Field of Science

Did your genes make you liberal?

"The new issue of the Journal of Politics, published by Cambridge University, carries the study that says political ideology may be caused by genetic predisposition."

"Scientists find 'liberal gene.'"
  --- NBC San Diego

"Liberals may owe their political outlook partly to their genetic make-up, according to new research from the University of California, San Diego, and Harvard University.  Ideology is affected not just by social factors, but also by a dopamine receptor gene called DRD4."
 -- University press release

As in the case yesterday of the study about sisters making you happy, these statements are all technically true (ish -- read below) but deeply misleading. The study in question looks at the effects of number of friends and the DRD4 gene on political ideology. Specifically, they asked people to self-rate on a 5-point scale from very conservative to very liberal. They tested for the DRD4 gene. They also asked people to list up to 5 close friends.

The number of friends one listed did not significantly predict political ideology, nor did the presence or absence of the DRD4 gene. However, there was a significant (p=.02) interaction ... significant, but apparently tiny. The authors do not discuss effect size, but we can try to piece together the information by looking at the regression coefficients.

An estimated coefficient means that if you increase the value of the predictor by 1, the outcome variable increases by the size of the coefficient. So imagine the coefficient between the presence of the gene and political orientation was 2. That would mean that, on average, people with the gene score 2 points higher (more liberal) on the 5-point political orientation scale.

The authors seem to be reporting standardized coefficients, which means that we're looking at increasing values by one standard deviation rather than by one point. The coefficient of the significant interaction 0.04. This means that roughly as the number of friends and presence of the gene increase by one standard deviation, political orientation scores increase by 0.04 standard deviations. The information we'd need to correctly interpret that isn't given in the paper, but a reasonable estimate is that this means that someone with one extra friend and the gene would score anywhere from .01 to .2 points higher on the score (remember, 1=very conservative, 2=conservative, 3=moderate, 4=liberal, 5=very liberal).

The authors give a little more information:
For people who have two copies of the [gene], an increase in number of friendships from 0 to 10 friends is associated with increasing ideology in the liberal direction by about 40% of a category on our five-category scale.
People with no copies of the gene were unaffected by the number of friends they had.

None of what I wrote above detracts from the theoretical importance of the paper. Identifying genes that influence behavior, even just a tiny bit, is important as it opens windows into the underlying mechanisms. And to their credit, the authors are very guarded and cautious in their discussion of the results. The media reports -- fed, no doubt, by the university press release -- have focused on the role of the gene in predicting behavior. It should be clear that the gene is next to useless in predicting, for instance, who somebody is going to vote for. Does that make it a gene for liberalism? Maybe.

I would point out one other worry about the study, which even the authors point out. They tested a number of different possible predictors. The chances of getting a false positive increases with every statistical test you run, and they do not appear to have corrected for multiple comparisons. Even with 2,000 participants (which is a large sample), the p-value for the significant interaction was only p=.02, which is significant but not very strong, so the risk that this will not replicate is real. As the authors say, "the way forward is to seek replication in different populations and age groups."


GamesWithWords said...

PS The paper indicates that the regression coefficients are beta-weights, but the actual numbers suggest that they are un-standardized coefficients instead. Either way, the effect is very small.

lbr said...

One reason why this result isn't very interesting is that many genes might have such a weak influence.