Perceptual and M o ~ o Skills, r 1978,47, 815-818. @ Perceptual and Motor Skills 1978
MATERNAL AGE AND CHILDREN'S ABILITY1 PATRICIA ZYBERT, ZENA STEIN, LILLIAN BELMONT Epidemiology of Brain Dirorders Research Unit New York State Psychiatric Instibrcte, and Division of Epidemiology School of Public Hedth, Columbia University Summary.-Associations of maternal age at birth and subsequent intelligence test scores of children were examined in a series of over 1500 young men from the Netherlands. All subjects were members of 2-child families and were resident in Amsterdam at age 19 yr. Possible confounding by birth order, spacing interval, social class and sex of sibling were considered. Significant correlations between maternal age and child's ability remained in three of the four possible birth order/social class combinations.
The effect of maternal age on intelligence test score in 2-child families was examined in a series of 19-yr.-old young men from the Netherlands. Consistent effects of birth order and family size such that test score became poorer both with later birth order and with larger family size have been reported for the total population elsewhere (Belmont & Marolla, 1973). There was a significant effect of birth order in this series as well (Belmont, Stein, & Zybert, in press). Since the effect of maternal age on test score is opposite to the effect of birth order, we chose to control for birth order while analyzing effects of maternal age. Generally, the children of older mothers have higher scores, whereas later birth orders (when the mother is older) have lower scores (Ikley, 1966; Record, McKeown, & Edwards, 1969; Davie, Butler, & Goldstein, 1972). Although the association between maternal age and IQ has been frequently reported (Lobl, et al., 1971; Miller, et al., 1974; National Center for Health Statistics, 1974; Broman, et al., 1975), it has in general not been possible to remove confounding.
METHOD The study population was drawn from a series of over 400,000 young men born in the Netherlands between 1944 and 1947 who were tested as part of their military preinduction examination at age 19 yr. Anonymous records from this examination were supplied to our research unit for a study of the effects of prenatal famine on subsequent physical and mental development (Stein, Susser, Saenger, & Marolla, 1975). The records include measures of physical, psychological and social status. Supplemental data including raw 'We thank the Department of Defense of the Netherlands for permission to use the military preinduction examination. This study was supported in part by NICHD Grants 06808 (to L. Belrnont) and 06751 (to M. Susser). Send reprint requests to Patricia Zybert, Division of Epidemiology, Columbia University School of Public Health, 604 West 168th Street, New York, N. Y. 10032.
816
P. ZYBERT. ET AL.
psychometric test scores and mother's, father's and sibling's dates of birth were later collected for selected individuals in order to study possible effects of spacing interval and/or maternal age on test score.2 The resulting study series included 1511 young men from 2-child families resident in Amsterdam at the time of military examination. About half had brothers ( N = 731) and half had sisters ( N = 780). Sex of sibling bore no relation to test score. There were 60 pairs where both brothers of a sibship were included in the series. Social dass is based on father's occupation. Findings are presented for each social dass, Manual and Non-manual, and Birth order (1 and 2) separately. The intelligence test score used is the unweighted sum of Dutch ratings (1-6) on the five tests admini~tered.~The range of Total Score is thus from a poor score of 5 to a high score of 30.
RESULTS Table 1 presents mean maternal ages at birth for the manual and nonmanual social classes. Mean maternal age did not vary significantly by social class or birthyear of subject but, as expected, was significantly lower for firstborn than for secondborn ( p < .001). TABLE 1
MATERNAL AGE A T BIRTH (MEAN, SD,AND RANGE) FOR FIRSTBORN AND SECONDBORN IN TWO-CHILD FAMILIES BY SOCIAL CLASS Social Class Manual Non-manual
Position First Second First Second
N
339 35 1 40 1 420
Maternal Age (in yr.) SD Ranae -
M 26.65 31.72 27.23 31.82
4.66 5.20 4.34 4.98
1842 1745 1639 2045
Correlation coefficients indicate a positive association between Total Score and maternal age; the poorest scores were achieved by men with the youngest mothers. With the exception of Manual Birth Order 2 all correlations were significant at the .O1 level: Manual Birth Order 1, r = .162, df = 337; 'A list of military ID numbers of men who were either firstborn of two and born in 1944 or secondborn of cwo and born in 1947 was generated from our original tape and sent to the Netherlands where IDS were linked with names. Using these, the Dutch registrar searched local records for birthdate and sex of sibling and b h h year of parents (as well as for possible military I D of brothers). Made anonymous again, the data were forwarded to New York. We were sent photocopies of examination records from which raw test scores were abstracted. T h e five tests administered were the Bennett Test of Mechanical Comprehension, Raven Progressive Matrices (Dutch modification) and tests of mathematics (arithmetic), language and grammar, and clerical aptitude. Correlations between tests ranged from .647 to .853 (Marolla, 1973). A composite score devised by the Dutch military was also provided.
MATERNAL AGE AND CHILDREN'S ABILITY
817
Manual Birth Order 2, r = .097, df = 349; Non-manual Birth Order 1, r 170, df = 399; Non-manual Birth Order 2, r = .153, df = 418.
=
DISCUSSION Effects of maternal age on intelligence test scores were examined in a large series of individuals from 2-child families. A significant positive correlation between maternal age at birth of child and test score at age 19 was found for both birth orders in the Non-manual social dass and for firstborn in the Manual social class. Scott, Illsley, and Thomson (1956) in Aberdeen as well as the Scottish Council for Research in Education (1953) have remarked on a pattern of small family size, older parents, and better housing with children above average in intelligence a t one end of the socioeconomic scale versus a pattern of large family size, low housing standards, and children of below average intelligence at the other. Scott, et 01. concluded however that the associations found "do not directly elucidate the problems of causation" (p. 342). By what mechanism would maternal age exert an influence on test score? Within their series of over 35,000 11-yr.-olds Record, McKeown, and Edwards (1969) attributed all apparent maternal age effects on intelligence (as well a s birth order effects) to social class. Younger mothers at a particular birth order tended to be of lower socioeconomic status than older mothers, perhaps even within their three divisions of socioeconomic status. In the National Child Development Study a positive relation between maternal age and reading attainment at age 7 yr. remained after effects of social class, birth order and family size were removed (Davie, Butler, & Goldstein, 1972). The authors suggest char apparent maternal age effects might reflect social or regional differences associated with maternal age fbr which their study design could not account. Controls for social class in our own study were strict. The study series was homogeneous in terms of social class by virtue of being drawn from an urban population (Amsterdam) and restricted to 2-child families. In addition, results were presented separately for sons of manual and non-manual workers. All subjects were 19-yr.-old males. W e have thus found significant associations of test score and maternal age in a very circumscribed study series which we cannot attribute to socioeconomic or regional variations. Sons of older mothers had higher test scores whether they were firstborn or secondborn, providing presumptive evidence that effects of maternal age are present within particular birth orders. In our study of spacing interval it was clear that maternal age was correlated with spacing. But while maternal age was related to test score, spacing interval was not (Belrnont, Stein, & Zybert, in press). The present study series was too small for more refined analyses which
818
P. ZYBERT, ET AL.
might have suggested causal mechanisms. It would be simplistic to assume that maternal age "causes" intelligence. Multivariate statistical analyses depend on larger and more comprehensive data than are generally available. Our current research using large data sets (Health Examination Surveys and Collaborative Perinatal Project) examines maternal age effects on IQ when other variables are controlled. BBLMONT, L.,
&
REFERENCES MAROLLA, F. A. Birth order, family size, and intelligence. Science,
1973, 182, 1096-1101. STEIN, Z. A., & ZYBERT, P. Child spacing and birth order: effect on intellectual ability in two-child families. Science. in press. BROMAN,S. H., NICHOLS, P. L., & KENNEDY.W. A. Preschool 1Q-+renut$ and early developmental correlates. Hillsdale, N. J.: Lawrence Erlbaum Assoc., 1975. DAVIB,R., BUTLW,N., & GOLDSTEIN, H. From birth to seven: a report of the National Child Development Study. London: Longman, 1972. ILLSLEY, R. Family growth and its effect on the relationship between obstetric factors and child functioning. In Lord Platt & A. S. Parkes (Eds.), Social and genetic influence on life and death. Edinburgh: Oliver e( Boyd, 1966. Pp. 29-42. LOBL, M., WELCHER, D. W., & MELLITS,E. D. Maternal age and intellectual functioning in offspring. Johns Hopkins Medical loitrnal, 1971, 128, 347-361. MAROLLA, F. A. Intelligence and demographic variables in a 19-year-old cohort in the Netherlands: an analysis of their relationship from the two-component theories of intelligence. Unpublished doctoral dissertation, New School for Social Research, New York, 1973. MILLER,F. J. W., COURT,S. D. M., KNOX,E. G., S( BRANDON,S. The school years in Newcastle upon Tyne. London: Oxford Univer. Press, 1974. NATIONALCENTERFOR HEALTHSTATISTICS.Family background, early development, and intelligence of children 6-11 years. Vital alnd Health Statistics, 1974, Series 11, No. 142. RECORD, R. G., MCKEOWN, T., & EDWARDS, J. H. The relation of measured intelligence to birth order and maternal age. Annals o f H U M Genetics, London, 1969, 33, 61-69. Scan, E. M., ILLSLEY, R., & THOMSON,A. M. A psychological investigation of primigtavidae: 11. Maternal social class, age, physique and intelligence. Journal of Obstetrics and Gymecology of the British Empire, 1956, 63, 338-343. S c o r n s ~COUNCILFOR RESEARCH IN EDUCATION. Social implications of the 1947 Scottish Mental Survey. London: Univer. of London Press, 1953. STEIN, Z., SUSSER,M., SAENGBR,G., & MAROLLA, F. Famine and human develophunger winter of 1944-1945. New York: Oxford Univer. Press, ment: the D ~ c h 1975.
BELMONT, L.,
Accepted September 18, 1978.
MATERNAL AGE AND CHILDREN'S ABILITY1 PATRICIA ZYBERT, ZENA STEIN, LILLIAN BELMONT Epidemiology of Brain Dirorders Research Unit New York State Psychiatric Instibrcte, and Division of Epidemiology School of Public Hedth, Columbia University Summary.-Associations of maternal age at birth and subsequent intelligence test scores of children were examined in a series of over 1500 young men from the Netherlands. All subjects were members of 2-child families and were resident in Amsterdam at age 19 yr. Possible confounding by birth order, spacing interval, social class and sex of sibling were considered. Significant correlations between maternal age and child's ability remained in three of the four possible birth order/social class combinations.
The effect of maternal age on intelligence test score in 2-child families was examined in a series of 19-yr.-old young men from the Netherlands. Consistent effects of birth order and family size such that test score became poorer both with later birth order and with larger family size have been reported for the total population elsewhere (Belmont & Marolla, 1973). There was a significant effect of birth order in this series as well (Belmont, Stein, & Zybert, in press). Since the effect of maternal age on test score is opposite to the effect of birth order, we chose to control for birth order while analyzing effects of maternal age. Generally, the children of older mothers have higher scores, whereas later birth orders (when the mother is older) have lower scores (Ikley, 1966; Record, McKeown, & Edwards, 1969; Davie, Butler, & Goldstein, 1972). Although the association between maternal age and IQ has been frequently reported (Lobl, et al., 1971; Miller, et al., 1974; National Center for Health Statistics, 1974; Broman, et al., 1975), it has in general not been possible to remove confounding.
METHOD The study population was drawn from a series of over 400,000 young men born in the Netherlands between 1944 and 1947 who were tested as part of their military preinduction examination at age 19 yr. Anonymous records from this examination were supplied to our research unit for a study of the effects of prenatal famine on subsequent physical and mental development (Stein, Susser, Saenger, & Marolla, 1975). The records include measures of physical, psychological and social status. Supplemental data including raw 'We thank the Department of Defense of the Netherlands for permission to use the military preinduction examination. This study was supported in part by NICHD Grants 06808 (to L. Belrnont) and 06751 (to M. Susser). Send reprint requests to Patricia Zybert, Division of Epidemiology, Columbia University School of Public Health, 604 West 168th Street, New York, N. Y. 10032.
816
P. ZYBERT. ET AL.
psychometric test scores and mother's, father's and sibling's dates of birth were later collected for selected individuals in order to study possible effects of spacing interval and/or maternal age on test score.2 The resulting study series included 1511 young men from 2-child families resident in Amsterdam at the time of military examination. About half had brothers ( N = 731) and half had sisters ( N = 780). Sex of sibling bore no relation to test score. There were 60 pairs where both brothers of a sibship were included in the series. Social dass is based on father's occupation. Findings are presented for each social dass, Manual and Non-manual, and Birth order (1 and 2) separately. The intelligence test score used is the unweighted sum of Dutch ratings (1-6) on the five tests admini~tered.~The range of Total Score is thus from a poor score of 5 to a high score of 30.
RESULTS Table 1 presents mean maternal ages at birth for the manual and nonmanual social classes. Mean maternal age did not vary significantly by social class or birthyear of subject but, as expected, was significantly lower for firstborn than for secondborn ( p < .001). TABLE 1
MATERNAL AGE A T BIRTH (MEAN, SD,AND RANGE) FOR FIRSTBORN AND SECONDBORN IN TWO-CHILD FAMILIES BY SOCIAL CLASS Social Class Manual Non-manual
Position First Second First Second
N
339 35 1 40 1 420
Maternal Age (in yr.) SD Ranae -
M 26.65 31.72 27.23 31.82
4.66 5.20 4.34 4.98
1842 1745 1639 2045
Correlation coefficients indicate a positive association between Total Score and maternal age; the poorest scores were achieved by men with the youngest mothers. With the exception of Manual Birth Order 2 all correlations were significant at the .O1 level: Manual Birth Order 1, r = .162, df = 337; 'A list of military ID numbers of men who were either firstborn of two and born in 1944 or secondborn of cwo and born in 1947 was generated from our original tape and sent to the Netherlands where IDS were linked with names. Using these, the Dutch registrar searched local records for birthdate and sex of sibling and b h h year of parents (as well as for possible military I D of brothers). Made anonymous again, the data were forwarded to New York. We were sent photocopies of examination records from which raw test scores were abstracted. T h e five tests administered were the Bennett Test of Mechanical Comprehension, Raven Progressive Matrices (Dutch modification) and tests of mathematics (arithmetic), language and grammar, and clerical aptitude. Correlations between tests ranged from .647 to .853 (Marolla, 1973). A composite score devised by the Dutch military was also provided.
MATERNAL AGE AND CHILDREN'S ABILITY
817
Manual Birth Order 2, r = .097, df = 349; Non-manual Birth Order 1, r 170, df = 399; Non-manual Birth Order 2, r = .153, df = 418.
=
DISCUSSION Effects of maternal age on intelligence test scores were examined in a large series of individuals from 2-child families. A significant positive correlation between maternal age at birth of child and test score at age 19 was found for both birth orders in the Non-manual social dass and for firstborn in the Manual social class. Scott, Illsley, and Thomson (1956) in Aberdeen as well as the Scottish Council for Research in Education (1953) have remarked on a pattern of small family size, older parents, and better housing with children above average in intelligence a t one end of the socioeconomic scale versus a pattern of large family size, low housing standards, and children of below average intelligence at the other. Scott, et 01. concluded however that the associations found "do not directly elucidate the problems of causation" (p. 342). By what mechanism would maternal age exert an influence on test score? Within their series of over 35,000 11-yr.-olds Record, McKeown, and Edwards (1969) attributed all apparent maternal age effects on intelligence (as well a s birth order effects) to social class. Younger mothers at a particular birth order tended to be of lower socioeconomic status than older mothers, perhaps even within their three divisions of socioeconomic status. In the National Child Development Study a positive relation between maternal age and reading attainment at age 7 yr. remained after effects of social class, birth order and family size were removed (Davie, Butler, & Goldstein, 1972). The authors suggest char apparent maternal age effects might reflect social or regional differences associated with maternal age fbr which their study design could not account. Controls for social class in our own study were strict. The study series was homogeneous in terms of social class by virtue of being drawn from an urban population (Amsterdam) and restricted to 2-child families. In addition, results were presented separately for sons of manual and non-manual workers. All subjects were 19-yr.-old males. W e have thus found significant associations of test score and maternal age in a very circumscribed study series which we cannot attribute to socioeconomic or regional variations. Sons of older mothers had higher test scores whether they were firstborn or secondborn, providing presumptive evidence that effects of maternal age are present within particular birth orders. In our study of spacing interval it was clear that maternal age was correlated with spacing. But while maternal age was related to test score, spacing interval was not (Belrnont, Stein, & Zybert, in press). The present study series was too small for more refined analyses which
818
P. ZYBERT, ET AL.
might have suggested causal mechanisms. It would be simplistic to assume that maternal age "causes" intelligence. Multivariate statistical analyses depend on larger and more comprehensive data than are generally available. Our current research using large data sets (Health Examination Surveys and Collaborative Perinatal Project) examines maternal age effects on IQ when other variables are controlled. BBLMONT, L.,
&
REFERENCES MAROLLA, F. A. Birth order, family size, and intelligence. Science,
1973, 182, 1096-1101. STEIN, Z. A., & ZYBERT, P. Child spacing and birth order: effect on intellectual ability in two-child families. Science. in press. BROMAN,S. H., NICHOLS, P. L., & KENNEDY.W. A. Preschool 1Q-+renut$ and early developmental correlates. Hillsdale, N. J.: Lawrence Erlbaum Assoc., 1975. DAVIB,R., BUTLW,N., & GOLDSTEIN, H. From birth to seven: a report of the National Child Development Study. London: Longman, 1972. ILLSLEY, R. Family growth and its effect on the relationship between obstetric factors and child functioning. In Lord Platt & A. S. Parkes (Eds.), Social and genetic influence on life and death. Edinburgh: Oliver e( Boyd, 1966. Pp. 29-42. LOBL, M., WELCHER, D. W., & MELLITS,E. D. Maternal age and intellectual functioning in offspring. Johns Hopkins Medical loitrnal, 1971, 128, 347-361. MAROLLA, F. A. Intelligence and demographic variables in a 19-year-old cohort in the Netherlands: an analysis of their relationship from the two-component theories of intelligence. Unpublished doctoral dissertation, New School for Social Research, New York, 1973. MILLER,F. J. W., COURT,S. D. M., KNOX,E. G., S( BRANDON,S. The school years in Newcastle upon Tyne. London: Oxford Univer. Press, 1974. NATIONALCENTERFOR HEALTHSTATISTICS.Family background, early development, and intelligence of children 6-11 years. Vital alnd Health Statistics, 1974, Series 11, No. 142. RECORD, R. G., MCKEOWN, T., & EDWARDS, J. H. The relation of measured intelligence to birth order and maternal age. Annals o f H U M Genetics, London, 1969, 33, 61-69. Scan, E. M., ILLSLEY, R., & THOMSON,A. M. A psychological investigation of primigtavidae: 11. Maternal social class, age, physique and intelligence. Journal of Obstetrics and Gymecology of the British Empire, 1956, 63, 338-343. S c o r n s ~COUNCILFOR RESEARCH IN EDUCATION. Social implications of the 1947 Scottish Mental Survey. London: Univer. of London Press, 1953. STEIN, Z., SUSSER,M., SAENGBR,G., & MAROLLA, F. Famine and human develophunger winter of 1944-1945. New York: Oxford Univer. Press, ment: the D ~ c h 1975.
BELMONT, L.,
Accepted September 18, 1978.
