October 29, 2009

Culture, Genes Affect Behavior

Evolutionary outcomes can be influenced by culture as well as genes, according to a new study released on Wednesday comparing societies around the world.

Natural and social sciences are rarely brought together, but the study analyzed the interaction across 29 countries of two sets of data, genetic and cultural.

The researchers discovered that the majority of people living in countries considered to be collectivist have a certain mutation within a gene that regulates the transport of serotonin, a neurochemical known to have a substantial affect on mood.

For example, in China and other east Asian nations, up to 80 percent of the population carry this so-called "short" allele, or variant, of a stretch of DNA known as 5-HTTLPR.

Previous research had revealed that the S allele is greatly connected to a variety of negative emotions, such as anxiety and depression.

What is even more serious is its association with the impulse to protect oneself and avoid harm.

On the flipside, in countries of European origin that tend toward self-expression and individuality over group goals, the long or "L" allele dominates, with only 40 percent of people carrying the "S" variant.

Published in Britain's Proceedings of the Royal Society B: Biological Sciences, the study provides a new take on how such a disparity could have taken place.

Without taking discredited ideas linking genetics and race into consideration, the researchers indicate that culture and genes may have worked together over time to mold the process of natural selection, assisting individuals and their societies with the ability to survive and thrive.

Scientists believe that cultures exposed to high levels of deadly pathogens, such as ancient cultures in Asia, Africa and Latin America might have leaned more toward collectivist norms in order to stave off disease.

Such a social transformation would have then favored the gradual dominance of the risk-avoidance S allele.

"We demonstrate that evolution is operating at least two levels," said Joan Chiao, a professor at Northwestern University in Chicago and lead author of the study.

"One is biological, which is well understood. But there is also a level where cultural traits may have been selected for themselves, emerging in congruence with the selection of different types of genes," she explained over the phone.

A good example of the "culture-gene co-evolutionary theory" involves the drinking of cow's milk, which is not something humans would intrinsically adapt to do.

Milk consumption eventually led to both the genetic selection of protein genes in cattle, and a gene in humans that encodes lactase, an enzyme that can break down the otherwise indigestible lactose in dairy.

When it comes to the collective cultures and the S allele, Chiao said, "we don't make a strong claim on the chicken-or-egg problem" of which came first.

"What we are proposing is that cultural and genetic selection actually operate in tandem, and that you can view human behavior as a product of culture-gene co-evolution," she said.

The study also contends that collectivist cultures could assist in protecting against the genetic risk of depression that comes with having the S allele.

"Such support seems to buffer vulnerable individuals from the environmental risks or stressors that serve as triggers to depressive episodes," said Chiao.

Since the United States and Europe have higher rates of anxiety and mood disorders even though they have the L allele, this may suggest that it comes from the stress of living in highly individualistic cultures, she said.

"People have treated natural selection as a rationale for looking for universal traits, across populations and species."

"But what really matters is the diversity across populations and species which may help us better understand how natural selection has operated at both individual levels and ecosystem levels," she said.


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