Why it's gross to kiss your sister

By Maggie Fox, Health and Science Editor

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.

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 --