This is going to sound sort of obvious, but here we go: A study from University College London published this week in Current Biology has discovered that there are actually differences in the brains of liberals and conservatives. Specifically, liberals' brains tend to be bigger in the area that deals with processing complex ideas and situations, while conservatives' brains are bigger in the area that processes fear.
According to the report: "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."
People with larger amygdalae respond to perceived threats with more aggression and "are more sensitive to threatening facial expressions." The anterior cingulate cortex, however, "monitors uncertainty and conflict." "Thus," says the report, "it is conceivable that individuals with a larger ACC have a higher capacity to tolerate uncertainty and conflicts, allowing them to accept more liberal views."
The London researchers say they're unsure whether the brain's structure causes political views or is the effect of them. Regardless, this puts the "Obama's a Muslim socialist" fearmongering at Tea Party rallies into a whole new light.