Got milk? If so, you also may get bloating, gas, or stomach pain after you drink a glass of it. That’s because most of the world’s adults—an estimated 68 percent—aren’t able to digest it.

Their condition is commonly called lactose intolerance. It stems from a lack of lactase, an enzyme that breaks down the milk sugar lactose. Lactase is present in young children but weakens in most people after weaning, says evolutionary geneticist Pascale Gerbault of University College London.

The enzyme continues to be produced, a trait scientists call lactase persistence, only in small populations of adults around the globe.

Though what sparked the digestion divide is uncertain, Gerbault says, one pattern may shed light: Milk tolerance in adults appears to be more