A study conducted by a fellow at Wellcome Trust found that liberals tend to have a larger anterior cingulated cortex, a brain region which helps govern decision making, particularly when the person is faced with conflicting information.
People with conservative views, on the other hand, tend to have larger amygdala, a primitive area of the brain which processes emotions -- fear in particular. The finding backs up previous studies concerning liberal vs. conservative behavior.
It has previously been shown that conservatives react to threats by showing more aggression than liberals do. It´s not clear whether the differences in brain structure cause the political differences or are caused by them.