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Path analysis of general and specific cognitive abilities in the Colorado adoption project: Early childhood
0191~8869188 53.00 +O.OO
Person. hdivid. D@?IVol. 9, No. 2, pp. 391-395. 1988 Printed in Great Britain
Pergamon Press plc
PATH ANALYSIS OF GENERAL AND SPECIFIC COGNITIVE ABILITIES IN THE COLORADO ADOPTION PROJECT: EARLY CHILDHOOD C. S. BERGEMAN,ROBERTPLOMIN,J. C. DEFRIES and D. W. FULKER Institute for the Study of Human Development, The Pennsylvania State University, University Park, PA 16802 and Institute for Behavioral Genetics, University of Colorado, Boulder, Colorado, U.S.A. (Received 30 April 1987)
Summary-A path model of genetic and environmental transmission was fitted to general and specific cognitive abilities (verbal, spatial, visual memory, and perceptual speed) data from biological, adoptive, and control parents and their children at ages 3 and 4. Using a maximum-likelihood estimation procedure, the significance of genetic influence, passive genotype-environment correlation, parental influence on the child’s environment, assortative mating, and selective placement was assessed. As predicted, the parameter estimate for genetic influence (a function of the heritabilities of the character during early childhood and adulthood and their genetic correlation) is significant for general cognitive ability at both 3 and 4. Moreover, estimates of the genetic parameter are significant for spatial ability at both 3 and 4 years of age, for verbal ability at 3, and for perceptual speed at 4. Thus, there is at least some continuity between early childhood and adulthood as well as some genetic differentiation for specific cognitive abilities. INTRODUCTION From a genetic perspective, the relationship between childhood and adulthood is a function of genetic variation in childhood, genetic variation in adulthood, and the genetic correlation between childhood and adulthood (Plomin and DeFries, 1981). The parent-offspring adoption design provides an opportunity to study genetic and environmental influences on complex human behavioral characteristics even when the parents are adults and the adoptees are children. Any resemblance between relatives within natural families is a function of both genetic and environmental influences shared by children and their parents. In contrast, adopted children share their genetic makeup with their biological parents, but they share only familial environmental influences with their adoptive relatives. In addition, comparisons between several traits in parents and their children in an adoption design can be used to assess the specificity of genetic and family environmental influences on the development of behavioral characteristics in early childhood. The present report is an extension of a series of previous articles reported from the Colorado Adoption Project (Plomin and DeFries, 1985a), which have focused on general IQ and specific cognitive abilities in infancy (Fulker and DeFries, 1983; Thompson, Plomin and DeFries, 1985) and in early childhood (Plomin and DeFries, 1985b; Rice, Fulker and DeFries, 1986b; DeFries, Plomin, and LaBuda, 1987; LaBuda, DeFries, Fulker and Plomin, 1986). Analyses of simple parent-offspring correlations for specific cognitive abilities at ages 3 and 4 led to an interesting hypothesis (Plomin and DeFries, 1985b): Genetic continuity from early childhood to adulthood is substantial for IQ, whereas specific cognitive abilities do not show this pattern of results independent from their relationship with IQ. For example, general cognitive ability (IQ) of adoptees in early childhood relates to IQ of biological parents, whereas specific cognitive abilities such as spatial ability of the adoptees do not relate to spatial ability of the biological parents, even though reliabilities are similar for general and specific cognitive abilities. This suggests that specific cognitive abilities may not yet be differentiated etiologically in early childhood. In other words, genetic effects that mediate continuity from childhood to adulthood may primarily involve general rather than specific cognitive abilities. Rice et al. (1986b), in a multivariate path analysis of specific cognitive abilities data from 4-yr-olds in the CAP, also found that four measures of specific cognitive abilities did not appear to be highly differentiated genetically-that is, individual differences in these measures appear to be largely influenced by the same genes. In order to test this hypothesis more rigorously, the present study fitted a path model of genetic and environmental transmission to general IQ and specific cognitive ability data, from biological, adoptive and control 391
392
C. S. BERGEMAN et al.
(nonadoptive) parents and their children at ages 3 and 4. This is the first analysis of specific cognitive abilities of 3-yr-olds in the CAP. The path model employed in the present analysis is the univariate adoption model formulated by Fulker and DeFries (1983). but it has been simplified to exclude a manifest variable (I) which was an index of home environment. In addition to providing tests of hereditary and environmental influences, the model includes parameters for passive genotype+nvironment correlation, assortative mating, and selective placement, as discussed later. A complete description of the path diagrams of genetic and environmental transmission in nonadoptive and adoptive families, and the corresponding expectations of variance and covariance can be found in Fulker and DeFries (1983). METHOD Subjects
The Colorado Adoption Project (CAP) is a longitudinal prospective study of genetic and environmental influences on individual differences in behavioral development (Plomin and DeFries. 1985a). Biological and adoptive parents are contacted through two Denver adoption agencies whose professional staff counsel unwed parents on a non-residential basis. Adoptive children are separated from their biological mothers shortly after birth, and are placed in adoptive homes, on average, 4 wk later. Control families participating in the CAP are matched to adoptive families on the basis of sex of proband, number of children in the family, age of father (+ 5 yr), occupational rating of the father, and total years of father’s education (+ 2 yr). The analyses summarized in this report are based upon data from 196 adoptive and 189 control children tested at age 3, and 177 adoptive and 161 control children at age 4, and their biological and adoptive parents. A concerted effort was made to test biological fathers of adoptive children; data from only 42 were available for the present analysis. A more detailed description of this sample and its ascertainment is provided elsewhere (Plomin and DeFries, 1983, 1985a). Measures
CAP parents are administered a 3-hr battery which includes 13 tests of specific cognitive abilities. A principal component analysis of the CAP adult data yields four readily interpretable factorsSpatial Visualization, Verbal Reasoning, Perceptual Speed, and Memory. Loadings on the first principal component (unrotated) are moderately high for all variables, (DeFries, Plomin, Vandenberg and Kuse, 1981). Since the first principal component from a highly similar battery employed in the Hawaii Family Study of Cognition was found to correlate over 0.70 with the WAIS full scale IQ (Kuse, 1977), the first principal component will be used as a measure of general cognitive ability in this report. The median test-retest reliability for the specific cognitive abilities is 0.80 (DeFries et al., 1981). The adopted and control children in the CAP are tested yearly between ages 1 and 4 in their homes. At ages 3 and 4, the Stanford-Binet IQ test and a battery of tests of specific cognitive abilities are administered. In addition, interview data are provided by parents and video recordings of parent-child interactions are obtained. The battery of specific cognitive abilities was designed by CAP staff to measure factors similar to those assessed by the adult battery (spatial, verbal, perceptual speed and visual memory). Descriptions of the test battery used at ages 3 and 4 have been reported by Singer, Corley, Guiffrida and Piomin (1984) and by Rice, Corley, Fulker and Plomin (1986a), respectively. The Stanford Binet IQ was used as a measure of the children’s general cognitive ability. Analysis
The path model for control families is illustrated in Fig. I. The model includes only two latent variables, additive genetic value G and environmental deviation E, in both parental and offspring generations, and these totally determine the phenotype P with paths. h and e, respectively. Although the model appears complex, the basic idea involves the simple point that control parents contribute both heredity and environment to their children. It is assumed that the measures are isomorphic in childhood and adulthood. Environmental transmission is modelled in terms of the impact of the parent’s phenotype on the child’s environment, through path w, (H., from the father,
Specific
cognitive abilities: early childhood Path
Nonadoptive
393
analysis
families
Fig. 1
and w2 from the mother). The complexities on the left side of the path model arise from consideration of assortative mating and genotype-environment correlation. These complex issues are presented in terms of reverse path analysis which is described by Fulker and DeFries (1983). This presentation facilitates the derivation of the three expected correlations among the three manifest variables (Li, 1975). Figure 2 illustrates the path model for adoptive families. Adoptees have one set of parents, PBM and P,,, who contribute genes, and another set, PAM and P,,, who contribute environmental influences. Because assortative mating may differ in wed and unwed couples, different assortative mating parameters, p and q, are included in the model. The extent of selective placement. X. is also determined. The five manifest variables in Fig. 2 yield 10 correlations for which expectations may be derived. The expectations for both the control and adoptive correlations are presented in the article by Fulker and DeFries (1983). The summary statistics to which the model was fitted are variances and covariances calculated separately for adoptive and nonadoptive families. In the case of nonadoptive families, five 3 x 3 matrices are formed, one for each variable analyzed (general, spatial, verbal, memory, and perceptual speed). For adoptive families, in which complete data are available for biological fathers, biological mothers, adoptive fathers, adoptive mothers and adopted children, five 5 x 5 matrices are formed. For those adoptive families in which data on biological fathers is not available, five 4 x 4 matrices were calculated.
Adoptive
families
Fig. 2
C. S. BERciEh4~xet al.
394
Path models were fitted to these observed covariance matrices using a maximum likelihood estimation procedure. The function was minimized using MINUIT (CERN, 1977), a package of optimization and error analysis routines made available by CERN, the European Centre for Nuclear Research (see Fulker and DeFries, 1983). RESULTS
The model was fitted to the data and their log-likelihoods were evaluated using chi-square. The results for general cognitive ability are shown in Tables 1 and 2. The parameter estimate for /I and its corresponding standard error implies that significant genetic continuity exists for IQ from childhood to adulthood. Assortative mating @ and q) is generally significant; selective placement (x) and environmental effect (w) of the parents’ IQ are not significant. However most of the variance remains unexplained. Consistent with previous findings in the CAP, these data suggest that significant genetic continuity exists for IQ from childhood to adulthood. Thus, for IQ, the results indicate the importance of genetic transmission at both 3 and 4 yr-maximum likelihood heritability estimates (h 2) are 0.14 at 3 years and 0.19 at 4 yr. The results of model-fitting for specific cognitive abilities, however, are mixed, as indicated in Tables 1 and 2. Spatial ability shows a pattern of results similar to those for IQ, yielding heritability estimates of 0.12 and 0.19 at 3 and 4 years, respectively. Verbal ability, however, yields evidence of significant influence at 3 but not at 4 years, with heritability estimates of 0.18 and 0.06, respectively. Perceptual speed, on the other hand, shows significant genetic influence at 4 but not at 3-heritability estimates are 0.07 and 0.10, respectively. Memory is not significantly heritable at either 3 or 4. As in the case of IQ, family environmental variance is low and nonsignificant for all measures at 3 and 4 yr. Assortative mating is positive for specific cognitive abilities for both biological parents of adopted children, (q), and adoptive and control parents, (p). Setting these parameter estimates equal in reduced models does not make a significant difference in the fit of the model, except for Table
h S WI P 4 XI x2 *I x4 e WI w F P
I. General
and specific cognitive
Spatial
0.37 + 0.09 0.06 of:0.02 0. I I + 0.06 0.29 + 0.05 0.34 f 0.22 0.00 + 0.06 0.05 * 0.07 0.02kO.13 0.07+0.17 0.91 0.12 0.06 20 25.72
0.35 + 0.16 0.02 + 0.02 0.03 & 0.07 0.09 i: 0.05 0.00 & 0.07 -0.04 * 0.07 0.09 f 0.07 -0.11 * 0.25 0.29*0.16 0.93 0.09 0.02 20 29.98
0.18
0.08
0.42 0.03 0.03 0.34 0.00 0.05 0.03 0.21 -0.02 0.89 0.08 0.02 20 16.06
0. I 1 0.02 0.06 0.04 0.30 0.07 0.07 0.14 0.14
and specific cognitive
Spatial
Verbal
0.44 * 0.09 0.05 * 0.02 0.10 + 0.07 0.23 + 0.05 0.32 f 0.19 0.08 f 0.07 0.12+0.07 -0.00~0.15 0.12 kO.16
0.44*0.12 0.01 + 0.02 -0.05 + 0.07 0.06 f 0.05 0.00 * 0.25 -0.01 f 0.07 0.15+0.07 -0.32 kO.18 0.2OkO.16 0.89 0.10 0.01 22.55 20 0.30
0.24 + 0.18 0.04 * 0.02 0.16f0.06 0.26 + 0.05 0.00 f 0.29 0.07 f 0.07 0.02 + 0.07 0.25 * 0.13 0.09 * 0.14 0.96 0.04 0.06 23.51 20 0.25
0.07 0.05 23.35 20 0.28 estimates
Memory
0.27 50.16 0.02 * 0.01 0.04 f 0.06 0.16kO.05 0.18 f0.13 -0.02 + 0.07 0.00 * 0.08 -0.16f0.12 -0.01 kO.15 0.96 0.05 0.02 20 .a5 21
0.15+0.70 -0.01 * 0.05 0.03 + 0.06 0.11 +0.05 0.16kO.13 -0.07 * 0.07 -0.09 * 0.07 -0.18 f 0.14 0.22 f 0.15 0.99 -0.12 -0.01 20 28.02
0.35 errors.
General
0.88
Parameter
f * + + + f * * _t
for full model with standard
errors.
at age 3 P Speed
0.70
Parameter estimates for full model shown with standard parameters, and standard errors are not given. Table 2. General
abilities
Verbal
General
0.10
E, w,, and the rge correlation
abilities
are derived
at age 4 PSpced
0.31 kO.14 0.01 + 0.01 0.00 f 0.06 0.19f0.05 0.18 kO.13 0.01 * 0.08 -0.03 * 0.09 -0.17kO.12 -0.02+0.15 0.95 0.02 0.01 21.97 20 0.35
Memory 0.01 f 0.02 0.00 + 0.00 0.01 + 0.06 0.11 kO.05 0.26+O.l2 -0.03 f 0.08 -0.05 + 0.07 -0.27 kO.14 0.14+0.15 0.99 -0.07 0.00 26.76 20 0.15
Specific cognitive abilities: early childhood
395
verbal ability. With regard to the selective placement parameters, each is non-significant as expected given the placement practices employed by the adoption agencies that participate in the CAP (Plomin and DeFries, 1985a). CONCLUSIONS Using CAP data, Thompson et al. (1985) found that infant specific cognitive abilities (ages 12 and 24 months) are predictive of parent’s general cognitive ability, but not parent’s specific abilities. That is, infant mental development is undifferentiated in the sense that components of infant mental functioning relate more to adult IQ than to adult specific cognitive abilities, and these infant-toadult relationships are mediated genetically. In early childhood (ages 3 and 4), there appears to be genetic continuity from childhood to adulthood not only for general IQ, but for spatial ability as well. In addition, there is evidence for significant genetic influences for verbal ability at 3 yr of age and for perceptual speed at 4. However, visual memory continues to show little genetic influence. From an environmental perspective, the results imply that the aspects of the family environment relevant to cognitive development are not “fine-tuned” according to parental specific cognitive abilities. Although most of the variance still remains unexplained, finding significant genetic influence for IQ and for specific cognitive abilities in early childhood is exciting, because it suggests genetic continuity from childhood to adulthood, as well as some possible differentiation of specific abilities. This pattern of results leads us to predict that genetic differentiation of specific cognitive abilities increases during middle childhood, when children embark upon formal schooling, in other words, we would expect to find greater evidence for parent-offspring resemblance for isomorphic cognitive abilities. Because children in the CAP are tested again at age 7 on a similar battery of tests of specific cognitive abilities, we will be able to test this hypothesis in future years as the sample of children tested at 7 becomes large enough for analysis. Acknowledgemenrs-This research was supported in part by grants from the National Institute of Child Health and Human Development (HD-10333 and HD-18426). the National Science Foundation (BSN-8200310 and BSN-8505692). and the Spencer Foundation. We are grateful to the families participating in the CAP, who have so generously contributed their time and effort, and to the adoption agencies who make the study possible-Lutheran Social Services of Colorado, and Denver Catholic Community Services.
REFERENCES Cem (I 977) MINUIT: A system for function minimization and anaIysis ofparameter errors and correlations. CERN, Geneva. DeFries J. C., Plomin R. and LaBuda M. C. (1987) Genetic stability of cognitive development from childhood to adulthood. Dev. Psychol.
23, 4-12.
DeFries J. C., Plomin R., Vandenberg S. G. and Kuse A. R. (1981) Parent offspring resemblance for cognitive abilities in the Colorado Adoption Project: Biological, adoptive and control parents and one-year-old children. Intelligence 5, 265-277. -._ -...
Fulker D. W. and DeFries J. C. (1983) Genetic and environmental transmission in the Colorado Adoption Project: Path analysis. Br. J. Math. Statist. Psychol. 36, 175-188. Kuse A. R. (1977) Familial resemblance for cognitive abilities estimatedfrom two test batteries in Hawaii. Unpublished Ph.D Thesis, University of Colorado, Boulder. LaBuda M. C., DeFries J. C., Fulker D. W. and Plomin R. (1986) Longitudinal stability of cognitive ability from infancy to early childhood: Genetic and environmental etiologies. Chid Dev. 57, 1142-I 150. Li C. C. (1975) Path Analysis: A Primer. Pacific Grove, Boxwood Press, CA. Plomin R. and DeFries J. C. (1981) Multivariate behavioral genetics and development: Twin studies. In Twin Research 3, Part B: Intelligence, Personality and Development (Edited by Gedda L., Parisi P. and Nance W. E.). Liss, New York. Plomin R. and DeFries J. C. (1983) The Colorado Adoption Project. Chld Dev. 54, 276-289. Plomin R. and DeFries J. C. (1985a) Origins of Individual Differences in Infancy: The Colorado Adoption Project. Academic Press, Orlando, FL. Plomin R. and DeFries J. C. (1985b) A parent-offspring adoption study of cognitive abilities in early childhood. Intelligence . 9, 341-356. Rice T.. Corley R.. Fulker D. W. and Plomin R. (1986a) The development and validation of test battery measuring specific cognitive abilities in four-year-old children. &duc. ksychol. Me&r. 46, 699-708. Rice T.. Fulker D. W. and DeFries J. C. (1986b) Multivariate oath analysis of specific cognitive abilities in the Colorado Adoption Project. Behav. Gener. 16, lb7-125. Singer S., Corley R., Guiffrida C. and Plomin R. (1984). The development and validation of a test battery to measure differentiated cognitive abilities in three-year-old children. Educ. Psychol. Measur. 44, 15-24. Thompson L. A., Plomin R. and DeFries J. C. (1985) Parent-infant resemblance for general and specific cognitive abilities in the Colorado Adoption Project. Intelligence 9, I-13.
Person. hdivid. D@?IVol. 9, No. 2, pp. 391-395. 1988 Printed in Great Britain
Pergamon Press plc
PATH ANALYSIS OF GENERAL AND SPECIFIC COGNITIVE ABILITIES IN THE COLORADO ADOPTION PROJECT: EARLY CHILDHOOD C. S. BERGEMAN,ROBERTPLOMIN,J. C. DEFRIES and D. W. FULKER Institute for the Study of Human Development, The Pennsylvania State University, University Park, PA 16802 and Institute for Behavioral Genetics, University of Colorado, Boulder, Colorado, U.S.A. (Received 30 April 1987)
Summary-A path model of genetic and environmental transmission was fitted to general and specific cognitive abilities (verbal, spatial, visual memory, and perceptual speed) data from biological, adoptive, and control parents and their children at ages 3 and 4. Using a maximum-likelihood estimation procedure, the significance of genetic influence, passive genotype-environment correlation, parental influence on the child’s environment, assortative mating, and selective placement was assessed. As predicted, the parameter estimate for genetic influence (a function of the heritabilities of the character during early childhood and adulthood and their genetic correlation) is significant for general cognitive ability at both 3 and 4. Moreover, estimates of the genetic parameter are significant for spatial ability at both 3 and 4 years of age, for verbal ability at 3, and for perceptual speed at 4. Thus, there is at least some continuity between early childhood and adulthood as well as some genetic differentiation for specific cognitive abilities. INTRODUCTION From a genetic perspective, the relationship between childhood and adulthood is a function of genetic variation in childhood, genetic variation in adulthood, and the genetic correlation between childhood and adulthood (Plomin and DeFries, 1981). The parent-offspring adoption design provides an opportunity to study genetic and environmental influences on complex human behavioral characteristics even when the parents are adults and the adoptees are children. Any resemblance between relatives within natural families is a function of both genetic and environmental influences shared by children and their parents. In contrast, adopted children share their genetic makeup with their biological parents, but they share only familial environmental influences with their adoptive relatives. In addition, comparisons between several traits in parents and their children in an adoption design can be used to assess the specificity of genetic and family environmental influences on the development of behavioral characteristics in early childhood. The present report is an extension of a series of previous articles reported from the Colorado Adoption Project (Plomin and DeFries, 1985a), which have focused on general IQ and specific cognitive abilities in infancy (Fulker and DeFries, 1983; Thompson, Plomin and DeFries, 1985) and in early childhood (Plomin and DeFries, 1985b; Rice, Fulker and DeFries, 1986b; DeFries, Plomin, and LaBuda, 1987; LaBuda, DeFries, Fulker and Plomin, 1986). Analyses of simple parent-offspring correlations for specific cognitive abilities at ages 3 and 4 led to an interesting hypothesis (Plomin and DeFries, 1985b): Genetic continuity from early childhood to adulthood is substantial for IQ, whereas specific cognitive abilities do not show this pattern of results independent from their relationship with IQ. For example, general cognitive ability (IQ) of adoptees in early childhood relates to IQ of biological parents, whereas specific cognitive abilities such as spatial ability of the adoptees do not relate to spatial ability of the biological parents, even though reliabilities are similar for general and specific cognitive abilities. This suggests that specific cognitive abilities may not yet be differentiated etiologically in early childhood. In other words, genetic effects that mediate continuity from childhood to adulthood may primarily involve general rather than specific cognitive abilities. Rice et al. (1986b), in a multivariate path analysis of specific cognitive abilities data from 4-yr-olds in the CAP, also found that four measures of specific cognitive abilities did not appear to be highly differentiated genetically-that is, individual differences in these measures appear to be largely influenced by the same genes. In order to test this hypothesis more rigorously, the present study fitted a path model of genetic and environmental transmission to general IQ and specific cognitive ability data, from biological, adoptive and control 391
392
C. S. BERGEMAN et al.
(nonadoptive) parents and their children at ages 3 and 4. This is the first analysis of specific cognitive abilities of 3-yr-olds in the CAP. The path model employed in the present analysis is the univariate adoption model formulated by Fulker and DeFries (1983). but it has been simplified to exclude a manifest variable (I) which was an index of home environment. In addition to providing tests of hereditary and environmental influences, the model includes parameters for passive genotype+nvironment correlation, assortative mating, and selective placement, as discussed later. A complete description of the path diagrams of genetic and environmental transmission in nonadoptive and adoptive families, and the corresponding expectations of variance and covariance can be found in Fulker and DeFries (1983). METHOD Subjects
The Colorado Adoption Project (CAP) is a longitudinal prospective study of genetic and environmental influences on individual differences in behavioral development (Plomin and DeFries. 1985a). Biological and adoptive parents are contacted through two Denver adoption agencies whose professional staff counsel unwed parents on a non-residential basis. Adoptive children are separated from their biological mothers shortly after birth, and are placed in adoptive homes, on average, 4 wk later. Control families participating in the CAP are matched to adoptive families on the basis of sex of proband, number of children in the family, age of father (+ 5 yr), occupational rating of the father, and total years of father’s education (+ 2 yr). The analyses summarized in this report are based upon data from 196 adoptive and 189 control children tested at age 3, and 177 adoptive and 161 control children at age 4, and their biological and adoptive parents. A concerted effort was made to test biological fathers of adoptive children; data from only 42 were available for the present analysis. A more detailed description of this sample and its ascertainment is provided elsewhere (Plomin and DeFries, 1983, 1985a). Measures
CAP parents are administered a 3-hr battery which includes 13 tests of specific cognitive abilities. A principal component analysis of the CAP adult data yields four readily interpretable factorsSpatial Visualization, Verbal Reasoning, Perceptual Speed, and Memory. Loadings on the first principal component (unrotated) are moderately high for all variables, (DeFries, Plomin, Vandenberg and Kuse, 1981). Since the first principal component from a highly similar battery employed in the Hawaii Family Study of Cognition was found to correlate over 0.70 with the WAIS full scale IQ (Kuse, 1977), the first principal component will be used as a measure of general cognitive ability in this report. The median test-retest reliability for the specific cognitive abilities is 0.80 (DeFries et al., 1981). The adopted and control children in the CAP are tested yearly between ages 1 and 4 in their homes. At ages 3 and 4, the Stanford-Binet IQ test and a battery of tests of specific cognitive abilities are administered. In addition, interview data are provided by parents and video recordings of parent-child interactions are obtained. The battery of specific cognitive abilities was designed by CAP staff to measure factors similar to those assessed by the adult battery (spatial, verbal, perceptual speed and visual memory). Descriptions of the test battery used at ages 3 and 4 have been reported by Singer, Corley, Guiffrida and Piomin (1984) and by Rice, Corley, Fulker and Plomin (1986a), respectively. The Stanford Binet IQ was used as a measure of the children’s general cognitive ability. Analysis
The path model for control families is illustrated in Fig. I. The model includes only two latent variables, additive genetic value G and environmental deviation E, in both parental and offspring generations, and these totally determine the phenotype P with paths. h and e, respectively. Although the model appears complex, the basic idea involves the simple point that control parents contribute both heredity and environment to their children. It is assumed that the measures are isomorphic in childhood and adulthood. Environmental transmission is modelled in terms of the impact of the parent’s phenotype on the child’s environment, through path w, (H., from the father,
Specific
cognitive abilities: early childhood Path
Nonadoptive
393
analysis
families
Fig. 1
and w2 from the mother). The complexities on the left side of the path model arise from consideration of assortative mating and genotype-environment correlation. These complex issues are presented in terms of reverse path analysis which is described by Fulker and DeFries (1983). This presentation facilitates the derivation of the three expected correlations among the three manifest variables (Li, 1975). Figure 2 illustrates the path model for adoptive families. Adoptees have one set of parents, PBM and P,,, who contribute genes, and another set, PAM and P,,, who contribute environmental influences. Because assortative mating may differ in wed and unwed couples, different assortative mating parameters, p and q, are included in the model. The extent of selective placement. X. is also determined. The five manifest variables in Fig. 2 yield 10 correlations for which expectations may be derived. The expectations for both the control and adoptive correlations are presented in the article by Fulker and DeFries (1983). The summary statistics to which the model was fitted are variances and covariances calculated separately for adoptive and nonadoptive families. In the case of nonadoptive families, five 3 x 3 matrices are formed, one for each variable analyzed (general, spatial, verbal, memory, and perceptual speed). For adoptive families, in which complete data are available for biological fathers, biological mothers, adoptive fathers, adoptive mothers and adopted children, five 5 x 5 matrices are formed. For those adoptive families in which data on biological fathers is not available, five 4 x 4 matrices were calculated.
Adoptive
families
Fig. 2
C. S. BERciEh4~xet al.
394
Path models were fitted to these observed covariance matrices using a maximum likelihood estimation procedure. The function was minimized using MINUIT (CERN, 1977), a package of optimization and error analysis routines made available by CERN, the European Centre for Nuclear Research (see Fulker and DeFries, 1983). RESULTS
The model was fitted to the data and their log-likelihoods were evaluated using chi-square. The results for general cognitive ability are shown in Tables 1 and 2. The parameter estimate for /I and its corresponding standard error implies that significant genetic continuity exists for IQ from childhood to adulthood. Assortative mating @ and q) is generally significant; selective placement (x) and environmental effect (w) of the parents’ IQ are not significant. However most of the variance remains unexplained. Consistent with previous findings in the CAP, these data suggest that significant genetic continuity exists for IQ from childhood to adulthood. Thus, for IQ, the results indicate the importance of genetic transmission at both 3 and 4 yr-maximum likelihood heritability estimates (h 2) are 0.14 at 3 years and 0.19 at 4 yr. The results of model-fitting for specific cognitive abilities, however, are mixed, as indicated in Tables 1 and 2. Spatial ability shows a pattern of results similar to those for IQ, yielding heritability estimates of 0.12 and 0.19 at 3 and 4 years, respectively. Verbal ability, however, yields evidence of significant influence at 3 but not at 4 years, with heritability estimates of 0.18 and 0.06, respectively. Perceptual speed, on the other hand, shows significant genetic influence at 4 but not at 3-heritability estimates are 0.07 and 0.10, respectively. Memory is not significantly heritable at either 3 or 4. As in the case of IQ, family environmental variance is low and nonsignificant for all measures at 3 and 4 yr. Assortative mating is positive for specific cognitive abilities for both biological parents of adopted children, (q), and adoptive and control parents, (p). Setting these parameter estimates equal in reduced models does not make a significant difference in the fit of the model, except for Table
h S WI P 4 XI x2 *I x4 e WI w F P
I. General
and specific cognitive
Spatial
0.37 + 0.09 0.06 of:0.02 0. I I + 0.06 0.29 + 0.05 0.34 f 0.22 0.00 + 0.06 0.05 * 0.07 0.02kO.13 0.07+0.17 0.91 0.12 0.06 20 25.72
0.35 + 0.16 0.02 + 0.02 0.03 & 0.07 0.09 i: 0.05 0.00 & 0.07 -0.04 * 0.07 0.09 f 0.07 -0.11 * 0.25 0.29*0.16 0.93 0.09 0.02 20 29.98
0.18
0.08
0.42 0.03 0.03 0.34 0.00 0.05 0.03 0.21 -0.02 0.89 0.08 0.02 20 16.06
0. I 1 0.02 0.06 0.04 0.30 0.07 0.07 0.14 0.14
and specific cognitive
Spatial
Verbal
0.44 * 0.09 0.05 * 0.02 0.10 + 0.07 0.23 + 0.05 0.32 f 0.19 0.08 f 0.07 0.12+0.07 -0.00~0.15 0.12 kO.16
0.44*0.12 0.01 + 0.02 -0.05 + 0.07 0.06 f 0.05 0.00 * 0.25 -0.01 f 0.07 0.15+0.07 -0.32 kO.18 0.2OkO.16 0.89 0.10 0.01 22.55 20 0.30
0.24 + 0.18 0.04 * 0.02 0.16f0.06 0.26 + 0.05 0.00 f 0.29 0.07 f 0.07 0.02 + 0.07 0.25 * 0.13 0.09 * 0.14 0.96 0.04 0.06 23.51 20 0.25
0.07 0.05 23.35 20 0.28 estimates
Memory
0.27 50.16 0.02 * 0.01 0.04 f 0.06 0.16kO.05 0.18 f0.13 -0.02 + 0.07 0.00 * 0.08 -0.16f0.12 -0.01 kO.15 0.96 0.05 0.02 20 .a5 21
0.15+0.70 -0.01 * 0.05 0.03 + 0.06 0.11 +0.05 0.16kO.13 -0.07 * 0.07 -0.09 * 0.07 -0.18 f 0.14 0.22 f 0.15 0.99 -0.12 -0.01 20 28.02
0.35 errors.
General
0.88
Parameter
f * + + + f * * _t
for full model with standard
errors.
at age 3 P Speed
0.70
Parameter estimates for full model shown with standard parameters, and standard errors are not given. Table 2. General
abilities
Verbal
General
0.10
E, w,, and the rge correlation
abilities
are derived
at age 4 PSpced
0.31 kO.14 0.01 + 0.01 0.00 f 0.06 0.19f0.05 0.18 kO.13 0.01 * 0.08 -0.03 * 0.09 -0.17kO.12 -0.02+0.15 0.95 0.02 0.01 21.97 20 0.35
Memory 0.01 f 0.02 0.00 + 0.00 0.01 + 0.06 0.11 kO.05 0.26+O.l2 -0.03 f 0.08 -0.05 + 0.07 -0.27 kO.14 0.14+0.15 0.99 -0.07 0.00 26.76 20 0.15
Specific cognitive abilities: early childhood
395
verbal ability. With regard to the selective placement parameters, each is non-significant as expected given the placement practices employed by the adoption agencies that participate in the CAP (Plomin and DeFries, 1985a). CONCLUSIONS Using CAP data, Thompson et al. (1985) found that infant specific cognitive abilities (ages 12 and 24 months) are predictive of parent’s general cognitive ability, but not parent’s specific abilities. That is, infant mental development is undifferentiated in the sense that components of infant mental functioning relate more to adult IQ than to adult specific cognitive abilities, and these infant-toadult relationships are mediated genetically. In early childhood (ages 3 and 4), there appears to be genetic continuity from childhood to adulthood not only for general IQ, but for spatial ability as well. In addition, there is evidence for significant genetic influences for verbal ability at 3 yr of age and for perceptual speed at 4. However, visual memory continues to show little genetic influence. From an environmental perspective, the results imply that the aspects of the family environment relevant to cognitive development are not “fine-tuned” according to parental specific cognitive abilities. Although most of the variance still remains unexplained, finding significant genetic influence for IQ and for specific cognitive abilities in early childhood is exciting, because it suggests genetic continuity from childhood to adulthood, as well as some possible differentiation of specific abilities. This pattern of results leads us to predict that genetic differentiation of specific cognitive abilities increases during middle childhood, when children embark upon formal schooling, in other words, we would expect to find greater evidence for parent-offspring resemblance for isomorphic cognitive abilities. Because children in the CAP are tested again at age 7 on a similar battery of tests of specific cognitive abilities, we will be able to test this hypothesis in future years as the sample of children tested at 7 becomes large enough for analysis. Acknowledgemenrs-This research was supported in part by grants from the National Institute of Child Health and Human Development (HD-10333 and HD-18426). the National Science Foundation (BSN-8200310 and BSN-8505692). and the Spencer Foundation. We are grateful to the families participating in the CAP, who have so generously contributed their time and effort, and to the adoption agencies who make the study possible-Lutheran Social Services of Colorado, and Denver Catholic Community Services.
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