The very thought of romantically kissing your sister or brother is repugnant! But why?

Of course there are strong societal taboos against sibling smooches. Sigmund Freud held that these taboos are all that prevent us from having incestuous sex. Now a joint research team from the University of California, Santa Barbara and the University of Hawaii begs to differ with the legendary psychiatrist.

Santa Barbara's John Tooby and Leda Cosmides and Hawaii's Debra Lieberman say there are two things we do that turn us off sexually to a sibling, even one who is only a half-sib:

• Spending time in the same household.
  
• Watching your mother care for your brother or sister.

In this study, 600 volunteers were asked many questions that were jumbled together so they couldn't figure out the topic that 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," Cosmides said, adding they also asked the volunteers about ethical dilemmas and sexual relationships with siblings. Included in the group were people who had never shared a home with their sibling, such as full- or half-siblings who were born 10 or even 20 years apart.

"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 concluded. The strongest effect was watching a 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 told Reuters. Women are especially sensitive to this.  

What about Freud? He theorized that we are sexually attracted to our relatives, siblings and parents, but it's only the force and taboos of culture and society that keep us from committing incest. Cosmides thinks she understands why Freud thought this. He had a surrogate mother--a wet nurse who breastfed and cuddled him. "Who their brain thinks is mom is different from who they consciously believe is mom," Cosmides explained to Reuters. "For them it is quite reasonable that they have an attraction to their mothers."

The study findings were published in the journal Nature.

--From the Editors at Netscape