In this paper, I consider a body of observational evidence not commonly studied by social scientists, namely the behavior of men and women (mostly men) in the military. I focus here on two issues: first the behavioral foundations for creating an effective military unit and second, evidence that infantrymen have historically been reluctant to fire on the enemy and how this reluctance has been overcome in the last half century through changes in military training. The evidence in each of these areas reinforces the appeal of the idea of cognitive modularity, the view that thought and behavior are influenced by different “mental organs.” With respect to behavior, these usually align in the counsel they provide. But not always, and focusing on circumstances where guidance conflicts—Prisoners Dilemmas are examples—offers a route towards building a behavioral science with greater explanatory and predictive success.