The study, a summary of which was published on Science Daily, asked identical and fraternal twin sisters, questions about their own selfless actions and how desirable they found noble or sacrificial acts performed by others. The participants were not aware of what the study was testing.

“Identical twins share the same genes 100%, while non-identicals have a 50% chance of sharing any given gene,” says lead author Tim Phillips, a psychology professor at the University of Nottingham. If the identical twins came up with the same answers more often than the fraternal twins did, then it’s more likely that genes were the reason. “And that’s exactly what we found,” says Phillips.

Of course, altruism wasn’t attractive to everyone. But because some people are genetically predisposed to find selfless men hot — perhaps assessing that they’re more likely to help with the vicissitudes of bringing up kids — that trait may have been able to survive in the human gene pool through generations. “According to our theory females would have often had a limited choice of quality, altruistic males due to the cost of child-rearing so there would have been competition to attract and retain such males,” says Phillips. “But this is at present informed speculation.”