Heritability and the Equal Environments Assumption: Evidence from Multiple Samples of Misclassified Twins

Abstract

Classically derived estimates of heritability from twin models have been plagued by the possibility of genetic-environmental covariance. Survey questions that attempt to measure directly the extent to which more genetically similar kin (such as monozygotic twins) also share more similar environmental conditions represent poor attempts to gauge a complex underlying phenomenon of GE-covariance. The present study exploits a natural experiment to address this issue: Self-misperception of twin zygosity in the National Longitudinal Survey of Adolescent Health (Add Health). Such twins were reared under one “environmental regime of similarity” while genetically belonging to another group, reversing the typical GE-covariance and allowing bounded estimates of heritability for a range of outcomes. In addition, we examine twins who were initially misclassified by survey assignment—a stricter standard—in three datasets: Add Health, the Minnesota Twin Family Study and the Child and Adolescent Twin Study in Sweden. Results are similar across approaches and datasets and largely support the validity of the equal environments assumption.

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Notes

  1. 1.

    Assortative mating is the non-random selection of mates in a population. For example, brunettes may be more likely to pair with other brunettes (positive assortative mating) or non-brunettes (negative assortative mating).

  2. 2.

    Technically, if their genetic similarity in appearance, for instance, is causing the twins to be confused and/or treated more similarly, then that is an effect of genes and thus should unproblematically be part of the overall “genetic” effect (Jencks 1980). However, this logic flies in the face of common sense understandings of what we mean by genetic effects and makes the estimates less externally valid to the rest of the non-twin population. Moreover, bias is introduced by any increased cross-sibling interaction that leads to increased similarity in phenotypes.

  3. 3.

    Ideally we would instrument misclassification. Birth weight differences temporally precede self-perception of zygosity and strongly predict it, thus fulfilling the first condition necessary for an instrument. However, birth weight differences are likely to have direct effects on the similarity in phenotypes we consider, net of misclassification status. Birth weight has been shown to affect a range of anthropometric measures (see, e.g., Conley et al. 2003 for a review), and recent work has shown that differences themselves, in fact, have predictive power for the differences between siblings (including twins) (see Conley and Rauscher 2013). Thus, birth weight differences violate the exclusion restriction and would thus fail as an instrument. Indeed, it is likely that any factor that would affect the probability of misclassification would also affect the phenotypes, thus we abandoned the hope for an instrumentation strategy and rely instead on simple comparisons between correctly and incorrectly classified groups.

  4. 4.

    Intra-class correlation is the proportion of the variance between pairs, measured as the variance between twin pairs divided by the sum of the variance within pairs and the variance between pairs. ICC = σB/(σBW).

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Acknowledgments

This research uses data from Add Health, a Program Project directed by Kathleen Mullan Harris and designed by J. Richard Udry, Peter S. Bearman, and Kathleen Mullan Harris at the University of North Carolina at Chapel Hill, and funded by Grant P01-HD31921 from the Eunice Kennedy Shriver National Institute of Child Health and Human Development, with cooperative funding from 23 other federal agencies and foundations. Special acknowledgment is due Ronald R. Rindfuss and Barbara Entwisle for assistance in the original design. Information on how to obtain the Add Health data files is available on the Add Health website (http://www.cpc.unc.edu/addhealth). No direct support was received from Grant P01-HD31921 for this analysis. This research was funded by the National Science Foundation’s Alan T. Waterman Award, SES-0540543.

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Correspondence to Dalton Conley.

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Edited by Chandra Reynolds.

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Conley, D., Rauscher, E., Dawes, C. et al. Heritability and the Equal Environments Assumption: Evidence from Multiple Samples of Misclassified Twins. Behav Genet 43, 415–426 (2013). https://doi.org/10.1007/s10519-013-9602-1

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Keywords

  • Equal environments
  • Twin misclassification
  • Heritability
  • ACE model