The phenomenon of social contagion—that information, ideas, and even behaviours can spread through networks of people the way that infectious diseases do—is both intuitively appealing and potentially powerful.

Consider the classic and familiar example of a standing ovation at the end of a theatrical performance. It often starts with a single individual or two standing up, and breaking with the larger crowd, who remain seated. Then slightly more audience members join in the ovation. And then a few more, and a few more until suddenly – whoosh – there’s a cascade and suddenly everyone is standing. Of course, not everyone who does so necessarily feels equally enthusiastic about the performance, but they stand anyway. Most of us, in such a situation, will eventually stand, without being asked to do so. And our having done so makes it all the more likely that those around us will as well. In simple terms, the ovation is 'catching' — it acts like a disease, coursing through a population of people in the theater just like a bacterial or viral infection. And that's social contagion.

This tendency to copy applies not just to behaviors, but also to beliefs, emotions, and cultural norms that we internalize. What we believe, how we act, and who we connect with are all influenced by those around us; and that, in turn, influences all of those people with whom we come in contact.

These webs of influence are what give rise to this season's 'must have' fashions, to the formation of ‘conventional’ political wisdom and to that strange synchrony where all of your friends send you the same viral video on the same day. But social contagions cause less obvious effects. For example, research suggests that your friend's friend's health-related behaviors can significantly shape your health-related outcomes. Just like your friend’s friend’s happiness, financial standing, voting behavior, and acts of kindness are also contagious, spreading throughout your social networks.

All of which leads to the question, how can we harness social contagion to promote social good?