Revulsion and taboo against sex with family members is a natural instinct and isn't taught, say US researchers, whose findings challenges some basic tenets of Freudian theory.
Cognitive psychologist Dr Leda Cosmides from the University of California Santa Barbara and colleagues report their findings in today's issue of the journal Nature.
"We went in search of a kin detection system because some of the most important theories in evolutionary biology said such a thing should exist," Cosmides was quoted by ABC online, as saying.
"It should regulate both altruism and incest disgust." The research team found that humans have an inbuilt system that does both.
"[Our] data shows that the degree to which we feel those things is governed by these cues that, for hunter-gatherers, predict whether somebody is a sibling. And it works regardless of your beliefs - who you are told who your siblings are," she says.
Cosmides and her colleagues tested 600 volunteers, asking them all sorts of questions jumbled together so they would not know what was being studied.
"We asked them how many favours did you do for this particular sibling in a month. We asked if this sibling needed a kidney, how likely would you be to donate this sibling a kidney."
And 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.
"If you co-resided with them for a long time as a child, you'd treat them as you'd treat any full sibling. This seems to operate non-consciously," Cosmides says.
Especially strong was the effect of watching one's mother care for a younger child.
"They would be very altruistic towards that baby and they'd be grossed out at the idea of sex with that baby as an adult," Cosmides says.
She says women are especially sensitive to this.
"One whiff of possible siblinghood and that's it for you if you are a woman," says Consmides.
The study contradicts the teachings of Sigmund Freud, who described Oedipal urges and conflicts, Cosmides says.
"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 says.