For example, research shows that when people are told that it is possible — but not certain — that going to work while sick would infect a co-worker, people are less willing to stay home than when they are reminded of the certainty that going to work sick would expose vulnerable co-workers to a serious chance of illness. Stressing the certainty of risk, in other words, more effectively motivates altruism than stressing the possibility of harm.

The lesson for the real world is that health officials should be explicit in telling the public that selfish responses to an epidemic, such as going to work while sick or failing to wash your hands, threaten the health of the community.

There are other ways to encourage selfless behavior. For example, another study examined the neural activity of people while they played a game in which they made either generous or selfish choices to award or withhold money. The researchers found that when subjects made selfish decisions, the brain’s reward center was activated, whereas when they made generous decisions, a region of the brain implicated in empathy lit up. This suggests that people are more likely to be altruistic if they are primed to think of others and to imagine how their behavior might benefit them.