When illness in one person is treated or prevented, others to whom that person is connected also benefit. Replacing an elderly man’s hip or providing better terminal care for a woman improves the health of their spouses. Getting people to lose weight or quit smoking encourages their friends to do likewise. Treating depression in one man makes those around him happier. Vaccinating part of a population benefits everyone.

All these effects are reflections of our embeddedness in vast social networks involving our fellow human beings. The benefits (and costs) of healthcare interventions can ripple through the network, creating additional benefits (and costs) for others both near and far.

But the better connected that people are—the more family and friends they have, and the more central they are in the network—the larger these effects. If we were to replace the hip of a hermit or get him to quit smoking no one else would have their …