We feel more altruistic to those who resemble us because in the past our early ancestors assumed that they were related, according to the study.
The instinct dates back to when there were no mirrors and people could learn what their kin looked like only by inspecting the faces of household members.
The study, published in Biology Letters, even found that we were more naturally drawn to people who looked like us than our own relatives, if the resemblance was strong enough.
The researchers came to the conclusion after a study of 70 identical adult twins who, although genetically the same, had over the years grown to look different from each other.
Then they manipulated the photographs of the participants by digitally mixing them with a model’s face so that the images would either resemble them or their co-twin.
Then they asked each one who they would prefer to rescue from danger and which one they would prefer a different sex sibling to marry.
In each case, the person most resembling themselves was preferred almost two thirds of the time–significantly higher than being down to chance alone.
Dr Paola Bressan, of the University of Padova, Italy, said: “Our work shows a stranger who resembles us elicits pro-social regard more than a stranger who resembles a close family member–even one as close as our identical twin, who is, incidentally, genetically identical.”