Psychological Bulletin Copyright 1990 by the American Psychological Association, Inc. 1990. Vol. 107, No. 2, 226-237 0033-2909/90/S00.75 Growing Up and Growing Apart: A Developmental Meta-Analysis of Twin Studies Kathleen McCartney Monica J. Harris University of New Hampshire University of Kentucky Frank Bernieri Oregon State University Developmental change in twin similarity was examined with age contrasts in a meta-analysis of twin studies from 1967 through 1985. Intraclass re were coded from 103 papers that included data for monozygotic or dizygotic twins, or for both, on personality or intelligence variables. Analyses indi- cated that there was a general tendency for some intraclass rs to decrease with age. In other words, as twins grow up, they grow apart. There were also developmental differences associated with compo- nents of variance for heritability, the shared environment, and the nonshared environment. Mecha- nisms through which the nonshared environment may operate are discussed. The purpose of this article is twofold. First, we offer meta- meta-analysis include (a) obtaining a combined estimate from analysis as a tool for developmental hypothesis testing. The field all available evidence, (b) identification of robust findings, and of developmental psychology is often criticized for being not a (c) the ability to determine whether age interacts with other study of development but rather a study of the child at a given variables of interest (e.g., methodological moderators). How- age (McCall, 1977). Meta-analysis can be used to combine stud- ever, meta-analysis does not permit the same degree of causal ies of subjects at different ages, as in a cross-sectional study. inference found in longitudinal studies (Cronbach. 1984). Spe- Second, we examined whether there are age-related differences cifically, differences among age groups can result from either age in twin similarity. On the basis of recent behavior genetics the- effects or cohort effects, the latter of which is only interesting ory and data (McCall, 1983; Plomin & Daniels, 1987; Rowe & from a historical perspective. Thus, like most methodologies, Plomin, 1981), we predict that as twins grow up, they grow developmental meta-analysis has both strengths and weak- apart; that is, twin similarity should decline with age. The im- nesses. portance of the nonshared environment is presumed to in- Twin studies are based on a philosophy and methodology that crease over time, as children's experiences move outside the make them perfect candidates for meta-analysis. Researchers family. have traditionally studied twins to compute heritability coeffi- Researchers have begun to realize that meta-analysis can be cients. The basic procedure is the same in all twin studies: First, used to summarize a body of research through the computation a sample of twins is obtained. Second, the zygosity of the twin of effect-size estimates. However, most researchers may be un- pairs is determined (i.e., monozygotic, or MZ, twins share aware that specific hypotheses can be tested by performing con- 100% of their genes and dizygotic, or DZ, twins share, on aver- trast analyses on the effect sizes of independent studies (Rosen- age, 50% of their genes). Third, individual difference measures thai, 1984). A developmental meta-analysis is thus denned as a are administered to all twins. Fourth, an intraclass correlation meta-analysis with age contrasts. between twins is computed, once for the MZ twins and once for The advantages of being able to perform age contrasts within the DZ twins. The expectation is that if a given trait or ability has a genetic component, then the intraclass correlation will be higher for the MZ twins than for the DZ twins. This research was supported by grants from the Harvard Graduate Consequently, every twin study contains the data for, and ide- Society and by National Institute of Mental Health Grant MH41807 awarded to Kathleen McCartney, who also acknowledges the support ally should report, the MZ and DZ intraclass correlations. of the Whitcomb family. These correlations can easily be used in a number of meta-ana- We would like to thank Robert Rosenthal for frequent statistical con- lytic procedures, including age contrasts; however, this has not sultations and Maura Kerrigan for her help in coding articles. We would been done. Typically, the twin method has been used to estimate also like to thank the reviewers, David Buss, Elizabeth Jordan, Carolyn components of variance for genetic and environmental influ- Mebert, Edward J. O'Brien, Sandra Scarr, Deborah Vandell, members ence. The assumptions of the method continue to be questioned of Murray Straus's research seminar, and especially Victor Benassi for (Goldsmith, 1983; Hoffman, 1985). The most critical assump- their comments on an earlier draft of this article. This article was pre- tion is that the degree of environmental similarity is the same sented by Kathleen McCartney at the International Conference on Psy- chology in Sydney, Australia, August 1988. for MZ and DZ twins. If MZ twins, because they look alike, Correspondence concerning this article should be addressed to Kath- experience more similar environments than DZ twins, then leen McCartney, Department of Psychology, Conant Hall, University of heritability computations would also reflect this environmental New Hampshire, Durham, New Hampshire 03824. similarity (Plornin, DeFries, & McClearn, 1980). 226 GROWING UP AND GROWING APART 227 However, twin studies are useful for purposes other than the Table 1 estimation of heritability. Twin studies are actually within-fam- Categorization of Identified Studies From Literature ily studies, in that more than one member of a family is studied. Search (Based on Abstracts) Longitudinal, within-family studies are needed to assess the rel- Category n ative importance of shared and nonshared experiences of sib- lings. Codable 130' 17 Several longitudinal, within-family studies do, in fact, suggest Uncodable that the older siblings become, the less like one another they Anecdotal or case studies 120 16 11 appear on various indexes of intelligence and personality. For Theoretical 81 No relevant data 114 15 example, three adoption studies have shown that adopted sib- Foreign language 83 11 lings, who are genetically unrelated, look moderately similar on Biological 141 18 intelligence tests and personality inventories in early childhood Animal 9 1 but do not resemble one another at all by later adolescence, Predated 9 1 Dissertations 52 7 presumably because the effect of the shared environment wanes Miscellaneous 25 3 (Kent, 1985; Loehlin, Willerman, & Horn, 1987;Scarr&Wein- berg, 1978). The Louisville twin study (Wilson, 1983) showed Total 764 100 that DZ twins became less similar over time on IQ. However, • Of these 130 papers, 103 were actually used in the meta-analysis. The MZ twins became more similar over time, perhaps because they remaining 27 papers did not contain data that contained intraclass rs seek and are exposed to highly similar environments. or that could be converted to intraclass rs. These 27 papers were receded In addition, a preliminary report of the Minnesota study of to the no-relevant-data category after the papers were read. twins reared apart and together showed that the common family environment accounted for little variance on 12 of 14 personal- were excluded because intraclass rs were not included and could not be ity scales (Tellegen, Lykken, Bouchard, Wilcox, Segal, & Rich, calculated; (d) foreign language papers; (e) studies on psychopathology 1988). Because the "environment," broadly calculated, did ac- or biology, which were excluded because the dependent variables were count for substantial variance, one possibility is that nonshared not in the targeted areas of personality and intelligence; (f) animal stud- experiences play a prominent role in development. ies; (g) predated studies, which were excluded because an updated study These findings were at first surprising to developmentalists, with more data was available; (h) dissertations, which were excluded who have traditionally focused on the role of the shared family because many were duplicated in later journal articles; and (i) miscella- environment in psychological development, as reflected in tra- neous studies that included articles we could not locate through interli- ditional socialization theory (see Maccoby & Martin, 1983). brary loan, papers from various conferences and proceedings, and pa- This perspective assumes that influential environmental experi- pere by Sir Cyril Burt, which have been shown to contain fraudulent data (Heamshaw, 1979). It should also be noted that unrefereed books ences are shared by siblings, because they take place within the and book chapters were excluded by the computer literature search pro- family (e.g., parental discipline, attitudes, and values). The pre- cedure. Thus, this search was limited to journal articles on the similar- diction that follows from this perspective is that the longer sib- ity of twins' personality and intelligence. Table 1 displays the number lings live together, the more like one another they should be- and percentage of studies per category. Note that 130 twin studies were come. read for this meta-analysis. Of these 130 papers, 103 actually contained A developmental meta-analysis of twin studies allows the intraclass re or data that could be converted to intraclass re; these 103 comparison of predictions concerning the importance of the papers were used in the