Self-interest is widely accepted as a powerful motivator by both academics and laypeople alike. However, research surrounding the self-interest motive paints a complicated picture of this most important psychological construct. Additionally, research on the social desirability of self-interest has revealed that despite its widespread acceptance, people do not readily accept that self-interest drives their own behaviors. This paper reviews the literature on self-interest and reveals several curious features surrounding its actual effect on helping behaviors, political attitudes and voting, and people's apparent ambivalence toward self-interest as a motive. It is possible that norms against the expression of self-interestedness evolved, creating ambivalence towards this widely accepted construct, subsequently affecting its expression on many human behaviors.