In addition to vasopressin, the hormone oxytocin contributes to the development of attachment behavior in monogamous prairie voles. This hormone plays a role in rewarding a male for mating and staying with a partner. Oxytocin also affects the social behavior of humans; the hormone appears to make us feel more warmly toward others, more trusting and generous than we might be otherwise. Some researchers have suggested that application of oxytocin via nasal sprays would provide therapeutic benefits by boosting our willingness to cooperate with others. With these points in mind, how did Carsten De Dreu and his coworkers use evolutionary theory to interpret the results shown in Figure 13.29?