Maintaining friendships is one of the most cognitively demanding things we do, according to Professor of Evolutionary Psychology Robin Dunbar. So why do we bother? Robin has spent his life trying to answer this deceptively simple question. For most of his twenties, he lived with a herd of five hundred gelada monkeys in the Ethiopian highlands. He studied their social behaviour and concluded that an ability to get on with each other was just as important as finding food, for the survival of the species. Animals that live in large groups are less likely to get eaten by predators. When funding for animal studies dried up in the 1980s, he turned his attention to humans. and discovered there’s an upper limit to the number of real friends we can have, both in the real world and on social media.