Studies on crowding out document that incentives sometimes backfire—decreasing motivation in prosocial tasks. In the present research, we demonstrated an additional channel through which incentives can be harmful. Incentivized advocates for a cause are perceived as less sincere than nonincentivized advocates and are ultimately less effective in persuading other people to donate. Further, the negative effects of incentives hold only when the incentives imply a selfish motive; advocates who are offered a matching incentive (i.e., who are told that the donations they successfully solicit will be matched), which is not incompatible with altruism, perform just as well as those who are not incentivized. Thus, incentives may affect prosocial outcomes in ways not previously investigated: by crowding out individuals’ sincerity of expression and thus their ability to gain support for a cause.