Deontic reasoning is reasoning about permission and obligation: what one may do and what one must do, respectively. Conceivably, people could reason about deontic matters using a purely formal deontic calculus. I review evidence from a range of psychological experiments suggesting that this is not the case. Instead, I argue that deontic reasoning is supported by a collection of dissociable cognitive adaptations for solving adaptive problems that likely would have confronted ancestral humans.