AMHERST, Mass. - Herbert Gintis, an economist at the University of Massachusetts, is reshaping traditional models of economic behavior, using research drawn from neuroscience, biology, psychology, anthropology, and political science. Gintis says his goal is to improve and expand current models which are based primarily on rational choice and reward. He is looking for analytic tools to explain why, for example, when people form into groups, they begin with a tendency to cooperate, impulsively strike back if other group members cheat, yet still maintain the ability to forgive. Eventually people return to cooperating if the situation changes, Gintis says.