Simon Jackman points me to this article by Ryota Kanai, Tom Feilden, Colin Firth, and Geraint Rees, who write:
In a large sample of young adults, we related self-reported political attitudes to gray matter volume using structural MRI. We found that greater liberalism was associated with increased gray matter volume in the anterior cingulate cortex, whereas greater conservatism was associated with increased volume of the right amygdala. These results were replicated in an independent sample of additional participants. Our findings extend previous observations that political attitudes reflect differences in self-regulatory conflict monitoring . . .
Here’s my take on this. Conservatives are jerks, liberals are wimps. It’s that simple. So these researchers can test their hypotheses by more directly correlating the brain functions with the asshole/pussy dimension, no?