In the comments here, we've been discussing this April "neuropolitics" study in Current Biology, which was supported by the actor Colin Firth and actually lists him as a co-author. Another celeb science fan, one supposes. Anyway, it finds that in a sample of 90 young British men and women, the liberals and the conservatives tended to have somewhat different brain structures in brain scans. Conservatives had more gray matter in the amygdala, and liberals had more in the anterior cingulate cortex (ACC). The authors weren't shy about speculating as to what this means:

We speculate that the association of gray matter volume of the amygdala and anterior cingulate cortex with political attitudes that we observed may reﬂect emotional and cognitive traits of individuals that inﬂuence their inclination to certain political orientations. For example, our ﬁndings are consistent with the proposal that political orientation is associated with psychological processes for managing fear and uncertainty. The amygdala has many functions, including fear processing. Individuals with a large amygdala are more sensitive to fear, which, taken together with our ﬁndings, might suggest the testable hypothesis that individuals with larger amygdala are more inclined to integrate conservative views into their belief system.... Similarly, it is striking that conservatives are more sensitive to disgust, and the insula is involved in the feeling of disgust. On the other hand, our ﬁnding of an association between anterior cingulate cortex volume and political attitudes may be linked with tolerance to uncertainty. One of the functions of the anterior cingulate cortex is to monitor uncertainty and conﬂicts. Thus, 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. Such speculations provide a basis for theorizing about the psychological constructs (and their neural substrates) underlying political attitudes. However, it should be noted that every brain region, including those identiﬁed here, invariably participates in multiple psychological processes. It is therefore not possible to unambiguously infer from involvement of a particular brain area that a particular psychological process must be involved.

For reporting on other caveats, see here. One major issue that immediately arises is causation: Does being liberal make the ACC bigger, or does having a bigger ACC make you liberal, or is something else going on? For more detail, see the Wikipedia page on this line of research, which is actually pretty helpful. This is published, peer reviewed science; it is consistent with other, peer reviewed science; but still, this whole area strikes me as highly uncertain and subject to misinterpretation. However, it is noteworthy that on this blog in February, when I asked readers for opinions about the documented liberalism of scientists, many of you basically gave the same answer as the one implied above. As I summarized your views then: "liberalism is associated with more shades-of-gray thinking and an appreciation of complexity, and this goes naturally with the pursuit of science." Nobody cited this paper, though it had already been in the news in a pre-press form. My question is this: Is this really the way it is all going? Will we soon not only psychoanalyze, but also brain-scan our politics, and our politics of science?