I think the case for big vs small governments is instead "not enough information." Big vs. small isn't really as important as are how the gov't is implemented and the culture/attitudes of the governed.

As far as the brain scans go, science isn't nearly at the point to even hazard guesses at random high-level behaviors like "will murder." We've really only had success when behaviors in insects just so happen to be controlled by a single gene. Here, I doubt there even is a single "murderer" gene, especially since "murder" is such a random, infrequently displayed behavior.