Drop something in front of a two-year-old, and she`s likely to pick it up for you. This is not a learned behavior, psychologist Michael Tomasello argues. Through observations of young children in experiments he himself has designed, Tomasello shows that children are naturally--and uniquely--cooperative. Put through similar experiments, for example, apes demonstrate the ability to work together and share, but choose not to. As children grow, their almost reflexive desire to help--without expectation of reward--becomes shaped by culture. They become more aware of being a member of a group. Groups convey mutual expectations, and thus may either encourage or discourage altruism and collaboration. Either way, cooperation emerges as a distinctly human combination of innate and learned behavior. In Why We Cooperate, Tomasello`s studies of young children and great apes help identify the underlying psychological processes that very likely supported humans` earliest forms of complex collaboration and, ultimately, our unique forms of cultural organization, from the evolution of tolerance and trust to the creation of such group-level structures as cultural norms and institutions.