Physical similarity and the equal-environment assumption in twin studies of psychiatric disorders

Abstract

The equal-environment assumption (EEA), upon which twin methodology is based, was examined for the impact of physical similarity on phenotypic resemblance in five common psychiatric disorders: major depression, generalized anxiety disorder, phobia, alcoholism, and bulimia. A population-based sample of 882 female-female twin pairs of known zygosity was rated for similarity of appearance by color photographs. Psychiatric diagnoses were made by clinical assessment of personal interviews of the twins. Structural equation modeling of the data using physical similarity as a form of specified common environment provided no evidence for a significant effect of physical resemblance on concordance for major depression, generalized anxiety disorder, phobia, and alcoholism, thereby supporting the validity of the EEA in twin studies of these disorders. Results for bulimia, on the other hand, suggest, within the limitations of this study, that physical similarity may significantly influence twin resemblance for this disorder.

Defining nicotine dependence for genetic research: evidence from Australian twins

“…inappropriately inflating estimates of trait heritability). Such characteristics, for example, physical twin similarity (Hettema et al 1995), self-perceived zygosity (Xian et al 2000), perceived zygosity and associated parental approach to rearing their twins (Kendler et al 1993(Kendler et al , 1994Kendler & Gardner, 1998 ;Cronk et al 2002), and physical and emotional closeness between the twins (LaBuda et al 1997 ;Kendler & Gardner, 1998;Cronk et al 2002) have been shown to be uncorrelated with zygosity differences in a number of psychiatric disorders such as alcohol and illicit drug dependence, major depression, anxiety, and externalizing disorders, supporting the validity of the equal environmental assumption in twin studies assessing these disorders. The equal environment assumption has also been found to be valid for DSM-III-R nicotine dependence in female (Kendler & Gardner, 1998) and male twins (Xian et al 2000).…”

A Twin Study of Chronic Fatigue

“…If this so-called equal environment assumption is violated, the estimates of genetic influence could be falsely inflated. However, this assumption has recently been examined for several psychiatric disorders and found to be quite tenable (52,53). Second, it is important to acknowledge that the method we used to identify the twin sample was not ideal.…”

Genetics and the Social Science Explanation of Individual Outcomes

“…Violations of this "equal environments assumption" (EEA) imply that estimated heritabilities overestimate true heritabilities. 5 The EEA provides the most principled grounds for radical doubt regarding twin studies, but various means of indirect assessment-for example, by studying DZ twins mistakenly believed by their families to be MZ-provide no empirical grounds to think violation of the assumption is commonly anywhere near enough to nullify findings of substantial heritability (e.g., Kendler et al 1993;Hettema, Neale, and Kendler 1995;Bradshaw 2007). Designs not involving twins have also not revealed radically divergent findings (Plomin et al 2001).…”