ABSTRACT
The biological study of social behavior has a long history, dating at least from Charles Darwin. In the 1960s two relatively new approaches brought social behavior into better ecological perspective. The study of cooperative social behavior benefited in the 1960s and 1970s from both approaches. Helping behavior is a type of aid giving that has been traditionally defined as parentlike behavior toward young that are not offspring of the helper. Long recognized as a contributing cause of helping is the difficulty of acquiring a mate. A distinctly different mode of origin of helpers is the persistence of nuclear families by prolonging the period of association of young with each other and with one or both of their parents, usually on their natal territory. The existence of an ontogenetic process of role or caste determination implies some variation among species in degree of reversibility. In asocial species the sequence, or "ontogenetic trajectory", is relatively invariant.
