How do siblings recognize each other?

Unconscious suppression in childhood decides

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Siblings usually have a special relationship: on the one hand they are willing to assist each other selflessly, on the other hand, there is a deep-seated inhibition to engage in sexual intercourse. But how are these instinctive behaviors fixed? Is it just social impact or are biological factors playing a role? A new study, now published in Nature, has identified crucial key stimuli.


For a long time fundamental theories in behavioral science and psychology have said that biological affinity forms the basis of family cohesion. The consciousness of "the same blood being" forms the basis not only for the cohesion and mutual support of the family members, but also causes deep-rooted taboos such as the sexual relationship among siblings. But how do our instincts realize that we are really dealing with a relative? Which key stimuli determine from childhood who we consider to be "associated"?

Unconscious stamping by key stimuli

A research team from the University of California at Santa Barbara has investigated this question in a study of more than 600 volunteers. The results confirmed what had been previously assumed: The childhood environment determines who we recognize as related and follows unconscious behavioral patterns that were already established in our hunter-gatherer ancestors. These behaviors are thus less the result of deliberate social imprinting by parents or fellow human beings, but rather patterns that have evolved as an unconscious response to the recognition of certain key stimuli to the relationship.

In older siblings, the sight of the mother caring for the younger siblings apparently activates a mechanism in the brain that triggers both altruism and sexual inhibition against the sibling. For younger siblings, on the other hand, it seems that the time they spend living together with their other siblings, from early childhood to puberty. This cohabitation triggers in the brain the behavior patterns of altruism and sexual inhibition both towards genetically related and to step-siblings. display

Not just culturally determined

According to the researchers, both altruism between siblings and their instinctive reluctance to have a sexual relationship with each other has developed through natural selection. "According to ancient wisdom, Darwinism applies to people only in the physical sense, not in the social sense, " explains John Tooby, a professor of anthropology at the University of California. "Now we see the development of a mechanism that regulates important aspects of human social behavior." Similar mechanisms have been found in many animal species, but their transmission to humans has always been controversial.

With the current study, the researchers have helped to end this debate. Because their data suggest that not purely cultural, conscious taboos are crucial for the development of these patterns of behavior, but rather unconscious, established in the course of evolution key stimuli and imprints. The results could also help protect potential incest victims better. "Siblings who have lived separated for a long time have not been exposed to these key stimuli, so their brains do not recognize the other as related."

(University of California - Santa Barbara, 16.02.2007 - AHE)