Humans aren’t sexually monogamous in the sense that many birds are. Geese form lifelong couples and virtually never mate with anyone except their partner. We are termed ‘socially monogamous’ by biologists, which means that we usually live as couples, but the relationships aren’t permanent and some sex occurs outside the relationship. There are three main explanations for why social monogamy evolved in humans, and biologists are still arguing which is the most important. It may be because human babies need a lot of looking after and stable couples can share the parenting burden. Or it could be because men want to stay close to prevent their partners from cheating. And it could also be a strategy that women evolved to discourage men from killing infants that they suspected were not theirs. Monogamy in humans is beneficial because it increases the chances of raising offspring, but it is actually very rare in mammals – less than 10 per cent of mammal species are monogamous, compared with 90 per cent of birds.