The difference between a social butterfly and an introverted recluse may be in the genes. But it may be the genes that scientists least suspect.

Researchers say the findings indicate that social behavior, such as shyness and boldness, may be shaped by seemingly unimportant DNA often referred to as "junk" genes.

The study, published in this week's issue of Science, is the first to show a link between junk genes, otherwise known as microsatellite DNA, and social behavior in different species.

"The variability in the microsatellite could account for some of the diversity in human social personality traits," says researcher Elizabeth Hammock, of Emory University, in a news release. "For example, it may help explain why some people are naturally gregarious while others are shy."

Researchers say the findings may also lead to a better understanding of human social behavior and disorders such as autism.

Read Web MD's "Is Shyness a Mental Disorder?"

Genes May Affect Shyness

In the study, researchers looked at how microsatellite DNA affects social behavior in male prairie voles, a type of rodent.

Previous research has shown that the male prairie vole is highly social, forms lifelong attachments to a mate, and shares parenting duties with the female. But the closely related montane vole does not bond with a mate nor contribute to parenting duties and seems socially indifferent.

To see if these differences may lie in seemingly nonfunctional genes, researchers bred two groups of prairie voles with short and long versions of the junk DNA.

When they examined the behavior of the male offspring, they found that micro-satellite or junk DNA length affected gene patterns in the brain. These changes corresponded to differences in social behavior.

For example, males with long junk DNA had higher levels of receptors in the brain involved in social behavior and parental care. These moles were more likely to form bonds with mates and spent more time with their offspring than those with shorter sequences in this DNA.

"Because a significant portion of the human [genetic makeup] consists of junk DNA and due to the way microsatellite DNA expands and contracts over time, microsatellites may represent a previously unknown factor in social diversity," says Emory researcher Larry J. Young, PhD, in the release.

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By Jennifer Warner, reviewed by Michael W. Smith, MD

SOURCES: Hammock, E. Science, June 10, 2005; vol 308: pp 1630-1634. News release, Emory University Health Sciences Center.