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What Can We Learn from Twin Studies? a Comprehensive Evaluation of The The Pennsylvania State University The Graduate School College of the Liberal Arts WHAT CAN WE LEARN FROM TWIN STUDIES? A COMPREHENSIVE EVALUATION OF THE EQUAL ENVIRONMENTS ASSUMPTION A Dissertation in Sociology by Jacob Felson c 2009 Jacob Felson Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy December 2009 The dissertation of Jacob Felson was reviewed and approved∗ by the following: John D. McCarthy Professor of Sociology Head of the Department of Sociology Dissertation Advisor Alan Booth Professor of Sociology Roger Finke Professor of Sociology and Religious Studies Katerina Bodovski Assistant Professor of Education ∗Signatures are on file in the Graduate School. ii Abstract Most evidence about the effects of genes on behavior comes from twin studies. The classic twin study method compares the similarity of monozygotic (MZ) twins on some trait with the similarity of dizygotic (DZ) twins on that same trait. If the rearing environments of MZ and DZ twins are analogous, then the magnitude of genetic influence on variation in a trait can be estimated based on the extent to which the correlation for MZ twins exceeds the correlation for DZ twins. If the rearing environments of MZ and DZ twins are not analogous, then the estimates of heritability generated from the classic twin study method may be confounded with differences between MZ and DZ twins in the similarity of their rearing environment. It has long been established that MZ twins grow up in a more similar environment than do DZ twins. However, behavior geneticists have generally asserted that the greater degree of similarity in the rearing environments of MZ relative to DZ twins does not bias the results of twin studies. Behavior geneticists claim that the similarity in outcomes is largely unaffected by similarity in rearing environments for MZ and DZ twins. This claim is known as the equal environments assumption (EEA). In this paper, I argue that the empirical support for the validity of the EEA is not as strong as some behavior geneticists have claimed. I reanalyze some of the data on which ritually cited research on EEA is based, and I find that the evidence is more mixed than latter-day interpretations have suggested. I expose some of the flaws in the research that is frequently cited by researchers in support of claims about genetic influence on behavior. Finally, I conduct the most comprehensive evaluation of the equal environments assumption to date. My analysis incorporates a larger and more diverse set of out- come variables than any previous research on the EEA. In my analysis, I also use a more extensive set of controls for environmental similarity than any previous study. Finally, unlike most if not all previous research on EEA, I generate estimates of genetic influence with and without controls for environmental similarity, in order to show the extent of EEA violation in a straightforward manner. The findings are not easily categorized as supporting or undermining the EEA. Some violations of EEA were found for some outcomes, but the bias caused by these violations is likely to be modest in most cases. Researchers should not conduct twin studies without including controls for environmental similarity. However, the findings also indicate that many of the results of twin studies are robust with these controls. The flaws of twin studies are not fatal, but rather seem no worse (and maybe better) than the flaws of the typical causal study that relies on observational data. iii Contents List of Tables v List of Figures vi Acknowledgements viii 1 Introduction 1 1.1 Planforthepresentstudy . .. .. 2 1.2 Estimating the effects of genes on variation in traits with twin studies 3 1.3 Evaluating the twin study as a natural experiment . ..... 5 2 Literature Review 9 2.1 Earlieststudies(1930–1960) . 10 2.2 Earlystudies(1965–1979) . 21 2.3 Recentstudies............................... 40 2.4 Studies of adopted children and studies of twins reared apart..... 47 3 Data 48 3.1 Measures.................................. 50 3.2 Handlingmissingdata . .. .. 63 4 Methods 64 4.1 Comparisons of means and variances by zygosity . .... 64 4.2 Examining measures of environmental similarity . ...... 64 4.3 Absolutevalueregressions . 64 4.4 Defries-Fulkerregressions. .. 65 4.5 Propensityscorematching . 67 5 Results 68 5.1 Comparisons of means and variances by zygosity . .... 69 5.2 Examining measures of environmental similarity . ...... 75 5.3 Absolutevalueregressions . 80 5.4 Defries-Fulkerregressions. .. 91 5.5 Propensityscorematching . 99 6 Conclusion 114 References 116 iv List of Tables 1 Descriptive statistics for reanalysis of Wilson (1934) . ........ 13 2 Model fit statistics for reanalysis of Wilson (1934) . ..... 16 3 Mean differences between co-twins on various measures, reproduced fromScarr(1968)............................. 23 4 Parental reports of environmental similarity, by perceived and diag- nosed zygosity, NMSQT twins study . 26 5 Twin reports of environmental similarity, by perceived and diagnosed zygosity, NMSQT twins study . 28 6 Regressions of absolute twin differences in cognitive test scores on measures of perceived, physical, and genetic differences, n=226, re- produced from Scarr and Carter-Saltzman (1979) . 37 7 Average absolute differences between co-twins on Raven’s Progressive Matrices, from Scarr and Carter-Saltzman (1979) . .. 39 8 Average absolute differences between co-twins on Revised Visual Re- tention Test, from Scarr and Carter-Saltzman (1979) . ... 39 9 Recent studies testing the equal environments assumption ...... 43 10 Recent studies: measures of environmental similarity . ....... 44 11 Recent studies: outcome measures . 45 12 Recent studies: methods and findings . 46 13 Descriptive statistics for physical attributes . ........ 51 14 Descriptive statistics for measures of Health . ..... 52 15 Descriptive statistics for measures of Health-Related Behaviors . 53 16 Descriptive statistics for measures of Mental Disorders ........ 54 17 Descriptive statistics for measures of Personality . ........ 55 18 Descriptive statistics for measures of Self-Actualization ........ 55 19 Descriptive statistics for measures of Well-Being . ....... 56 20 Descriptive statistics for measures of Sociability . ........ 57 21 Descriptive statistics for measures of Marriage and Family ...... 57 22 Descriptive statistics for measures of Social Class . ....... 58 23 Descriptive statistics for measures of Socio-political Attitudes . 58 24 Descriptive statistics for measures of Social Obligations........ 59 25 Descriptive statistics for measures of religion . ....... 60 26 Components of scales measuring environmental similarity....... 61 27 Components of scales measuring environmental similarity, continued . 62 28 Standardized outcome variances by zygosity . ... 72 29 Standardized outcome variances by zygosity, continued . ....... 74 v List of Figures 1 Comparing environmental similarity of DZ twins to that of MZ twins, basedonreanalysisofWilson(1934) . 18 2 Correlations between absolute differences on traits and parents’ re- ports of environmental similarity, following Loehlin and Nichols (1976) 27 3 Correlations between absolute differences on traits and twins’ reports ofenvironmentalsimilarity. 29 4 Heritability estimates for components of National Merit Qualifying Test: unmatched and matched twin pairs, NMSQT data . 31 5 Heritability estimates for personality traits: unmatched and matched twinpairs,NMSQTdata. .. .. 32 6 Heritability estimates for personality traits: unmatched and matched twin pairs, continued, NMSQT data . 33 7 Heritability estimates for vocational interests: unmatched and matched twinpairs,NMSQTdata. .. .. 34 8 Standardized means of outcome variables by zygosity . .... 69 9 Standardized means of outcome variables by zygosity, cont....... 70 10 Standardized means of outcome variables by zygosity, cont....... 71 11 Ninety-five percent confidence intervals of environmental similarity . 75 12 Agreement between twins on measures of environmental similarity . 77 13 Twin reports about equality in relationship growing up . ...... 78 14 Perceptions of equality in relationship growing up, by zygosity . 79 15 Percent change in coefficient for zygosity when controlling childhood environment................................ 81 16 Percent change in the coefficient for zygosity when controlling for sta- tus inequality in relationship growing up . 82 17 Percent change in the coefficient for zygosity when controlling for cur- rentcontact ................................ 83 18 Percent change in the coefficient for zygosity when controlling for the proportionoflifelivedtogether . 85 19 Percent change in the coefficient for zygosity when controlling for vir- tualcontact ................................ 86 20 Percent change in the coefficient for zygosity when controlling for psy- chologicalintimacy ............................ 87 21 Percent change in the coefficient for zygosity when controlling for neg- ativityintwinrelationship . 88 vi 22 Percent change in the coefficient for zygosity when controlling for ad- vice exchanged between twins . 89 23 Percent reduction in heritability when controlling for childhood simi- larity.................................... 92 24 Percent reduction in heritability when controlling for inequality in twinrelationshipgrowingup. 93 25 Percent reduction in heritability when controlling for current contact 94 26 Percent reduction in heritability when controlling for proportion of life spenttogether............................... 95 27 Percent
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