Anyone who has found herself in a supermarket late on Thursday when a new checkout opens will have no trouble understanding why evolutionary biologists have struggled to explain the development of altruism in humans. In On Natural Selection, Darwin asserts: ‘In social animals [nature] will adapt the structure of each individual for the benefit of the community, if each in consequence profits by the selected change.’ Yet, practically, how could that adaptation first develop outside family groups? How could a lone altruist achieve anything but loss?

It required game theory to calculate how cooperating with strangers could help survival. Classic game theory asks you to imagine yourself in solitary confinement in a Ruritanian prison, charged with plotting against the state. The state is holding another prisoner in a different cell. The state does not have enough evidence to convict either of you (for this to work, you have to recall a time before the ‘war on terror’, when lack of evidence might have constituted a problem for the state). If you both confess, you will both get ten years. If only you confess, she will get twenty years and you will go free, and vice versa. If neither of you confesses, you will both get out after just six months. Do you confess?