London, March 23: We tend to trust people who we look like but are not sexually attracted to, according to a study that shows the extent to which we use the face to work out how genetically related someone else is to us.
The study entitled 'Trustworthy but not lust-worthy' shows that we are attracted by more than just someone with whom we feel something in common and underlines current evolutionary theory, which predicts we should trust our relatives but not have romantic relationships with them because of the dangers of inbreeding.
The conclusions of Dr Lisa DeBruine of the Universities of St Andrews and Aberdeen are published today in the Proceedings of the Royal Society of London, Biological Series.
Students were shown pairs of face photographs, one that was subtly altered to resemble the student looking at the image and one that resembled another person.
The similarities were not noticed by the 150 undergraduates who were asked to decide between pairs of faces as to which was the most trustworthy. The majority selected the 'lookalike' face from each pair. But when asked to decide which they found the most attractive for a relationship, the students preferred the face they did not resemble.
Dr DeBruine, said: 'This supports the idea that people detect facial resemblance. It means to them that this person is 'family' and they are more trusting of them.
'However the similar faces were also deemed sexually unattractive by the students. These results back the notion that people trust kin but avoid them in a sexual setting due to the costs of inbreeding.'