Gene-Environment Interplay in Twin Models

Abstract

In this article, we respond to Shultziner's critique that argues that identical twins are more alike not because of genetic similarity, but because they select into more similar environments and respond to stimuli in comparable ways, and that these effects bias twin model estimates to such an extent that they are invalid. The essay further argues that the theory and methods that undergird twin models, as well as the empirical studies which rely upon them, are unaware of these potential biases. We correct this and other misunderstandings in the essay and find that gene-environment (GE) interplay is a well-articulated concept in behavior genetics and political science, operationalized as gene-environment correlation and gene-environment interaction. Both are incorporated into interpretations of the classical twin design (CTD) and estimated in numerous empirical studies through extensions of the CTD. We then conduct simulations to quantify the influence of GE interplay on estimates from the CTD. Due to the criticism's mischaracterization of the CTD and GE interplay, combined with the absence of any empirical evidence to counter what is presented in the extant literature and this article, we conclude that the critique does not enhance our understanding of the processes that drive political traits, genetic or otherwise.

Fig. 1

Path diagrams that violate the independence assumption in the classical twin design. Consistent with conventions of path analysis, squares denote manifest or observed variables, whereas circles denote latent variables. Single-headed arrows indicate causal effects, whereas double-headed arrows indicate correlational effects. Interactions between latent variables are indicated by small circles with no variance. All these models are unidentified.

Fig. 2

Biases in A and C estimates as a function for gene–common environment correlation.

Fig. 3

Biases in A and E estimates as a function for gene–unique environment correlation.