Students shown pictures of people with similar facial features said they looked trustworthy but unattractive, a study has found.
A male face is altered to bear a resemblance to a female participant
Research at Aberdeen University involved 144 students being shown a series of paired faces.
One of the images was altered to look like them, while the other resembled another person.
Dr Lisa DeBruine, who conducted the research, said people trust kin but avoid them in a sexual setting.
The similarities in the images were not spotted by the students who were asked to decide which were the most trustworthy.
However, when they were asked to decide which they found most attractive for a relationship, they chose the face they did not resemble.
Dr DeBruine said: "Basically this supports the idea that people, perhaps unwittingly, detect facial resemblance.
"It means to them, on some level, 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."
Dr DeBruine carried out the research at McMaster University in Canada and is continuing her work at Aberdeen and St Andrews universities.
A total of 78 female and 66 male undergraduates took part in the research in 2003.
They had their photographs taken earlier for a separate experiment and these were used during the research.
As well as trustworthiness, students were asked to select faces they preferred for a short-term relationship, such as a single date or a one-night stand.
Long-term relationships included someone you might want to move in with or marry.
The results of the study, Trustworthy but not lust-worthy: Countext-specific effects of facial resemblance, appear in the Biological Series of the Proceedings of the Royal Society of London.