Abstract Authority, and the behavioral response to authority, is central to many important questions in public economics, but has received insufficient attention from economists. In particular, research has not differentiated between legitimate power and the presumption of expert knowledge, what we call authority “to” and authority “in.” In this paper we report on the results of a series of lab experiments designed to distinguish the effects of the two sources of authority on contributions to a public project. The results suggest that authority “to” and authority “in” interact in ways not heretofore understood. Penalizing non-social behavior without expert explanation does not increase voluntary contributions, nor does expert explanation without the threat of penalty, but together they induce more contributions than any other combination of policies. We interpret these findings to indicate that the reaction to an authority depends on whether that authority is perceived to be legitimate.