This chapter examines the evolutionary roots of the proximate psychological mechanisms that underlie cooperation. The idea that there are specific biological mechanisms behind at least some aspects of cooperation is supported by recent work in behavior genetics. One common technique in behavior genetics is to compare identical twins to fraternal twins. Another study, using a different technique, found a relationship between voter turnout and two specific genes. Hormones provide another window onto the proximate psychological mechanisms underlying cooperation. The chapter first considers the most basic form of cooperation, reciprocity, before discussing its relation to culture, the avoidance of individuals prone to free riding, and detection of cheaters. It also explores indirect reciprocity, generosity as performance, and hard-to-fake signals.