Humans are a remarkably social species. They form and live in groups and recurrently have to decide whether to cooperate or compete with others within and among groups. Cooperation has been essential for group survival and prosperity across human history. In hunter-gatherer societies, people need to form alliances in hunting to alleviate the risks from predator attacks. Likewise, modern societies require groups of people to cooperate in large ventures. Yet, social situations often involve a conflict between one’s short-term personal interest and the long-term collective interest (i.e., social dilemmas; Dawes, 1980; Van Lange et al., 2013). In such mixed-motive situations, what is good for an individual may often harm the collective, and this makes people tempted to free ride and harvest the benefits from others’ cooperation. Indeed, many societal problems and global issues (e.g., traffic problems, environmental pollution, and resource depletion) involve such conflicts of interests. Solving these problems often requires individuals to cooperate by paying a personal cost to benefit another person or the group.