I am generally interested in the evolution of cooperative behavior in both human and nonhuman animals. Theoretically, I am interested reciprocal altruism, particularly, long-term vs. short-term reciprocity. Empirically, my recent research focuses on human friendship, trying to understand how people choose their friends, how they maintain their friendship, and what kind of psychologies are evolved for this type of cooperative relationship. I take a cross-cultural perspective to explore my question. I am currently looking at three cultural settings, US, China, and Japan. My previous works (in China) includes food sharing behavior in snub-nosed monkeys and conservational biology of Chinese white dolphin.