The Equal Environments Assumption in the Post-Genomic Age: Using Misclassified Twins to Estimate Bias in Heritability Models

Abstract

While it has long been known that genetic-environmental covariance is likely to be non-trivial and confound estimates of narrow-sense (additive) heritability for social and behavioral outcomes, there has not been an effective way to address this concern. Indeed, in a classic paper, Goldberger (1979) shows that by varying assumptions of the GE-covariance, a researcher can drive the estimated heritability of an outcome, such as IQ, down to zero or up close to one. 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 than, say, dizygotic twins, represent poor attempts to gauge a very complex underlying phenomenon of GE-covariance. Methods that rely on concordance between interviewer classification and self-report offer similar concerns about validity. In the present study, we take advantage of a natural experiment to address this issue from another angle: Misclassification of twin zygosity in a nationally-representative study (Add Health). Since such twins were reared under one “environmental regime of similarity” while genetically belonging to another group, this reverses the typical GE-covariance and allows us bounded estimates of heritability for a range of outcomes of interest to medical and behavioral scientists.