Am. Acad. Child Adolesc. Psychiatry, 30:3 , May 1991
aI., 1990). It has often been assumed that internalizing disorders have a better prognosis . However, more research is necessary to test this adequately. Within the two broad diagnostic groups of internalizing and externalizing problems, longitudinal data may also lead to better distinctions. It is clear, however, that more longitudinal studies of child psychiatric patients are needed to test the validity of different approaches to measurement and classification of childhood psychopathology.
Different Longitudinal Strategies There are three main approaches to linking measures obtained at different points in time (Robins, 1980). First, there is the "real-time" prospective study in which individuals who are selected and studied at Time 1 are reexamined at subsequent occasions after certain time intervals. The Berkeley Growth Study (MacFarlane et aI., 1962) was one of the first studies of this kind. Second, there is the "catchup" or follow-up study. The study by Robins (1974) is an early example of this approach. The author demonstrated the strong links between childhood conduct disorder and adult antisocial personality disorder in her 30-year followup of over 500 children referred to a child guidance clinic. In a "catch-up" study, the time interval is usually long , and the number of measurements do not usually exceed two. This kind of study uses preexisting records that may suffer from biases inherent in unstandardized assessment procedures and unknown sample selection factors. Furthermore, data relevant to the research may be missing from the original records . Whereas the "catch-up" study uses preexisting data on a previously selected sample, a third major approach to linking measures over time, the "follow-back" or retrospective study , selects a current sample of cases known to have a particular outcome, such as hyperactivity . Data are then collected on past behavior or events either from people's recollection or by employing records, such as medical reports on perinatal problems. Follow-back studies, however, are vulnerable to biases in both the selection of current cases and in the recollection or availability of past data. By using referred cases, information on nonreferred cases are missed that, although also having the outcome of interest, may differ in ways related to not being referred. From the three approaches mentioned above, the "realtime" prospective strategy is usually superior for studying individual functioning across time, although this approach is liable to sample attrition and takes a long time before results can be obtained.
Different Populations Several classifications of longitudinal research can be employed . Beside the three main types of design, it is also possible to differentiate among longitudinal methodologies according to the subject population . One can distinguish longitudinal research based on normal , unselected, and representative populations from that based on nonrepresentative, especially selected, or deviant groups (Mednick et aI., 1981).
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General Population Samples
Several "real-time" prospective studies, reviewed by Verhulst and Althaus (1988), have investigated the course of problem behavior in children from large general population samples . The main advantage of prospective studies of representative, general population samples is the generalizability of findings. A major drawback of general population samples for studying the course or etiology of psychopathological conditions, however, is that very large samples are needed to catch even a few subjects with a particular condition. Most existing studies on the continuity of problem behavior in children were limited to samples drawn from one locality . Furthermore, the majority of studies have focused on very broad categories of functioning. Most studies report substantial continuity of problem behavior across time. In a 4-year longitudinal study of children, aged 4 to 12, it was found that parent reported problem behavior was equally stable for all ages as well as for both sexes (Verhulst et aI., 1990). It was concluded that problem behavior, especially aggression, is not more likely to be outgrown by young children than by older children . It was also found that adolescence was not characterized by rapid changes in the level of problems in this general population sample. Unrepresentative Samples
A number of studies have followed longitudinally assessed, small, unrepresentative samples. Examples are the studies by Kagan and Moss (1962) and Thomas and Chess (1976) who employed intensive repeated measurements. These studies give detailed accounts of the individual behavioral development of the children studied. In contrast to the longitudinal study of representative samples, however, the generality of the findings of these studies is limited because of the unrepresentativeness of the sample. High-Risk Samples
One way of increasing the number of subjects with deviant behaviors is to select children who are at risk because of adverse early experiences, such as perinatal problems (Werner, 1989), discontinuous caretaking (Hodges and Tizard, 1989a, b), or parental deviance, such as schizophrenia (Rosenbaum, 1988), depression (Cytryn et aI., 1984), or alcoholism (Rydelius, 1981). Although the great advantage of the high-risk design over the representative general population design is the higher yield of deviant individuals, high-risk studies may suffer from distortions in their samples of subjects having the target disorders. For instance, it may be that in children who have a schizophrenic parent and who develop this disturbance, the causes may be different from those in children who develop schizophrenia without having schizophrenic parents. Patient Samples
A fourth category of longitudinal research is formed by samples of identified patients . This Special Section focuses especially on this category because of its direct relevance for clinical practice . The longitudinal study of patients or
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individuals can give us information on outcomes, treatment effects, or changes in diagnosis. Although there are numerous studies on the short-term effects of specific types of treatment, little is known about the long-term effects of different treatments. Furthermore, when large samples of patients from different localities are studied, it is difficult to standardize treatment methods and to operationalize the criteria for selecting patients for a particular treatment. It is therefore unclear which outcomes merely reflect the course of a psychopathological condition and which reflect the results of particular interventions. Another problem in outcome studies of identified patients is the lack of adequate control groups, which should include normal children as well as those having other psychiatric diagnoses. For instance, Weiss et al. (1979) found that over 95% of their hyperactive subjects reported using soft drugs in the previous 5 years . However, the same proportion in their control group of normal children also reported using soft drugs. For this Special Section , several salient child psychiatric conditions have been chosen for closer discussion by a number of experts: Gillberg on autism, Klein and Mannuzza on hyperactivity, Kovacs and Goldston on depression, and Loeber on antisocial behaviors. Outcome studies of other specific child psychiatric conditions, such as obsessive-compulsive disorder (Berg et aI., 1989), anxiety disorders (Gittelman, 1986; Rutter and Garmezy, 1983), language disorders (Maughan et aI., 1985; McGee et aI. , 1986; Stevenson et aI., 1985) as well as studies that focused on general patient populations with a wide spectrum of psychiatric conditions (Blotcky et aI., 1984; Robins, 1974; Zeitlin, 1986), will not be discussed here because of space limitations. Conclusion
The developmental perspective is necessary in seeking the origins of psychopathology and in describing the course of psychopathological manifestations in children and adolescents . A necessary approach to the study of developmental variations and of causal mechanisms and processes underlying the course of psychopathology is the longitudinal method. The most salient characteristic of this method, which consists of assessments of the same individuals at certain intervals over a period of time, is that it provides the opportunity of investigating variations over time within individuals. This makes it a powerful and, for some purposes, indispensable approach for testing causal hypotheses, for describing the course of problem behavior, and for analyzing the effect of treatment and prevention. Although the examples given in the present review of recent longitudinal research relevant to child and adolescent psychiatry demonstrate the considerable activity in this field, there are a number of obstacles hampering its progress. First, there is a lack of widely accepted standardized assessment procedures and a lack of valid diagnostic constructs for childhood psychopathology. An exception is antisocial behavior . The key variables in the longitudinal investigation of delinquency are relatively discrete and agreed upon (e.g ., stealing , truancy , fire setting) . This may be a reason why I .Am.Acad. Child Adolesc. Psychiatry, 30 :3, May 1991
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longitudin al research on delinquen cy has been more fruitful than that on other childhood psychopathological conditions (see Farrington [1991] and Loeber [1991] in this Special Section). Second, there is a paucity of hypotheses or theories about possible etiological mechani sms that can be tested adequately. Third , there are obstacles inherent to the prospective longitudinal approach itself, such as the long period the study takes, the effects of sample attrition and the confounding of aging and period effects. Alternative approaches, such as the accelerated longitudinal design, may provide the opportunity to investigate within-individual variations while minimizing the adverse effects of the longitudinal method. It is clear that the complex nature of the etiological mechanisms itself and the methodological and statistical problems connected with the longitudinal method call for considerable efforts. Without such efforts, we will slow down the progress in our understanding of the developmental course of psychopathological conditions of childhood and adolescence and of the etiology , the preventi on , and the treatment of these conditions.
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linquency Prevention, eds. J. Q. Wilson & G . C. Loury. New York: Springer-Verlag. - - (1991), Longitudinal research strategies: advantages, problems and prospects. J. Am. Acad . Child Adolesc . Psychiatry, 30:369374. - - West, D. J. (1981), The Cambridge study in delinquent development (United Kingdom). In: Prospective Longitudinal Research: An Empirical Basis fo r the Primary Prevention of Psychosocial Disorders , eds. S. A. Mednick & A. E. Baert . Oxford : Oxford University Press. Gersten, J. C., Langner, T. S ., Eisenberg, I. G., Simcha-Fagan, O. & McCarthy, E. D. (1976), Stability and change in types of behavioral disturbance of children and adolescents. J . Abnorm . Child Psychol., 4:111-1 27. Gittelman, R. ( 1986), Childhood anxiety disorders: correlates and outcome. In: Anxiety Disorders of Childhood, ed . R. Gittelman. New York: Guilford Press. - - Mannuzza, S. , Shenker, R. & Bonagura, N. (1985), Hyperactive boys almost grown up: I. psychiatric status. Arch. Gen. Psychiatry, 42:937- 947 . Hodges, 1. & Tizard, B. (l989a), IQ and behavioural adjustment of ex-institutional adolescents. J. Child Psychol . Psychiatry , 30:5375. - - - - (l989b), Social and family relationships of ex-institutional adolescents. J. Child Psychol. Psychiatry, 19:99-11 8. Janson, C. G. (198 1), Some problems of longitudinal research in the social sciences. In: Longitudinal Research: Methods and Uses in Behavioral Science. ed. F. Schulsinger, S. Mednick & J . Knop. Boston: Martinus Nijhoff Publishing. Kagan, J. & Moss, H. A. (1962), Birth to Maturity: A Study of Psychological Development. New York: Wiley. Kovacs, M. , Goldston, D. (1991), Cognitive and social cognitive development of depressed children and adolescents. J . Am . Acad . Child Adolesc. Psychiatry, 30:388- 392. - - Feinberg, T . L. , Crouse-Novak, M. A. , Paulauskas , S. L. & Finkelstein , R. (I 984a) , Depressive disorders in childhood: I. a longitudinal prospective study of characteristics of recovery. Arch . Gen. Psychiatry, 4 1:229-237. -- -- -- - Pollock, M. & Finkelstein , R. (I 984b), Depressive disorders in childhood: II. a longitudinal study of the risk for subsequent major depression. Arch. Gen. Psychiatry , 4 1:643649 . Loeber, R. (1991), Antisocial behavior: more enduring than changeable? J . Am. Acad. Child Adolesc. Psychiatry, 30:000-000. - - Dishion, T. J . (1983), Early predictors of male delinquency: a review. Psychol. Bull. , 94:68- 99. MacFarlane, 1. W., Allen, L. & Honzik, M. P. (1962) , A Developmental Study of the Behavior Problems of Normal Children between Twenty-one Months and Fourteen Years. Berkele y, CA: University of California Press. Maughan, B. , Graig, G. & Rutter, M. (1985) , Reading retardation and antisocial behavior: a follow-up into employment. J. Child Psychol. Psychiatry, 26:74 1-758. McGee, R. , Williams, S., Share , D. L. , Anderson J . & Silva, P. (1986), The relationship between specific reading retardation, general reading backwardness and behavioural problems in a large sample of Dunedin boys: a longitudinal study from five to eleven years. J . Child Psychol. Psychiatry, 27:597-610. Mednick, S. A., Griffith, J. J. & Mednick, B. R. (1981), Problems with tra ditio nal strategies in mental he alth re se arch . In : Longitudinal Research: Methods and Uses in Beha vioral Science , eds, F. Schulsinger, S. A. Mednick & J . Knop. Boston: Martinus Nijhoff Publishing. Mezzich, A. c., Mezzich, J. E. & Coffman, G. A. (1985), Reliability of DSM-lll vs DSM-ll in child psychopathology. J . Am . Acad. Child Adolesc. Psychiatry, 24:273-280. Quinton, D. & Rutter, M. (1985), Parenting behaviour of mothers raised in care. In: Longitudinal Studies in Child Psychology and Psychiatry, ed. A. R. Nicol. Chichester, U.K. : Wiley , pp . 157201. Richman, N., Stevenson, J. E. & Graham, P. J. (1982), Preschool to School: A Behavioural Study. London: Academic Press.
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Robins, L. N. ( 1974), Deviant Children Grown Up: A Sociological and Psychiatric Study ofSociopathic Personality, New York: Krieger. - - (1980), Longitudinal methods in the study of normal and pathological development. In: Studies of Children, ed. F. Earls. New York: Prodist, pp. 34-83. Rosenbaum , J. (1988), Children at risk for schizophrenia: converging lines of evidence. Schizophr, Bull ., 14:613- 631. Rutter , M. (1981), Long itudinal studies: a psychiatric perspective. In: Prospective Longitudinal Research: An Empirical Basis f or the Primary Prevention of Psychosocial Disorders, ed . S. A. Mednick & A. E. Baert . Oxford: Oxford University Press, pp. 326-336. - - Garmezy , N. (1983), Developmental Psychopathology . In: Carmichael' s Manual of Child Psychology (Vol. 4): Social and Personality Development , ed. E. M . Hetherington . New York: Wiley. Rydelius , P. A. (1981), Children of alcoholic fathers : a longitud inal prospective study. In: Prospective Longitudinal Research: An Empirical Basisfor the Primary Prevention of Psychosocial Disorders, eds. S. A. Mednick & A. E. Baert . Oxford: Oxford Univer sity Press. Sameroff, A. J . & Chandler, M. J. (1975), Reproductive risk and the continuum of caretaking casualty . In: Review of Child Development Research, Vol. 4 , ed. F. D. Horowitz. London: University of Chicago Press, pp. 187-244. Scarr, S. & Weinberg , R. A. (1983), The Minnesota adoption studies: genetic differen ces and malleabil ity. Child Dev., 54:260-267 . Stevenson , 1. , Richman , N. & Graham , P. (1985), Behav ioural problems and language abilities at three years and beha vioural deviance at eight years. J. Child Psychol . Psychiatry, 26:215-230 . Thomas, A. & Chess, S. (1976), Evolution of behavior disorders into adolescence. Am. J. Psychiatry, 133:539-542. Thorley, G . (1984), Review of follow-up and follow-back studies of childhood hyperactivity. Psychol. Bull., 96:116-132. Tizard, B. & Rees , J. (1974), A comparison of the effects of adoption,
restoration to the natural mother, and continued institutionalization on the cognitive development of four-year-old children. Child Dev. , 45:92-99. - - - - (1975), The effect of early institutional rearing on the behaviour problem s and affectional relationships of four-year-old children. J . Child Psychol. Psychiatry, 16:61-73 . - - Hodges, J. (1978), The effec t of early institutional rearing on the development of eight-year-old children. J. Child Psychol. Psychiatry, 19:99-118. Verhulst , F. C. , Althaus, M. (1988), Persistence and chan ge in behavioral/emotional problems reported by parents of childre n aged 4-14: an epidemiological study. Acta Psychiatr , Scand . [Suppl. 339], 77 . - - Koot, J. M ., (1991), Stability of externalizing behaviors in an epidemiological sample . In: Developmental Psychopathology, ed . H. Remschmidt & M. Schmidt . Gottingen: Hogrefe & Huber Publishers. - - Berden, G. F. M. G . & Sander s-Woudstra, J. A. R. (1985), Mental health in Dutch children: II. the prevalence of psychiatric disorder and relationship between measures . Acta Psychiatr . Scand. [Suppl. 342], 72. - - Koot, J. M. & Berden , G. F . M . G. (1990), Four-year follow up of an epidemiol ogical sample. J . Am. Acad. Child Adolesc. Psychiatry, 29:440-448. Weiss, G. , Hechtman , L. , Perlman, T . , Hopkins, J. & Wener, A . (1979), Hyperactives as young adults: a controlled , prospective tenyear follow-up of 75 children. Arch . Gen. Psychiatry , 6:675-681 . Werner , E. E. (1989), High risk children in young adulth ood: a longitudinal study from birth to 32 years. Am . J . Orthopsychiatry, 59:72-81 . Werry , 1. S., Methven, R. J ., Fitzpatrick, J. & Dixon , H. (1983), The interrater reliability of DSM-/ll in children. J . Abnorm. Child Psychol. 11:341-354. Zeitlin, H. (1986), The Natural History of Psychiatric Disorder in Children, Oxford: Oxford University Press.
From Early Issues of the JAACP May we never grow complacent, the sedative of progress, but keep alive our critical judgments and utilize the value of conflict, which can provide the ore of progress if the heat generated is great enough to refine it but not so great as to destroy it. Child psychiatry has generated a great deal of heat in the decades just past, but it has been a refining process.
JM CP, Vol. 2, No. I January, 1963
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Longitudinal Research in Child and Adolescent Psychiatry FRANK C. VERHULST , M.D .,
AN D
HANS M. KOOT, M.S.
Abstract. An indispens able approach to the study of variations in individual development and of causal mechanisms and processes underlying the course of psychopatholog y is the longitudinal method . In this introductory review , the strengths and weaknesses of longitudinal research are discussed, and factors hampering progress in this field are outlined. The many advantage s of this approach warrant continuing efforts to develop strategies that minimize its drawback s. J . Am . Acad. Child Adolesc. Psychiatry, 1991, 30, 3:361-368 . Key Words: longitudinal research, child/adole scent psychopathology. The study of continuities and discontinuities in individual development and of factors that influence developmental processes is crucial to our understanding of normal as well as abnormal functioning of children . (For brevity, "child" will be used to include adolescents.) Compared with adults, children undergo more rapid and conspicuous changes in biological, cognitive, emotional, and social functioning . Children's adaptive and maladaptive behaviors, therefore, need to be viewed in relation to these developmental changes. Research concerning the individual course of child psychiatric conditions and knowledge of the processes underlying changes in individual development is necessary for the following reasons: (1) to assess which problems in children tend to persist and which do not; (2) to assess which early factors predict adult psychopathology; (3) to evaluate the necessity and efficacy of treatment and prevention; (4) to reveal causative mechanisms ; and (5) to assess the validity of diagnostic constructs in terms of outcome. In child psychiatry , clinicians are regularly confronted with parents' questions concerning the prognosis of their child's maladaptive behavior as well as the necessity of a certain kind of treatment and the prediction of the effect of treatment. Information is therefore needed on the natural course of problem behaviors in children and on factors that
Accepted November 8. 1990. Dr. Verhulst is Professor and Director of Child and Adolescent Psychiatry and Mr. Koot is research psychologist. Sophia Children's Hospital. Erasmus University, Rotterdam, The Netherlands . This research was supported by a grant from the Netherlands' Health Research Promotion Program (SGO). The authors are grateful to Drs . Thomas M . Achenbach and Guy F .M.G. Berden for their helpful comments . Reprint requests to Dr. Verhulst. Department of Child Psychiatry . Academic Hospital Rotterdam , Sophia Children' s Hospital . Gordelweg 160, 3038 GE Rotterdam, The Netherlands. 0890-8567/91 /3003-0361 $03. OO/O© 1991 by the American Academy of Child and Adolescent Psychiatry . l.Am.Acad. Child Adolesc.Psychiatry. 30:3, May 1991
influence the development of these conditions . A necessary approach to the study of developmental variations and of causative mechanisms and processes underlying the developmental course of psychopathology is the longitudinal method, which consists of assessments of the same individuals at certain intervals over a period of time (Janson, 1981; Robins, 1980). Although the longitudinal approach to the study of psychological and somatic development has been employed since the 1920s (Rutter , 1981), its impact on our understanding of the development and causative mechanisms of childhood psychopathological conditions is still rather rudimentary. One of the major obstacles to longitudinal research has been the choice of the variables studied . A lack of standardized assessment procedures and a lack of operational definitions of syndromes and diagnostic categories have severely limited the comparability of results across different studies and across time periods (Rutter, 1981; Zeitlin, 1986). Unfortunately, this has hampered the accumulation of knowledge of children's psychopathology . Robins (1974), for example , used preexisting records in her classical follow-up study of children who attended a child guidance clinic but acknowledged that the case records were too inconsistent to provide comparisons between psychiatric diagnoses at earlier and later ages . Longitudinal research is not possible without standardized assessment procedures and valid diagnostic constructs . It is recognized, however , that longitudinal research may be an important step in the validation of diagnostic constructs . One way of testing the validity of measures of psychopathology is the power to predict over time. Stability across time provides some support for the validity of measures or diagnostic constructs. Knowing which problem behaviors are stable or predicting other problems across time may guide priorities for prevention and interventio n strategies. Low stability does not necessarily indicate poor validity. Certain problem behaviors may be transient but may never361
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theless cause considerable concern during the time they are present. This special section highlights current longitudinal research activities rele vant to the field of child psychopathology . Because longitudinal research strategies are employed in such a wide variety of studie s concerned with disorders of psychosocial development in children and adults, a comprehensive review of these studies will not be attempted. Instead, the focus is on methodological issues concerned with the linking of measures obtained at different points in time as well as on the longitudinal course of major conditions, such as antisocial behavior, pervasive developmental disorders, hyperactivity , and depression. In this introductory review , the advantages of longitudinal over cross-sectional research are discussed. Next, problems of assessment and diagnostic definitions in longitudinal research are elaborated, and, finally, critical features of longitudinal studies using different strategies and different subject samples are discussed.
Longitudinal versus Cross-sectional Research An advantage of the cross-sectional design over the longitudinal design is that the former is relatively inexpensive and that it takes a much shorter time before result s can be obtained. In addition , it is usually possible to obtain more representative samples for cross-sectional studies than for longitudinal studies, which require the continued cooperation of subjects over long periods. However, the longitudinal method has many advantages that make it an indispensable approach for some purposes .
Assessment of Developmental Changes In cross-sectional research , individuals of different ages are compared at the same point in time . The prevalence of psychiatric disorders or measures of certain psychological characteristics can be determined for different age groups . Inferences about developmental change can be made if we assume that the behavior of the older group indicates how the younger group will eventually behave (Achenbach, 1978). However, this is seldom the case. For example, findings of lower IQ scores for older adults than for young adults were initially interpreted as indicating a developmental decline in adult ability (Achenbach, 1982; Baltes , 1968) . Yet, when subsequent longitudinal studies failed to show declining ability, it became evident that the cross-sectional differences in IQ scores were due to cohort differences . Because the older cohorts had received less education, they obtained lower scores than younger people. Research on child psychopathology also shows that cross-sectional designs cannot simply be used to estimate longitudinal changes . In a study by Gersten et al. (1976), for example, parent-reported problem behaviors were assessed in a representative sample of 6- to 16-year-olds from Manhattan, New York City , at an interval of 5 years. When cros s-sectional and longitudinal analyses were compared on 12 problem scales, the conclusions concerning the developmental course of problem behaviors disagreed for nearly half the comparisons. 362
Identification of Causal Mechanisms The cross-sectional appro ach can be used to identify variables associated with the conditions being studied . In a numbe r of cross-sectional studies , poor marital relations , parental hostility, and lack of parental empathy were found to be associated with psychi atric problems in children (Richman et al., 1982; Verhulst et al., 1985). However, from these data, the direction of the causation, if any, cannot be determined. Having a difficult child may evoke marital discord or negative reactions in parents. Yet , the reverse may also be true, namel y, that marital discord , parental hostility, or lack of empathy may evoke maladaptive behaviors in the child . In order to move from simple statistical associations between two variables to the testing of psychological and biological processes that are involved in causal mechanisms, we need longitudinal assessments. For instan ce , the con clusion from research by Farrington and West (1981) that family adversity was present before the occurrence of delinquen cy could only be drawn from longitudinal design. The time dimension inherent in the longitudinal design allows the assessment of change in behavior as a result of exposure to some hypothesized etiological variable. Kovacs and Goldston (1991), in this Special Section, discuss the relation between childhood depression and deficits in school performance and social behavior. The authors suggest that depressive episodes in children may have detrimental effects on their cognitive and social development. However, they acknowledge that the available evidence is not sufficient to disentangle temporal-causal relationships. Behavioral sciences are handicapped by obstacles to using the experimental-manipulative approach for testing causal mechanisms. However, longitudinal methods are often the best available means for investigating "natural experiments" that may reveal causal mechanisms. One approach is to look for situations where children's environments change markedly. In order to test the effect of an environmental change, assessments of the child's functioning before and after the change are needed. Berden et al. (1990), for example, investigated the effect of negative life events on the behavioral functioning of an epidemiological sample of children assessed twice over a 2-year interval. To assess life events, a modified version of the Coddington (1972) ques tionna ire was used . Cont rolling for the level of problem behavior at the initial assessment, negative life events showed a significant, though small, relation with an increase in problems across the 2-year period. However, to understand the causal mechanisms, more refined techniques are needed to pinpoint the temporal relationship between life events and changes in problem behavior as well as possible "moderator" variables , such as parental attachment, famil y routines , social support, and IQ. Another test of the effects of change in children's circumstances has been provided by Hodges and Tizard (1989a, b) who conducted a longitudinal study of institutionalized children. Before the age of 4 months, the children were admitted to an institution where they remained without interruption until at least 2 years of age . One group was then adopted , a second group was restored to their biological f.Am.Acad. Child Adolesc .Psychiatry, 30:3, May 1991
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parents, and a third group was in residential care at the time of assessment. A weakness of the study is that the children were not assessed before their placement. Although the authors found little support that the three groups differed much initially, the possibility of selective placement or other uncontrolled variables cannot be ruled out. Therefore, strictly speaking, this study of children's problem behavior in natural settings does not meet the criteria for a truly controlled experiment . However, the study provides some interesting results. The children were assessed at ages 4, 8, and 16 (Hodges and Tizard, 1989a, b; Tizard and Hodges, 1978; Tizard and Rees, 1974, 1975). The restored group was exposed to the influences of their disturbed and disadvantaged families. Most children in this group were reclaimed by their mothers after they were married, usually not to the putative father. These children showed more antisocial behavior and apathy compared with the adopted children . The adopted children, most of whom were living in stable homes, were able to form strong attachments to their adoptive parents. The adoptive families differed little from a normal comparison group; whereas , they differed greatly from the families of the restored group . It could be concluded that early institutional care did not prevent children from making close attachments to parents . However, adopted children resembled the restored children in their being more oriented toward adult attention and in their having more difficulties with peers and fewer close relationships than comparison children. These difficulties appear to have their origins in the children's early experience in institutional care and seem somewhat resistant to later positive influences. This study demonstrates the advantage of the longitudinal approach that made it possible to determine the heterogeneity of outcome in children with similar adverse backgrounds. The relatively favorable outcome of adopted children would not have been apparent if retrospective data were used, for instance , in a study of identified psychiatric patients. This study also shows nicely that the beneficial effect of adoption could only be demonstrated by comparing the adopted children's functioning with that of children in a comparison group of normal children and in the group of children restored to their biological parents. Determination of Timing
Longitudinal data are essential for questions concerning the timing of associations. An example of the importance of a developmental approach to the investigation of intellectual changes is given by Scarr and Weinberg (1983) who assessed similarities in parent-child and sibling intellectual functioning for two groups of adopted children. The first group consisted of transracially adopted children, aged 4 to 18 years, and the second was comprised of adopted adolescents, aged 16 to 22 years. It was found that younger siblings were intellectually quite similar to each other whether they were genetically related or not. The IQ correlations of adopted siblings (average age, 7 years) were as high as those of biological siblings (average age, 10 years) reared together. However, the IQ scores of adolescents with an average age of 18 years were similar to those of their parents and siblings only if they were biologically related. The l.Am.Acad. Child Ado/esc. Psychiatry, 30:3, May 1991
adopted adolescents' and their siblings' IQ correlations indicated that they were not more similar than biologically unrelated children from different families, notwithstanding the fact that they were reared together for 18 years in the same family. The authors concluded that younger children are more influenced by differences among their family environments than older adolescents who are freer to seek their own environments. However, the authors acknowledged that their hypothesis cannot be tested fully because they used cross-sectional data. The samples they studied may have been biased in some respects. There may have been factors other than age in the adolescents themselves or in the family environment that accounted for the lower correlations in the older age group . In a cross-sectional design , it is not possible to study groups of children from different ages with all relevant factors controlled for simply because it is not really known what factors are relevant. More definitive answers on the relative contribution of family environments to intellectual development may be obtained if the groups of adopted and biologically related siblings are followed longitudinally from early childhood into adolescence. Escape from Adverse Events
Although it is important to investigate the mechanisms involved in the detrimental outcome of adverse circumstances or events, it is equally important to study children who are resilient to these circumstances and who escape their influences. It is known that perinatal hazards have the greatest negative impact when they interact with psychosocial disadvantages (Sameroff and Chandler, 1975). In a longitudinal study of the perinatal complications and adverse rearing conditions in a birth cohort from the island of Kauai, Hawaii, Werner (1989) stressed the importance of regarding both vulnerability and resiliency as changeable over time. Thus, what may be a risk factor at one developmental phase may not be at another. For instance, males were more vulnerable than females when exposed to biological insults and adverse caregiving in the first 10 years of life, whereas the reverse was true in the second decade. This study demonstrates nicely that changes in the relative impact of risk factors within individuals over time can only be studied longitudinally . Another study demonstrating the resiliency to psychosocial adversities in some individuals is the one by Quinton and Rutter (1985). In this study, women institutionalized in childhood were followed up. The adult outcome of the institution-reared women, as a whole, was substantially worse than that for a comparison group of noninstitutionalized women. However, the developmental course in some exinstitutionalized women had been positively modified by favorable school experiences in childhood and by the characteristics of their spouse and marriage in adult life. Although the authors had access to questionnaire data on teacher-reported problems, most of the conclusions on school experiences were drawn from retrospective recollections and not from true longitudinal data . It is possible that recollection biases or other factors, such as the individuals' social 363
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and communication skills or feelings of self-esteem and self-efficacy before their school experiences, are partly responsible for these findings . Thi s study clearly generated valuable hypotheses, which however, can be tested more rigorousl y through prospective longitudinal research . Identificati on of Predictive Factors Longitudinal strategies may reveal relevant factors that predict later dysfunctioning. From an extensive review of prospective and retrospective studies , Loeber and Dishion (1983) found that the child' s conduct problems and poor academic performance as well as the parents' family management and parental criminality were the strongest predictors of delinquency in males. However , Farrington (1987) concluded that more prospective longitud inal studies are needed to investigate the early predictors of repeated delinquent acts . Especially for determining the sequential patterns of a delinquent developm ent, the longitudinal method is indispensable. Using the Child Behavior Checklist (Achenbach and Edelbrock , 1983) , a 4-year follow-up of a Dutch general population sample , originally aged 4 to 12 years, identified the problem behavio rs that best predicted the follow ing outcome variables: referral to mental health services, referral to special education services, and trouble with the police (Verhulst and Koot , 1991). Logistic regression analyses showed that aggressive and other extern alizing behaviors predicted referral to mental health services and cont act with the police . Attention problems/hyperactivity and internalizing problems predicted referral to mental health services as well as referral to special education services. It was concluded that parental ratings of children 's behavioral! emotional problem s were good predictors of outcomes over a 4-year period . Advantages and Disadvantag es of Longitudinal Research Designs In the discussion of the qualit ies of longitudinal and crosssectional research design s , the strengths of the longitudinal approach have been illustrated. One of the most crucial advantages of longitudinal data is that they provide the opportunity of investigat ing variations over time within individuals as well as investigating variations between individuals; whereas, cross-sectional data allow only the study of variations between individuals. Summing up, the advantages of longitudinal data are that they provide the following opportunities: I . To assess changes in individual development by comparing the same subjects at differ ent times; only in this way can the age of onset and the term ination of problem behavior as well as change s in its manifestation be determined. 2. To identify causal mechani sms by demonstrating that changes in one factor are followed by changes in another ; without the time dimension , it is not possible to determine the direction of the causal chain. 3. To study " escape" from adverse environmental circumstances or events and to evalu ate the factors that 364
make children resilient to adversity . 4. To investigate how far later functioning can be predicted by earlier functioning or events . Longitudinal studies also have disadvantage s. Farrington (199 1), in this Special Section, discusses the major problems of longitudinal surveys: confounding aging and period effects; the long delay between the start of the study and the availability of the first result s; the attrition of the sample during the study; and the effects of the repeated testing of subjects. Also , there is the problem of obtaining funding and keeping personnel across such a long time span. Farrington (199 1) offers an alternative approach to circumvent these problems by empl oying a strategy, which is intermediate between a single longitudinal and a cross-sectional survey, that is called the' 'accelerated longitudinal " design . This approach has the advantage that the follow-up period can be greatly reduced ; whereas, many of the adva ntages of the longitudin al approach can be kept.
Assessment and Definition of Disorders A major problem in the longitudinal study of childhood psychopathology is the lack of standardized assessment procedures and the uncertainty concerning operational definitions for child psychiatric syndromes. In DSM-III-R (American Psychiatric Association, 1987), for instance , major shifts have been introdu ced in many of the definitions of childhood disorders from the third to the third-revised edition . This diagnostic confusion is an especially serious handicap to the longitudinal study of childh ood disord ers, since diagno stic criteria used at the time of the initial assessment may be obsolete by the time of the next measurement. In the prospective longitudinal study of hyperactive boys by Gittelman et al. (1985) for instance , the inves tigators initially used DSM-ll criteria and were not able to assess all the beha vioral criteria listed in DSM-Ill-R . Instead of treating diagn ostic terms , such as hyperactivity, depression, or conduct disorder, as fixed, tangible, and diagnosable entities, it may be better to approach the problem of how to identify a comm on set of pathological phenomena empiri cally and to use longitudinal data for validation. Many researchers investigating childhood psychopath ological conditions longitudinally emphasize the problem of the lack of comp arability of the results betwe en studies or stress the import ance of a proper classification (Kovacs et al., 1984a, b; Thorley, 1984). It is now clearly recognized that childhood disorders cannot be merely regarded as miniature versions of adult disorders. To advance assessment and diagnosis of psychopathological conditions in children, it is important to take account of the following features that are specifi c to child psychopathology: ( I) with the possible exception of very deviant conditions, most childhood disorders involve quantitative variation s on normal development; (2) developmental changes necessitate age-specific norm s; (3) data must be obtained from multiple informants interacting with children in different situations. In DSM-III-R, it is acknowledged that a dimensional , " polythetic" approach is preferable to describe a number of childhood categories . The l .Am.Acad . Child Adolesc .Psychiatry ,30 :3 , May 1991
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DSM has greatly influenced research on psychopathology, but most of its childhood categories are not based on empirical data, show poor reliability (Mezzich et aI., 1985; Werry et al., 1983), and do not provide for developmental changes in the manifestation of symptoms. Another general approach is the empirical-statistical approach by which syndromes are derived from multivariate analyses of data on large samples of children referred to mental health services. This approach may be less necessary for disorders involving very conspicuous but rare symptoms, such as infantile autism and Tourette's syndrome. Achenbach and Edelbrock (1983 , 1986, 1987) constructed syndromes derived empirically from parent , teacher, and selfreport ratings. Although primarily dimensional, this approach can be used to categorize individuals as normal versus deviant by using cutoff points on quantitative scales. In this way, an individual child's scores can be calibrated developmentally by comparing the scores with those of normative samples of peers . To circumvent the rapid obsolescence of diagnostic constructs in longitudinal research, it may be helpful to use standardized assessment methods to obtain reliable information on basic critical features of children's maladaptive functioning irrespective of current nosological systems. Of course, it is also helpful to include current diagnoses in the assessment procedure as well. It is extremely important, however, to avoid premature closure on empirical processes that may advance assessment methodologies and classification of child psychopathology and in which longitudinal research may play an important role. There is a dual relationship between assessment and classification, on the one hand, and longitudinal research, on the other . Not only are reliable assessment procedures and valid classification systems required for longitudinal research, but the latter is also important for testing the validity of measures. For instance, in a longitudinal study of parentreported problems in children, aged 4 to 12, from the general population, the 4-year stability of syndrome scores derived from the Child Behavior Checklist (Achenbach and Edelbrock, 1983; Achenbach et al., 1989) was tested (Verhulst et aI., 1990). High stability over the 4-year period was found for syndromes designated as aggressive and attention problems; whereas, lower but still highly significant 4-year stability was found for six of the seven other syndromes. Kovacs et al. (l984a ,b) used a clinician-administered interview schedule in their longitudinal study of depressed children. They found a differential risk for major depression at follow-up between children with DSM-III diagnoses of major depression and dysthymic disorder, on the one hand,
and adjustment disorder with depressed mood on the other. The latter diagnostic group was associated with only a minimal risk for later major depressive episodes; whereas the former two groups were strongly related to later major depression. This difference in predictive power supports the validity of the distinction between the two groups of tested diagnoses. A similar distinction based on different outcomes can be made for the broad categories of externalizing or conduct disorders and internalizing or emotional disorders, with the latter being somewhat less persistent (Verhulst et J. Am. Acad. Child Adolesc. Psychiatry, 30:3 , May 1991
aI., 1990). It has often been assumed that internalizing disorders have a better prognosis . However, more research is necessary to test this adequately. Within the two broad diagnostic groups of internalizing and externalizing problems, longitudinal data may also lead to better distinctions. It is clear, however, that more longitudinal studies of child psychiatric patients are needed to test the validity of different approaches to measurement and classification of childhood psychopathology.
Different Longitudinal Strategies There are three main approaches to linking measures obtained at different points in time (Robins, 1980). First, there is the "real-time" prospective study in which individuals who are selected and studied at Time 1 are reexamined at subsequent occasions after certain time intervals. The Berkeley Growth Study (MacFarlane et aI., 1962) was one of the first studies of this kind. Second, there is the "catchup" or follow-up study. The study by Robins (1974) is an early example of this approach. The author demonstrated the strong links between childhood conduct disorder and adult antisocial personality disorder in her 30-year followup of over 500 children referred to a child guidance clinic. In a "catch-up" study, the time interval is usually long , and the number of measurements do not usually exceed two. This kind of study uses preexisting records that may suffer from biases inherent in unstandardized assessment procedures and unknown sample selection factors. Furthermore, data relevant to the research may be missing from the original records . Whereas the "catch-up" study uses preexisting data on a previously selected sample, a third major approach to linking measures over time, the "follow-back" or retrospective study , selects a current sample of cases known to have a particular outcome, such as hyperactivity . Data are then collected on past behavior or events either from people's recollection or by employing records, such as medical reports on perinatal problems. Follow-back studies, however, are vulnerable to biases in both the selection of current cases and in the recollection or availability of past data. By using referred cases, information on nonreferred cases are missed that, although also having the outcome of interest, may differ in ways related to not being referred. From the three approaches mentioned above, the "realtime" prospective strategy is usually superior for studying individual functioning across time, although this approach is liable to sample attrition and takes a long time before results can be obtained.
Different Populations Several classifications of longitudinal research can be employed . Beside the three main types of design, it is also possible to differentiate among longitudinal methodologies according to the subject population . One can distinguish longitudinal research based on normal , unselected, and representative populations from that based on nonrepresentative, especially selected, or deviant groups (Mednick et aI., 1981).
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General Population Samples
Several "real-time" prospective studies, reviewed by Verhulst and Althaus (1988), have investigated the course of problem behavior in children from large general population samples . The main advantage of prospective studies of representative, general population samples is the generalizability of findings. A major drawback of general population samples for studying the course or etiology of psychopathological conditions, however, is that very large samples are needed to catch even a few subjects with a particular condition. Most existing studies on the continuity of problem behavior in children were limited to samples drawn from one locality . Furthermore, the majority of studies have focused on very broad categories of functioning. Most studies report substantial continuity of problem behavior across time. In a 4-year longitudinal study of children, aged 4 to 12, it was found that parent reported problem behavior was equally stable for all ages as well as for both sexes (Verhulst et aI., 1990). It was concluded that problem behavior, especially aggression, is not more likely to be outgrown by young children than by older children . It was also found that adolescence was not characterized by rapid changes in the level of problems in this general population sample. Unrepresentative Samples
A number of studies have followed longitudinally assessed, small, unrepresentative samples. Examples are the studies by Kagan and Moss (1962) and Thomas and Chess (1976) who employed intensive repeated measurements. These studies give detailed accounts of the individual behavioral development of the children studied. In contrast to the longitudinal study of representative samples, however, the generality of the findings of these studies is limited because of the unrepresentativeness of the sample. High-Risk Samples
One way of increasing the number of subjects with deviant behaviors is to select children who are at risk because of adverse early experiences, such as perinatal problems (Werner, 1989), discontinuous caretaking (Hodges and Tizard, 1989a, b), or parental deviance, such as schizophrenia (Rosenbaum, 1988), depression (Cytryn et aI., 1984), or alcoholism (Rydelius, 1981). Although the great advantage of the high-risk design over the representative general population design is the higher yield of deviant individuals, high-risk studies may suffer from distortions in their samples of subjects having the target disorders. For instance, it may be that in children who have a schizophrenic parent and who develop this disturbance, the causes may be different from those in children who develop schizophrenia without having schizophrenic parents. Patient Samples
A fourth category of longitudinal research is formed by samples of identified patients . This Special Section focuses especially on this category because of its direct relevance for clinical practice . The longitudinal study of patients or
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individuals can give us information on outcomes, treatment effects, or changes in diagnosis. Although there are numerous studies on the short-term effects of specific types of treatment, little is known about the long-term effects of different treatments. Furthermore, when large samples of patients from different localities are studied, it is difficult to standardize treatment methods and to operationalize the criteria for selecting patients for a particular treatment. It is therefore unclear which outcomes merely reflect the course of a psychopathological condition and which reflect the results of particular interventions. Another problem in outcome studies of identified patients is the lack of adequate control groups, which should include normal children as well as those having other psychiatric diagnoses. For instance, Weiss et al. (1979) found that over 95% of their hyperactive subjects reported using soft drugs in the previous 5 years . However, the same proportion in their control group of normal children also reported using soft drugs. For this Special Section , several salient child psychiatric conditions have been chosen for closer discussion by a number of experts: Gillberg on autism, Klein and Mannuzza on hyperactivity, Kovacs and Goldston on depression, and Loeber on antisocial behaviors. Outcome studies of other specific child psychiatric conditions, such as obsessive-compulsive disorder (Berg et aI., 1989), anxiety disorders (Gittelman, 1986; Rutter and Garmezy, 1983), language disorders (Maughan et aI., 1985; McGee et aI. , 1986; Stevenson et aI., 1985) as well as studies that focused on general patient populations with a wide spectrum of psychiatric conditions (Blotcky et aI., 1984; Robins, 1974; Zeitlin, 1986), will not be discussed here because of space limitations. Conclusion
The developmental perspective is necessary in seeking the origins of psychopathology and in describing the course of psychopathological manifestations in children and adolescents . A necessary approach to the study of developmental variations and of causal mechanisms and processes underlying the course of psychopathology is the longitudinal method. The most salient characteristic of this method, which consists of assessments of the same individuals at certain intervals over a period of time, is that it provides the opportunity of investigating variations over time within individuals. This makes it a powerful and, for some purposes, indispensable approach for testing causal hypotheses, for describing the course of problem behavior, and for analyzing the effect of treatment and prevention. Although the examples given in the present review of recent longitudinal research relevant to child and adolescent psychiatry demonstrate the considerable activity in this field, there are a number of obstacles hampering its progress. First, there is a lack of widely accepted standardized assessment procedures and a lack of valid diagnostic constructs for childhood psychopathology. An exception is antisocial behavior . The key variables in the longitudinal investigation of delinquency are relatively discrete and agreed upon (e.g ., stealing , truancy , fire setting) . This may be a reason why I .Am.Acad. Child Adolesc. Psychiatry, 30 :3, May 1991
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longitudin al research on delinquen cy has been more fruitful than that on other childhood psychopathological conditions (see Farrington [1991] and Loeber [1991] in this Special Section). Second, there is a paucity of hypotheses or theories about possible etiological mechani sms that can be tested adequately. Third , there are obstacles inherent to the prospective longitudinal approach itself, such as the long period the study takes, the effects of sample attrition and the confounding of aging and period effects. Alternative approaches, such as the accelerated longitudinal design, may provide the opportunity to investigate within-individual variations while minimizing the adverse effects of the longitudinal method. It is clear that the complex nature of the etiological mechanisms itself and the methodological and statistical problems connected with the longitudinal method call for considerable efforts. Without such efforts, we will slow down the progress in our understanding of the developmental course of psychopathological conditions of childhood and adolescence and of the etiology , the preventi on , and the treatment of these conditions.
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From Early Issues of the JAACP May we never grow complacent, the sedative of progress, but keep alive our critical judgments and utilize the value of conflict, which can provide the ore of progress if the heat generated is great enough to refine it but not so great as to destroy it. Child psychiatry has generated a great deal of heat in the decades just past, but it has been a refining process.
JM CP, Vol. 2, No. I January, 1963
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