Imagine that you’re walking along a quiet street and you see a wallet lying on the pavement. Would you take it? Now imagine a slightly different situation – the wallet has a red circle drawn around it. While many people would be tempted in the first scenario, almost no one would touch the wallet in the second. The key difference is that the lone wallet was most likely dropped accidentally by a passer-by buy the encircled wallet was clearly placed and marked by someone, who may well still be watching. And there is nothing that keeps people more honest than the presence of a watchman.

Social experiments like these are of great interest to biologists because they tell us more about