WASHINGTON (Reuters) - Researchers who wanted to find out why it is not only taboo to kiss your sister, but also disgusting, said on Wednesday they have discovered why in a discovery that challenges some basic tenets of Freudian theory.

A pair of Kissing Gourami fish kiss at the Shanghai Ocean Aquarium on Valentine's Day in Shanghai February 14, 2007. Researchers who wanted to find out why it is not only taboo to kiss your sister, but also disgusting, said on Wednesday they have discovered why in a discovery that challenges some basic tenets of Freudian theory. REUTERS/Nir Elias

The instinct evolved naturally and cannot be taught, John Tooby and Leda Cosmides of the University of California Santa Barbara wrote in their report in the journal Nature.

Spending time in the same household and watching your mother care for your brother or sister is all it takes. This is all subconscious, of course, reported the researchers, who worked with Debra Lieberman of the University of Hawaii.

“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 said in a telephone interview.

“It should regulate both altruism and incest disgust.”

Humans have an inbuilt system that does both, they found.

“This 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 said.

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 favors 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.”

FORBIDDEN FRUIT

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 said.

Especially strong was the effect of watching one’s mother care for a younger child. “They would be very altruistic toward that baby and they’d be grossed out at the idea of sex with that baby as an adult,” Cosmides said.

Women are especially sensitive to this, added Cosmides, a cognitive psychologist. “One whiff of possible siblinghood and that’s it for you if you are a woman,” she said.

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.

Cosmides said Freud had a possible reason for his own feelings -- he had a wet nurse who cuddled and breastfed him: “Who their brain thinks is mom is different from who they consciously believe is mom. For them it is quite reasonable that they have an attraction to their mothers.”