Christopher Dawes on Psychological Traits as Intermediaries for Genes

Christopher Dawes visited UIUC this week as part of an ongoing colloquium on the links between genetics and politics. Dawes’ argument is that psychological traits, which he largely defines as personality, are the mechanism linking genetic predispositions and observable political outcomes such as ideology or voting behavior. (My apologies to Dawes’ coauthors. I cannot find a working paper online, and I neglected to record their names.)

Dawes builds his argument by drawing together several strains of research on the micro-foundations of political behavior. First, there is reasonable support that genetic factors play an important, though not determinate, role in political opinion and behavior. Similarly, research in personality has established consistent correlations between some personality traits, but by no means all, and political behaviors. Moreover, personality appears very heritable, at rates similar to height. Dawes attempts to test whether personality acts as an intermediary between genetic predispositions and political behavior.

Before proceeding to his results, I have several concerns about the methods Dawes applies. Dawes uses a dataset composed of monozygotic (identical) and dizygotic (fraternal) twins. As a natural experiment, I find twin studies convincing, and the data clearly indicate that MZ twins are consistently more similar than DZ twins. If we accept that twin pair zygosity is truly random, then the difference is causal: being an MZ twin pair causes greater similarity in behavior. But this conclusion is different than concluding that genetics, and not environmental factors, are responsible. Being an MZ twin may place individuals on a very different path than that of a DZ twin. For example, we know that MZ twins are more frequently dressed alike as children. This emphasis on similarity may influence later behavior. Seeing her identical twin vote may engage the “similarity” response and lead to participation in the second twin. From a strictly experimental view, the treatment had an effect, but parceling out that effect into a genetic component and an environmental component, which are causal, is beyond the scope of the design. Most twin studies, Dawes’s work included, simply assume that these events cannot occur. The “Equal Environments Assumption” posits that MZ and DZ twins do not experience systematically different environmental stimuli. In a working paper on the topic, I show how if the EEA fails, our findings will be biased towards a larger genetic component. I am concerned that the pattern of a large genetic component coupled with a small shared environmental component (which Dawes finds in his research) is evidence that the EEA frequently does not hold in practice.

Even if we assume that MZ and DZ twins experience similar environments, Dawes is attempting to trace causal pathways after the treatment, a practice known as mediation analysis. In a perfect experiment, both genetics and personality would be randomly assigned independently. We could then condition on either genetics or personality and draw causal conclusions about the results we saw. If we take the twin study as a natural experiment, only the percentage of shared genetics are randomly assigned. Attempting to explain the causal effect of non-randomly assigned quantities, i.e. personality, falls outside of the scope
of the design. At the risk of being trite, as the well known expression says, “Correlation is not causation.” Observed relationships between personality and political behavior, even in a twin study, are correlations, not necessarily causal. (For a more detailed review of the difficulties of mediation analysis, see Green, Ha, and Bullock (2010).)

Returning to the article at hand, Dawes’ results are consistent with previous findings that both personality traits and political behavior are related to genetics. He also concludes that personality traits correlate with political behavior and are therefore mediators, causal agents that are consequents of both genetics and environmental factors. To his credit, Dawes conditions these results on the Equal Environments Assumption and an assumption that gene-environment interactions do not occur. But if these assumptions do not hold, and we do not know that they do, Dawes’ findings are ambiguous. Concerning the dangers of mediation analysis, Dawes is not unaware of the issue, but I am concerned that this research extends twins studies beyond their defensible conclusions.

So where does this leave us on this research and twin studies in general? My intuition is that Dawes is basically correct: genetic predipositions manifest as personality traits, which in turn define default modes of behavior, including political choices. I remain skeptical that the methods employed demonstrate this causal chain. For my own part, I am working on extending randomization inference to twins studies to address these methodological issues. While the basic limitations of twin studies still apply, randomization inference provides a principled method to engage with hypotheses concerning deviations from the assumptions of the standard twins study analysis. Instead of simply assuming an additive model where environment does not interact with genetics, I purpose testing hypotheses on the amount of possible interactive effects under different models, in effect a sensitivity analysis of the classical models. Rather than condition our results on assumptions, we can then provide a range of plausible levels of gene-environment interaction that are consistent with our data, a form of analysis I would much prefer to see in this research and other genetics research.