For every hundred babies born in Europe, one or two of them are quickly followed by a twin. From a cold evolutionary point of view, twins look like a win for their mother. History, after all, is written by individuals who are best at passing on their genes, so having more children at once seems like a good strategy. So why are twins so rare?

The standard answer is that giving birth to twins, and raising them, is difficult. Complications during pregnancy and delivery could kill both mother and children. In a classic 1990 paper, a bird specialist called David Anderson expanded on this idea. He suggested that twins are the result of an evolutionary bet-hedging strategy gone wrong.

Anderson knew that