Principled behavior seems to defy evolutionary logic. Principled people consistently abide by their principles, ignore tradeoffs or compromises, and pursue the principles for transcendental reasons, such as that they are “right”, decreed by God, or part of an eternal debt to the emperor. Here, we explain principled behavior as a combination of what we call “committed agents” and “impersonators”. Committed agents are individuals whose extreme psychology compels them to never deviate from a maxim and who are especially trustworthy for it. Imitators non-consciously masquerade as committed agents to garner trust. Given that observers can only determine whether a person is genuinely committed on the basis of their behavior, impersonators must appear to never deviate from the maxim, never think about deviating, pursue the maxim for the reason motivating committed agents, and justify ambiguous or compromising decision as conforming to the principle. We use this account to explain key features of principled behavior as well as seemingly unrelated phenomena, including cognitive dissonance, foot-in-the-door effects, moral licensing, sacred values, the expanding moral circle, and beliefs in supernatural punishment. Principled behavior consists of the behavior of rare extreme individuals and strategic attempts by others to pass as them.