Researchers who wanted to find out why it is not only taboo to kiss your sister, but also disgusting, said last week they have discovered why in a find that challenges some basic tenets of Freudian theory.
The instinct evolved naturally and cannot be taught, John Tooby and Leda Cosmides of UC Santa Barbara wrote in their report in the journal Nature.
They tested 600 volunteers, asking them all sorts of questions jumbled together so they would not know what was being studied. They asked about all sorts of ethical dilemmas, including questions about sexual relationships with siblings.
Among the volunteers were people who had never shared a home with their siblings -- for instance, full- or half-siblings born 10 or even 20 years apart.
What determined incest disgust and altruism was the same -- how much time an older sibling spent watching his or her mother care for a younger one, or how much time the two spent together in the same household.
The study contradicts the teachings of Sigmund Freud, who described Oedipal urges and conflicts, Cosmides said.
"He thought you are attracted to your relatives and your siblings and parents and it takes the force of culture and society to keep you from committing the incest that is in your heart," she said.