Max M. Krasnow, a postdoctoral scholar at University of California, Santa Barbara's Center for Evolutionary Psychology, says acts of generosity -- incurring a cost to benefit someone else when no future return is likely -- are seen as maladaptive by biologists and irrational by economists.
If traditional theories in these fields are true, such behaviors should have been weeded out long ago by evolution or by self-interest, Krasnow says.
"When past researchers carefully measured people's choices, they found that people all over the world were more generous than the reigning theories of economics and biology predicted they should be," Krasnow says in a statement. "Even when people believe the interaction to be one-time only, they are often generous to the person they are interacting with."
The study, published in the Proceedings of the National Academy of Sciences, shows generosity emerges naturally from the evolution of cooperation.
"Believing that you will never meet this individual again, you might choose to benefit yourself at his expense -- only to find out later that the relationship could have been open-ended," the researchers say. "If you make this error, you lose out on all the benefits you might have had from a long-term, perhaps lifelong, cooperative relationship."