Giving birth to twins is rough, especially in rural regions. They tend to be born smaller and weaker than single babies, and their mothers have more complications during childbirth. So why did twinning evolve?

A new study in Gambia finds that women who have twins also tend to have single babies that are heavier than average at birth, which makes them more likely to survive. Since the 1950s, the U.K. Medical Research Council has been collecting data and providing medical care in Gambia. It’s a highly unusual data set, says evolutionary anthropologist Rebecca Sear of Durham University in the United Kingdom, with a length and thoroughness that’s “unheard of for populations without good access to medical care.”

Twins are at an evolutionary disadvantage for themselves and their mothers, but their siblings might be better off in the long run.

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