Cooperation eases our way in the world, contributing to extraordinary and mundane human achievements alike. Yet even the nicest do-gooders sometimes act with self-interest. A study published recently in Nature sought to understand the mental processes that tip a person from generous to greedy. “By default are we selfish animals who have to exert willpower in order to cooperate?” asks David Rand, a psychologist at Harvard University who led the study. “Or are we predisposed to cooperate, but when we stop to think about it the greedy calculus of self-interest takes over?”

To peer into this aspect of human nature, Rand and his colleagues gave study participants 40 cents, then asked them to decide how much to keep for themselves and how much to donate to a common pool that would later be doubled and split evenly among those who donated. Those who quickly made up their minds donated more than those who took longer, suggesting that quick decisions based on intuition were more generous than slower, deliberative decisions.

In a follow-up study, researchers prompted participants either to trust their instincts or to mull them over when deciding. Consistent with the earlier finding, donations were higher for the intuition group.

This result suggests that our first impulse is to cooperate, but it does not necessarily mean we are genetically hardwired to do so, Rand says. Instead it may reflect a habit learned from a lifetime of fruitful cooperative experiences. The work also suggests that donation seekers would do well to leave their facts and statistics behind when courting potential donors—that pitch could backfire by promoting a ruminative, miserly mind-set.