A growing body of evidence shows that those at the bottom of the socioeconomic ladder are more likely to die prematurely than those at the top. The pattern isn’t unique to humans: Across many social animals, the lower an individual’s social status, the worse its health.

That’s probably proof that these gaps aren’t fully explained by risk factors commonly attributed to humans, such as smoking, drinking or access to medical care, says Jenny Tung, associate professor of evolutionary anthropology at Duke University.