And so the survival of our species has depended on the evolution of innate responses to keep these so-called free riders mostly in check, to make sure that there are enough people willing to run into burning buildings to save lives and a lot fewer who light fires. Evolution has equipped us with tools to help tip the balance toward cooperation. And we need to use all of those tools in our current predicament.

What are these responses shaped by our evolution? The first is that cooperation is more likely when a group faces a shared enemy. That we already have, in the form of this nasty virus.

Another is that people are more likely to cooperate if they anticipate future interactions with the same people. This is one reason people are likely to wear a mask at work with familiar colleagues but skip it when they shop in a grocery store.

Repeated interactions also tend to foster the reciprocation of kindness, and hence encourage more and more altruistic behavior. Vaccine and mask mandates by companies, schools, and other places where people see one another repeatedly are sound practices not only because they indicate respect for customers and employees, but also because they promote the reciprocal altruism that leads to optimal public-health practice more generally.