A parent-offspring adoption study of cognitive abilities in early childhood

INTELLIGENCE 9, 341-356 (1985)

A Parent-Offspring Adoption Study of Cognitive Abilities in Early Childhood ROBERT PLOMIN AND J. C. DEFRIES

Umverstty of Colorado, Boulder

Do cognmve abdlttes m early childhood relate genetically and/or environmentally to Isomorphic abdmes in adulthood') Are specific cognmve abdmes differentiated m early childhood m terms of their predicuon of adult cognmve abdmes '~ The present study, the first behavioral genetic analysis of speofic cogmttve abthtles in early chddhood, explored these questions using parent-offspnng data for 186 adopted children and 151 nonadopted chddren tested in the longitudinal Colorado Adoption Project at 3 years of age and 162 adopted children and 138 nonadopted children tested at 4 years The chddren's Stanford-Bmet IQ and scores for four specific cognmve abihtles (verbal, spatial, perceptual speed, and memory) were correlated w~th corresponding measures for their parents--the biological and adoptwe parents of the adopted children and the natural parents of the nonadopted chddren Slgmficant correlations were found between biological mothers' IQ and the IQ of their adoptedaway offsprmg at 3 and 4 years of age, suggesting genetic influence for IQ However, specific cogmtwe abdmes yielded no slgmficant correlations between biological mothers and their adopted-away offspnng These results suggest that substantial genetic contmmty exists for IQ from early chddhood to adulthood, but not for speofic cognmve abdmes

Although It is widely acknowledged that individual differences an cognitive development revolve m u c h more than IQ, little is k n o w n about the development of specific cognitive ab!htles, especially in early childhood, that is, d u n n g the preschool years following infancy. Etiologies, developmental course, and longterm predlctiveness could well differ for specific cognitive abilities such as verbal, spatml, perceptual speed, and memory. The purpose o f the present study was to analyze data on c o g n m v e abllmes in early childhood from the Colorado Adoption Project (CAP), using a p a r e n t - o f f s p r i n g design to compare general and spec|fic cognitive a b i l m e s m adopted and nonadopted 3- and 4-year-olds and their biological, adoptive, and nonadoptive parents. The present study thus extends into early childhood two previous C A P articles in this journal that described p a r e n t - o f f s p r i n g resemblance in infancy: One reported resemblance between parental IQ and 12-month-old Bayley scores (DeFrles, Plomtn, Th~s work was supported m part by grants from the Nauonal Insutute of Chdd Health and Human Development (HD-10333) and the National Science Foundation (BNS-7826204 and BNS-8200310) Correspondence and requests for repnnts should be sent to Robert Plomm. Instatute for Behavioral Genetics, Campus Box 447, Umverstty of Colorado, Boulder, CO 80309

341

342

PLOMIN AND DEFRIES

Vandenberg, & Kuse, 1981). and another explored parent-offspnng resemblance for parental cognitlve abilities and item clusters from the Bayley tests at 12 and 24 months (Thompson, Plomin, & DeFiles, 1985). From a genetic perspective, this parent-offspring design is like an "instant" longitudinal study from childhood to adulthood. Because the biological parents have been studied as adults and their adopted-away offspring have been studied as children, resemblance between cognmve abilities of biological parents and their adopted-away children IS due not only to the 50% genetic similarity shared by first-degree relatives, but also to genetic continmty between childhood and adulthood. That Is, even if heredity substantially affects individual differences in cognitive abilities in chddhood and in adulthood (as determmed, for example, by a twin study comparing identical and fraternal twins), no resemblance between biological parents and their adopted-away children would be observed unless the genetic effects on childhood cognitive abdmes were correlated w~th the genetic effects on cognitive abihties in adulthood. Thus, the CAP parent-offspring design assesses genetic contInmty from chddhood to adulthood as well as genetic variance in chddhood and adulthood. This feature of the design facilitates exploration of the question whether general or specific cogmtive abdmes m chddhood relate genetically to isomorphic abihtles in adulthood. The design is also useful for explonng the differentiation of specific cognitwe abdities by asking, for example, whether children's spatial ability relates more strongly to parental spatial ability than ~t does to parental IQ. These two questions are the focus of the present study. For general cogniuve ability (g or IQ), a cnucal change from Infancy to early childhood is its increasing abdity to predict adult IQ. For example, In the Berkeley Guidance Study (Honzik, MacFarlane, & Allen, 1948), scores on the first testing at I year 9 months on the Cahfornla Preschool Schedule correlate only .07 with Wechsler-Bellevue IQ scores at 18 years; 3-year-old scores correlate .35 with IQ scores at 18 years and 4-year-old scores correlate .42 In the Berkeley Growth Study, Bayley (1954) reported even greater longitudinal correlations: .49 from 3 to 18 years and .66 from 4 to 18 years. One possible explanation of the increasing long-term predictiveness of IQ scores in early childhood revolves genetics. From a genetic perspective, the relationship between childhood and adulthood ts a function of genetic variance in childhood, genetic variance m adulthood, and the genetic correlation between chddhood and adulthood (Plomin & DeFiles, 1981). Genetw correlation refers to the extent to which genetic deviations that affect IQ in childhood covary with genetic deviations that affect adult IQ. Increasing predictiveness of adult IQ from childhood IQ as compared to infancy IQ could be due to greater genetic varmnce in early childhood than in infancy or it could be due to greater genetic covarmnce between early childhood and adulthood as compared to genetic covarmnce between infancy and adulthood. It should be noted that this discussion pertains only to IQ

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as it is measured, which usually means the Stanford-Binet Intelligence Scale (Terman & Merrill, 1972) in early childhood. In other words, this approach does not address the issue of the processes by which genetic devmtlons that affect IQ in childhood may or may not covary with genetic deviations that affect adult IQ; for example, genetic discontinuity may be due to lack of overlap in the content of child and adult IQ tests. Rather than asking why genetic variance or covanance exists, we ask the prior question: To what extent do they exist? Some behavioral genetic research on IQ suggests that genetic vanance increases from infancy to early childhood. The longitudinal Louisville Twin Study (Wilson, 1983) yields Identical and fraternal twin correlations of .88 and .79, respectively, at 3 years and .83 and .71 at 4 years. These twin correlations suggest that genetic variance accounts for approximately 20% of the variance of IQ scores at 3 years and 25% of the variance at 4 years. In contrast, about 15% of the vanance in infancy Is heritable; by the early school years, genetic variance accounts for about 50% of the variance of IQ scores in the Louisville Twin Study. No other twin studies of IQ have been reported for 3- and 4-year-olds. Results similar to those of the Louisville Twin Study were found in the classic adoption study of Skodak and Skeels (1949). The IQ correlation between biological mothers and their adopted-away children at the average age of 4 years is .28; the correlation is .00 in infancy and .35 in the early school years. Although selective placement inflates these correlations--for example, educational level of the adoptive parents and biological parents correlates .25 in the Skodak and Skeels' study, the results are consistent with those of the Louisville Twin Study in pointing to greater genetic influence on IQ scores in early childhood than in infancy. Two other adoption studies of IQ in early childhood are relevant. An early study (Snygg, 1938) reported an IQ correlation o f . 15 for biological mothers and their adopted-away offspring at 3 years and a correlation of .05 for adoptees at 4 years. However, it is likely that restriction of range curtailed the mother-child correlations In this study because IQ tests were administered primarily to mothers who had not passed high school entrance exmainatlons; the average IQ of the biological mothers was only 78 (no information concerning variance was reported). The second study did not include data on biological mothers but rather compared mother-child correlations in adoptive and nonadoptlve homes when the children were 4 years old (Flsch, Bilek, Deinard, & Chang, 1976). The IQ correlation between adoptive mothers and their adopted children is .07, whereas in nonadoptive famihes, m which parents share heredity as well as family environment with their children, the mother-child correlation Is .35. Parent-offspring correlations for IQ m nonadoptlve families were reported for the Fels study (McCall, Appelbaum, & Hogarty, 1973): The mean parent-offspring correlations were .21 for children 28-33 months of age in 40 families and .54 for children from 40-45 months in 71 families. The difference in these correlations

344

PLOMINAND DEFRIES

suggests substantml genetic influence Other adoption studies of IQ involved older chddren (Burks, 1928; Freeman, Holzmger, & Mitchell, 1928; Horn, Loehlin, & Wdlerman, 1979; Leahy, 1935; Scarr & Wemberg, 1977. 1978). Although much remains to be learned about individual differences on IQ tests in chddhood, the domain of specific cognitive abdmes is altogether uncharted. Relatively independent specific cogmtive abdlties such as verbal, spatial, and memory can be ~solated early in life--perhaps in infancy (Lewis, 1983; Stott & Ball, 1965; Thompson et al., 1985), probably by 3 years of age (Bayley, 1970; Hurst, 1960; Singer, Corley, Guiffrlda, & Plomin, 1984), and certainly by 4 years (Meyers, Dingham, Orpet, Sitkel, & Watts, 1964; R~ce, Corley, Fulker, & Plomin, 1985). Although studies have occasionally attempted to predict later IQ from specific scales and items in early chddhood (e.g., Bradway & Thompson, 1962; Moore, 1967), little is known about the long-term longitudinal stabdlty of specific cognitive abilities per se from early chddhood; more systematic research on this topic has been reported in infancy than in early childhood (e.g., Plomm & DeFries, 1985). The etiology of individual differences in specific cognmve abilmes in early childhood has scarcely been addressed, although two twin studies have included some relevant data The Louisville Twin Study (Wilson, 1975) reported twin correlations separately for verbal and performance IQ scores for 4-year-olds. The correlations for verbal IQ suggested little genetic influence: The identical twin correlation is .76 and the fraternal twin correlation Is .73. For performance IQ, the twin correlations are .76 and .53, respectively, suggesting substantml genetic influence. However, the only other twin study of specific cogmtive abdities in early childhood yields conflicting results (Mittler, 1969). For a sample of 28 pairs of identical twins and 64 pairs of fraternal twins tested at 4 years of age on subtests of the Illinois Test of Psychohnguistlc Abdit~es (ITPA), twm correlations suggested greater genetic influence on verbal abihties than on memory. Five verbal ITPA subtests (auditory and visual reception, auditory and visual association, and verbal expression) yield an average ldentlcal twm correlation of .70 and a fraternal twin correlation of .44; two memory ITPA subtests (auditory and visual sequences) yield average twin correlations of .41 for identical twins and .49 for fraternal twins. Other twin studies of specific cogmtive abilities involve older children (Foch & Plomm, 1980; Munsmger & Douglass, 1976; Plomin & Vandenberg, 1980; Segal, in press). There are no previous adoption studies of specific cognitive abilities in early childhood. The longitudinal Colorado Adoption Project (CAP) has applied its parentoffspring design to the study of general cognitive abdlty (DeFnes et al., 1981) and specific cogmtive abilities (Thompson et al., 1985) in infancy A general hypothesis emerged from these analyses of CAP infancy data: Insofar as components of infant mental development predict adult cognitive abihties, the infant measures primarily predict adult general cognitive ability, not adult specific cognitive abilities (Thompson et al., 1985). For example, infant Bayley factor

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scores and measures of infant communicative competence correlate primarily with parental g. not with parental specific cognitive abdmes, even though parental factor scores for specific cognitive abilities are as reliable as the measure of general cognitive ability. Moreover, this relationship between infant specific cognitive abilities and adult general cognitive ability is genetically mediated to some extent because the same pattern of parent-offspring correlations occurs for adopted infants and their biological mothers. This finding implies that genetic factors that affect infant cognitive functioning also affect adult IQ, but not adult specific cognitive abilities. Thus, CAP analyses in infancy suggest that 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 that these mfant-toadult relationships are in part mediated genetically. For early childhood, it would not be surprising if a different pattern of results emerged. Specifically, it would seem reasonable to expect greater genetic differentiation of specific cognitive abilities In early childhood than in Infancy. In the CAP, this would be seen as correlations between children's specific cognitive abilities and isomorphic cognitwe abilities in their parents. Although this introduction has emphasized genetic effects on cognitive abdlties in early childhood, it should be noted that the adoption design provides novel estimates of environmental influence as well. The CAP design includes three types of parents and children: pairs who share both heredity and family environment, pairs who share heredity alone, and pairs who share family environment alone. Resemblance for the latter group, adoptive parents and their adopted children, directly estimates the contribution of family environment to cognitive abilities in early childhood, at least those aspects of the family environment related to parental cognitive abilities. Furthermore, comparisons between different abilities in adoptive parents and their adopted children can be used to assess the specificity of these family environmental influences on the development of cognitive abilities in early childhood METHOD

Subjects The Colorado Adoption Project (CAP) Is a prospective, longitudinal adoption study in which adopted and matched nonadopted children are assessed at 1, 2, 3, and 4 years of age during visits to the adoptive and nonadoptive homes. The biological and adoptive parents of the adopted children and the parents of the nonadopted children are tested on a 3-hour battery of diverse measures that includes assessments of cognitive abilities. The present study is based on 186 adopted children and 151 nonadopted children tested at 3 years of age and 162 adopted and 138 nonadopted children tested at 4 years of age as well as the biological, adoptive, and nonadoptive parents of these children.

346

PLOMIN AND DEFRIES

The CAP sample is described in detail m a book presenting the CAP infancy results (Plomin & DeFrles, 1985). Importantly, the CAP sample is qmte representative of metropolitan Caucasian famlhes in the U.S. For socioeconomic status, the sample is representatwe of the entire U.S. population for variance, although its mean Is about one standard deviation above the national average. Selective placement--the matching of adoptive parents to biological parents--is negligible for all traits including cognitive abilities and demographic characteristics such as socioeconomic status and education.

Measures Test batteries to assess verbal, spatial, perceptual speed, and memory factors were developed for 3-year-olds (Stager et al., 1984) and for 4-year-olds (Rice et al., 1985). The verbal tests include a picture vocabulary test and a test of word fluency (for example, thinking of things that make noise); the spatial tests include a simplified version of a block design test and a figure-ground test in which the child is asked to point to ammals embedded in a complex background; and the perceptual speed tests involve a form-discrimination test m which the child points to the picture in a row of four that is identical to the stimulus ~tem at the left end of the row and a test in which the child finds a standard stimulus (a cow) m rows of hne drawings of five ammals. At 3 years, a recognition memory test was employed in which children were asked to remember line drawings of familiar objects. (An associative memory task developed by Stager et al. was not used m the present study.) At 4 years, immediate and delayed versions of the recognition memory test was used. In addmon, an associative memory task was employed at 4 years in which the child is shown pairs of ammals and then asked to recall the other member of the pair when only one member is displayed. Each battery requires 30 to 40 mm to administer. For both test batteries, factor analyses supported the a priori placement of the tests on four factors of verbal, spatial, perceptual speed, and memory The tests of each ability were z-scored for the entire sample and summed at each age to produce scale scores assessing the specific cognitive abilities of verbal, spatial, perceptual speed, and memory The four scales are not orthogonal: In the present sample, the average intercorrelatlon among the four scales IS .23 for the 3-year battery and .33 for the 4-year battery These mtercorrelatlons are lower than those found for the McCarthy Scales of Children's Abilities (McCarthy, 1 9 7 2 ) the average mtercorrelation among the verbal, perceptual, quantitatwe, and memory scales of the McCarthy test is .71 at 3 years and .64 at 4 years. Test-retest rehabdities for the verbal, spatial, perceptual speed, and memory scales are .75, .57, .70, and .57, respectively, at 3 years as derived from the test-retest correlattons for the individual tests (Singer et al., 1984) and the test intercorrelatlons; at 4 years, the test-retest rehablhties are 77, 78, .81, and 56 (Rice et al., 1985). These test-retest reliabflmes are comparable to the rehabilities of subscores of IQ tests m early childhood--the average reliability of

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the Stanford-Binet subscores is 60 (Terman & Merrill, 1972) and for the lVlcCarthy scales, the average rehabihty ~s .81 (McCarthy, 1972). IQ was assessed at 3 and 4 years using the Stanford-Binet Intelligence Scale Form L-M (Tennan & Memll, 1972). The Stanford-Binet manual reports testretest reliabiliues of .83 to .91 for different IQ levels for children from 2 to 5 years of age. Thus, the reliabilittes of the brief measures of specific cognmve abdiues are only slightly lower than those of Stanford-Bmet IQ. One of the reasons for focusmg on these four spectfic cognittve abdiues m young chddren is that the parents of the CAP chddren have been admimstered a battery of 13 cognitive tests that tap simdar cognmve abdmes. Examples of 1terns for each test are avadable elsewhere (Plomm & DeFnes, 1985) and the reliabdities, sex and age adjustment, and the factor structure have been pubhshed previously by DeFnes et al. (1981). The average internal consistency of the tests is .85 and their average test-retest rehability ts .76. The b~olog~cal, adoptive, and nonadoptive parents do not differ in variance for the cogniuve tests. Age ts s~gnificantly related to cognitive scores, however, and ts confounded w~th parental type because the b~ologtcal parents are about 10 years younger than the other parents. For th~s reason, the cognmve scores were adjusted for age, age squared, and sex separately for each group of parents. The resultmg standard scores thus do not differ m terms of means or variances for the three types of parents The pnnclpal component loadings for the theee types of parents are wrtually idenucal and suggest factors of verbal ability (highest-loading tests, vocabulary and two verbal fluency measures); spatial ability (paper form board, card rotauons, hidden patterns, and ~denttcal p~ctures); perceptual speed (s~mple subtraction and multiphcat~on and letter series d~scriminat~on), and memory (figure recognihon and assocmtive memory revolving names and faces). Rotated principal component scores for these four dimensions are used as measures of specific cognmve abilihes, and component scores on the first unrotated pnnc~pal component (which accounts for 36% of the total variance) are used as a measure of general cognmve abdity or IQ. For both parents and children, mdxvldual scores three standard devmtions or more above or below the mean were ehmmated from analyses to ensure that the results would not be affected by rare outhers. Indwlduals who missed more than two tests were excluded (5% of the sample); the group mean was substituted for missing scores when one or two tests were missing (2% of the sample).

RESULTS

Preliminary Analyses Assortative mating and selective placement can affect esttmates of quant~tatwe genetic parameters in adoption studies (Plomm, DeFnes, & McClearn, 1980). Assortative mating inflates parent-offspring correlations and thus overestimates

348

PLOMIN AND DEFRIES

family environmental influence based on correlations between adoptive parents and adopted children as well as genetic influence based on correlations between biological parents and their adopted-away offspring (DeFiles & Plomin, 1978). Age-adjusted assortative mating correlations for IQ a r e . 23 for nonadoptlve parents and .29 for adoptive parents. For specific cognitive abilities, assortative mating correlations are highest for verbal ability (.27 and .36) and lower for spatial ability (.04 and .07), perceptual speed (. 15 and .09), and memory (.22 and .05). Only 48 of the biological fathers were tested; however, for this small sample of biological couples, the magnitude of assortative mating is similar to that of nonadoptive and adoptive couples. Although the effects of selective placement can be taken into account m analyses of adoption data, when adoptive parents resemble biological parents, the clean separation of genetic and environmental mfluences is attenuated. Fortunately, selective placement ts neghgtble in the CAP: Biological mothers' IQ correlates .04 with the adoptive mothers' IQ and - . 0 3 with the adoptive fathers' IQ. Selective placement is also negligible for specific cognitive abilities. Biological mothers' factor scores on verbal, spatial, perceptual speed, and memory correlate .05, .07, - . 0 4 , and - . 10, respecttvely, wtth adoptive mothers' factor scores and . 10, - . 0 8 , - . 0 4 , and .00 with adoptive fathers' factor scores

Descriptive Statistics Table 1 lists means and standard deviations tor cognitive measures for adopted and nor.adopted children at 3 and 4 years. At 3 and 4, variances of the children's Stanford-Blnet IQ scores are less than the standardization norms; the CAP means TABLE 1 Means and Standard Devlanonsfor Cogmtlve Measures at 3 and 4 Years of Age" Adopted Chddren Measure 3-Year Stanford-Bmet Verbal Spatial Perceptual Speed Memory 4-Year Stanford-Bmet Verbal Spatial Perceptual Speed Memory

Nonadopted Children

M

SD

N

M

SD

N

103.9 - 11 - 16 - 06 - 17

13.8 15 14 15 09

186 182 181 177 179

107 4 15 24 14 23

14 1 1.6 15 16 l0

151 147 148 148 147

106 4 - 14 - 08 - 03 - 21

Il 7 21 17 17 18

162 159 159 155 157

109 8 17 06 .09 25

11 5 l9 17 17 17

134 138 138 138 136

"Each speofic cognitive ablhty score is the sum of individualtests, z-scored for the adopted and nonadopted children combined

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are slightly higher than the norms. At both ages, the variances for adopted and nonadopted children are quite slmdar; the means for adopted chddren are shghtly but significantly lower than the means for nonadopted children--the difference amounts to about one fifth of a standard deviation, F(1,314) --- 4.22, p < .05 at 3 years, and F(1,278) = 7.24, p < .01 at 4 years. No mean IQ differences emerged for boys and girls. The measures of specific cogmtlve abdtties also show reasonable vanabd~ty at both 3 and 4 years of age. Vartances for adopted and nonadopted chddren are s~mdar. Nonadopted chddren tend to score shghtly higher than adopted children, on average, the difference amounts to about one fifth of a standard dewauon. At 3 years, the difference is significant for the spatial scale, F(1,314) = 5.16, p < .05, and memory, F(I,314) = 12.27, p < .01, but only for the memory scale at 4 years, F(1,278) = 7.43, p < .01. Significant mean differences for boys and girls occurred for only 1 of the 16 comparisons between adopted and nonadopted boys and girls on the four specific cogmttve abilities at 3 and 4 years: Girls scored slgmficantly higher (about one quarter of a standard deviation) than boys on the perceptual speed scale at 4 years, F(1,278) = 4.65, p < .05.

Parent-Offspring Correlations for IQ Table 2 hsts parent-offspnng correlations for IQ at 3 and 4 years At both ages, the biological mothers' IQ scores correlate slgmficantly with their adopted-away offspring, suggesting sigmficant genetic influence. Although the biological mother-adoptee correlations o f . 18 and .22 at 3 and 4 years, respectwely, are only modest m absolute magmtude, if taken at face value, they imply substantml genetic influence on the relationship between IQ in early chxldhood and in adulthood, as explained in the d~scuss~on section. The s~gnlficant correlations for adoptive parents and their adopted chtldren imply s~gmficant effects of the family environment as reflected in parental IQ. Parent-offspring correlations in nonadoptlve homes should be approximately the sum of the "heredity" correlation of biological parents and their adoptedaway offspring and the "famdy environment" correlation of adopuve parents and their adopted chddren. However, although the nonadoptwe mother-child TABLE 2 Parent-Offspnng Correlations for IQ BIological

3-Year Stanford-BmetIQ N 4-Year Stanford-BmetIQ N *p < 05

Adoptive

Nonadoptlve

Mother

Father

Mother

Father

Mother

Father

18" 183 .22* 160

18 45 41" 36

14" 184 18" 161

15" 179 12 157

16" 146 21" 129

11 148 09 131

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PLOMIN AND DEFRIES

correlations are significant, the magnitude of the nonadoptlve parent-offspnng correlations IS lower than expected on the basis of the data for the other parentoffspnng comparisons. Such a finding would be expected if selective placement were important because selective placement would inflate parent-child correlations for both biological and adoptive relationships; however, selective placement is negligible in the CAP. Other possibilities are considerably more speculative For example, it is possible that adoptive parents have greater environmental impact on their adopted children than do nonadoptwe parents on their children, however, comparisons of environmental assessments in adoptive and nonadoptlve famihes in the CAP yield few differences (Plomln & DeFnes, 1985). Moreover, the parent-offspnng IQ correlation in nonadoptlve families is lower than that observed in the two other studies of nonadoptlve parents and their offspring m early childhood (Flschet al., 1976; McCall et al., 1973). We thus ascribe the lower-than-expected parent-offspring correlations in the nonadoptive families to chance; this concluston Is buttressed by model-fitting analyses of these IQ data (Rice, Fulker, & DeFrles, in press) In summary, these results suggest some resemblance between parents and offspnng for IQ. The significant parent-child correlations for biological relationships and for adoptive relationships indicate that both heredity and family environment play a role in the development of IQ in early childhood. As discussed earlier, finding genetic influence using the parent-offspring design of the CAP imphes not only genetic influence on IQ in childhood and adulthood, but also genetic continuity between childhood and adulthood

Parent-Offspring Correlations for Specific Cognitive Abilities Isomorphic parent-offspnng correlations for specific cognitive abllmes--for example, correlations between parents' verbal scores and children's verbal scores--are presented in Table 3. In general, none of the specific cognitive abilities shows a pattern of results as consistent as the results for IQ For biological mothers and their adopted-away offspnng--a direct estimate of genetic influence--the mean parent-offspnng correlations are .08 at 3 years and .03 at 4 years. Lower reliabilitles for the measures of specific cognitive abilities than for IQ could In part explain the difference in parent-offspring resemblance for IQ and specific cognitive abxlmes although, as indicated earlier, the measures of speofic cogmtive abilities are only slightly less rehable than IQ. Nonetheless, it should be pointed out that, although the parent-offspring correlations are significant at both 3 and 4 years for IQ of biological mother and their adopted-away children and none is sigmficant for specific cognitive abdltles, these parentoffspring correlations are not significantly different The slgmficant parent-offspring correlations in the nonadopttve families indicate some familial resemblance for verbal, spatial, and perceptual speed at both ages. Familial resemblance must be medmted either genetically or environmentally; however, few sigmficant correlations emerge for biological parents or for adoptive parents. Nonetheless, for verbal ability at 3 and 4, the positive correla-

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TABLE 3 Parent-Offspnng Correlationsfor Specific Cogmttve Abdmes Blological

3-Year-Olds Verbal Spatial Perceptual Speed Memory N 4-Year-Olds Verbal Spatml Perceptual Speed Memory N

Adoptive

Nonadoptwe

Mother

Father

Mother

Father

Mother

Father

12 11 O0 10 177180

02 12 - 09 27* 4243

07 - 05 05 07 174-

09 15" 12 - 07 174-

16* 14' 14" 11 142-

21* 15" 08 - 06 144-

180

177

144

145

08 09 - 04 - 01 155159

07 - 24 14 - 16 3536

11 - 01 06 10 154158

16* 06 04 - 05 152155

22* 08 15" 00 131134

04 21" 06 - 11 133135

*p < 05 tlons for biological mothers and for adoptive parents suggest a trace of genetic and family environmental influence, although both are weaker than for IQ. For spatial ability, the picture is even less clear, although biological mother correlations are positive. For perceptual speed, no evidence of genetic influence emerges from the biological mother correlations However, for both spatial and perceptual speed, the average nonadoptive parent correlation is greater than the average adoptive parent correlation, leaving open the possiblhty of some shght genetic influence. In summary, isomorphic comparisons between specific cognitive abilities in parents and their offspring suggest that specific cognitive abilities at 3 and 4 years of age, with the possible exception of verbal ability, show little influence of family environment or of heredity. From the environmental perspective, this tmplies that aspects of the family environment relevant to cognitive development are not fine-tuned according to parental specific cognitive abilities. For example, parents with good memory skills do not transmit those skills to their children environmentally even though parental IQ relates environmentally to children's IQ. From the genetic perspective, these results suggest little genetic continuity from childhood to adulthood for specific cognitive abilities.

Parent-Offspring Cross-Correlations Between IQ and Specific Cognitive Abilities If specific cognitive abilities are undifferentiated in childhood in terms of their prediction of adult cogmtlve abthttes, specific cognitive abilities in childhood would be expected to correlate as much with parental IQ as they correlate with

352

PLOMIN AND DEFRIES

isomorphtc speofic cognitive abthties of the parents. Table 4 lists parent-offspring cross-correlattons between children's specific cognitive abllmes and parental IQ which, when compared to the results in Table 3, support this hypothesis. On the average, chtldren's spectfic cognitive abilities correlate as much with parental IQ as they correlate with isomorphic parental specific cognitive abilities. The average correlation between children's specific cognitive abilities and nonadoptive mothers' IQ (Table 4) is . 13; the average nonadoptive motherchild correlation for isomorphtc specific cognitive abihttes (Table 3) is also . l 3 For nonadoptlve fathers, the corresponding average correlations are . 11 and .07. For biological mothers, these average correlations are .09 and .06. More detailed comparison of Tables 3 and 4 suggests that verbal ability m childhood might be related slightly more strongly to parental verbal abihty than to parental IQ. On the other hand, childhood memory, which shows virtually no relationship to parental memory (Table 3), shows some genetic relat~onshtp to adult IQ (Table 4). In general, when children's spectflc cogmtlve abilities correlate wtth parents' cognitive abilities, they correlate as much with parents' general cognitive abihty as they do with lsomorphtc specific cognitive abilities of the parents. These results are consistent with the hypothesis that specific cognttive abthtles in childhood--with the possible exception of verbal ability--are not yet differentiated. DISCUSSION The present study addressed two major questions concerning cognitive abthtIes in early chllhood: Do cognitive abilities in early childhood relate genetically to TABLE 4 Parent-Offspnng Correlations Between Children's Specific Cognitive Abilities and Parents' IQ Biological Mother 3-Year-Olds Verbal Spatial Perceptual Speed Memory N 4-Year-Olds Verbal Spatial Perceptual Speed Memory N

*p < 05

Father

01 17" 00 16* 175179 06 12 07 12 153157

Adoptive

-

-

Nonadoptive

Mother

Father

Mother

Father

12 24 15 14 4243

12 03 06 02 175180

05 05 00 04 170175

12 14" 09 12 142143

10 12 16" 12 144145

08 05 31" 22 3536

07 02 - 01 03 155158

II - 02 - Ol 04 151154

17" 16" 10 17" 131133

13 19" 06 01 133135

-

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isomorphic abilities in adulthood? Are specific cognitive ablhtles differentiated in early childhood in terms of their prediction of adult cognitive abilities? The results suggest that the answer to the first question is yes for IQ, possibly yes for verbal ability, and probably no for spatial, perceptual speed, and memory. The general answer to the second question is no, again with the possible exception of verbal ability. As indicated earlier, the parent-offspring adoption design of the Colorado Adoption Project (CAP) is hmited In terms of finding genetic influence for cognitive abilities In childhood because significant resemblance between biological parents and their adopted-away children has three prerequisites. The cognitive abilities must be heritable in childhood and in adulthood and genetic continuity must exist between childhood and adulthood. However, when sigmficant parent--offspring resemblance is observed, these limitations metamorphose into an advantage because they enable us to conclude that both the child and adult cognitive abilities are hentable and that genetic continuity exists between childhood and adulthood. Viewed in this light, the significant parent-offspring IQ correlations found for biological mothers and their adopted-away children at 3 and 4 are exciting, these results suggest that some genetic continuity exists for IQ from childhood to adulthood. The IQ correlations between biological mothers and their adopted-away offspnng in early childhood are lower than those found In studies of older children and adolescents (reviewed by Plomin, 1985), which suggests that genetic influences on IQ contmue to increase after early childhood. Although the parent-offspring correlations in the present study are low, the significant IQ correlations between mothe,s and their adopted-away offspring suggest significant genetic continuity How much continuity? The expectation for the correlation between biological parents and their adopted-away children is half the product of the square root of childhood hentabdlty, the square root of adulthood hentabihty, and the genetic correlation between childhood and adulthood (for details, see Plomin & DeFrles, 1985) The results of the Louisvllle Twin Study (Wilson, 1983), reviewed earlier, suggest a herltabihty of about .20 at 3 years and .25 at 4 years. Adult IQ data suggest a hentablhty of about .50 (Plomln & DeFrles, 1980). If we accept these figures for hentabdlty and solve for the genetic correlation, the biological mother-adopted child correlations of . 18 at 3 years and .22 at 4 years yield genetic correlations of unity at 3 and 4 years In other words, even though less of the IQ variance in childhood is due to genetic variance than in adulthood, the genetic variance that affects childhood IQ covanes completely with IQ-relevant genetic variance in adulthood. These IQ results support an amplification model of developmental genetics which posits that any genetic variance that exists early in development continues to affect traits later m development (Plomxn & DeFnes, 1985) Another Implication of these results is that, by Itself, genetic continuity from childhood to adulthood for IQ should lead to a stability correlation of about .35 and .45, respectively, at 3 and 4 years and adulthood (about twice the correlation between the biological mothers and their offspring) As indicated m the lntroduc-

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tlon, longitudinal studies of IQ from childhood to adulthood yield stability correlations of about .40 from 3 years of age and about .50 from 4 years of age. This suggests that most of the phenotypic stabihty observed for IQ from childhood to adulthood is mediated genetically. The finding that specific cognitive abihties in early childhood show httle genetic continuity with isomorphic cognitive abilities in adulthood suggests that if genetic variance accounts for some of the rehable vananc, ~ on tests of specific cognitive abilities in early childhood, this genetic variance uoes not covary with genetic variance that affects specific cogmtive abilities m adulthood. The adopted and nonadopted children in the Colorado Adoption Project are being tested again at 7 years of age m order to determine whether specific cognitive abilities show greater differentiation during middle childhood. Limitations of the present study should be acknowledged One hmltatlon revolves the sample. Although the sample is reasonably representative of U.S. famlhes in terms of demographic characteristics (especially for variance, which is most critical in quantitative genetic analyses), the sample consists primarily of middle-class Caucasmn famlhes. Other populations could yield different results. Another limitation is the s~ze of the sample. Although the sample ~s relatively large, it nonetheless hmits the analyses to detecting only major genetic and environmental effects. For example, the sample of about 170 b~ological mothers yields 84% power (p < .05, one-tailed) to detect parent-offspring correlations of .20 (Cohen-, 1977), the magnitude of the parent-offspring correlation for IQ. However, the sample prowdes only 37% power to detect parent-offspring correlations o f . 10, meaning that more than half the t~me we wdl fail to detect correlations of that magnitude. The power of such analyses is improved when maximum-hkelihood, model-fitting approaches are used. Model-fitting approaches are particularly useful because they analyze all of the data simultaneously, they make assumptions explicit, and they permit tests of the relative fit of different models. The basic model for analysis of the CAP data is described by Fulker and DeFries (1983). A longitudinal extensmn of this model has been apphed to the CAP IQ data and, as m the present study, the results indicate slgmficant genetic influence on IQ at both 3 and 4 years of age (LaBuda, DeFries, Fulker, & Plomin, 1985). A multwariate model-fitting analysis has also been applied to the CAP specific cognitive ability data in early childhood and it also yields conclusions similar to those reported herein (Rice et al., m press). Although the model-fitting analyses found some evidence for genetic influence for specific cognitive abflmes, the major conclusion was that a general factor of genetic influence pervades the relationship between specific cogmtlve abilities m parents and their offspring In early childhood. As always m the behavioral sciences, the major limitation involves measurement. Although the batteries of specific cognmve abilities tests constructed for the CAP represent a successful first attempt to assess verbal, spatial, perceptual speed, and memory abilities in 3- and 4-year-old children, these are outcome-

AN ADOPTION STUDY IN EARLY CHILDHOOD oriented measures

355

selected for no particular theoreucal grounds other than the

psychometric jUStlficatton that they reflect major categories of tradltmnal cognitive tests. Process-oriented, theoretically derived measures of cognition--such as t h o s e s o u g h t b y i n f o r m a t i o n - p r o c e s s i n g

researchers (e.g., Sternberg, 1985)-

represent an important direction for future research on cognitive abilities m early childhood.

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