Is there an evolutionary benefit of friendship?

Chris Smith put this question to astronbiologist Lewis Dartnell...

Lewis - One of the central things that psychologists have been trying to understand about human behaviour is why we're just so good at helping each other out. We're very cooperative, we're very altruistic to each other and one of the best explanations for why people help each other out and indeed why animals of the same species help each other out in general is called reciprocal altruism. So if I help you out, then a bit later you help me out and we've both increased our fitness and helped our own survival. The problem with that though is you have to run some kind of ledger, or log about who has helped you in the past and who has cheated you by taking your help and then not returning the favour later. So you don't want to do too much of that, of trying to remember from all the other people in your community or in your society who cheated you or who's helped you in the past. So if you develop a particularly close relationship or association with another individual, i.e. you develop a friendship, you no longer really have to bother remembering every time you've helped them or they've helped you. You just come to trust them that they'll return the favour later. It sort of simplifies that whole behavioural problem.

Xander - Sounds like a way of trying to not have to pay your friends back for things!

Lewis - I think you can assume that by the time you've forged this close relationship with someone, which we call in humans "friendship", you kind of trust them. And if someone cheats you enough times and doesn't return that favour for the 16th week running, the friendship breaks down. They cease to be your friend, right?