They assume the existence of groups and prosocial behaviors evolve by between-group selection, just as Darwin envisioned. Thus, MLS theory represents a synthesis and unification of previously discrepant theories. Second, not only does MLS theory include its so-called alternatives as special cases, but it goes further to explain other cases that previously were difficult to imagine. In particular, human groups emerge as important units of selection, despite the fact that their members are genetically unrelated and do not always base their behavior on expected return benefits. Ironically, the same theory of multilevel selection that led to the first and second rise of altruism also leads to its second fall.
Altruism is only one of several mechanisms that can evolve to increase the fitness of groups.
Thus, the case for groups as adaptive units is becoming increasingly secure while the case for altruism, as opposed to social control, remains tenuous. My talk will focus on this problem, which I regard as the current frontier of evolutionary research on altruism. First, it is very important to see social control as a form of altruism within the framework of MLS theory, even though the self-sacrificial component of the altruism is often very small. Second, high-cost altruism can evolve when altruists can segregate themselves sufficiently from nonaltruists. Plausible segregation mechanisms exist and have been demonstrated in social psychology experiments.