10/03/2012 03:16 pm ET Updated Oct 03, 2012

Genes Predict Women's Desire To Be Thin, Study Says

If you're a woman living in the U.S., you can hardly turn a corner without walking smack into an image idealizing thinness, whether it's a model on a billboard or a magazine cover splashed with another probably photoshopped celeb who's likely a size zero anyway. But why is it that some women and girls, when confronted with those images, internalize the desire to be thin, while others brush them off? According to a new study, in over 40 percent of cases, women's genetics might be to blame.

In the new paper, published in the International Journal of Eating Disorders on Wednesday, Michigan State University clinical psychology graduate student Jessica Suisman and her colleagues focused on the concept of "thin-ideal internalization," or the extent to which media images get under a woman's skin and begin to affect her behavior.

"Someone with high thin-ideal internalization will walk past a rack of women's magazines, compare herself with the models, feel inferior, think that she has to make changes to look more like them. And that will propel unhealthy dieting behavior and even disordered eating," said Cynthia Bulik, director of the University of North Carolina Eating Disorders Program and author of "The Woman in the Mirror," who was not involved in the study.

"Someone low on the thin-ideal internalization would walk right by, say 'whatever,' and would not let the societal messages influence her self-esteem or self-perception at all," she continued. Women who fall into the former group, or who internalize these influences, are at greater risk for eating disorders or general disordered eating.

To get at the question of what drives susceptibility to the thin-ideal, the authors of the new study interviewed more than 300 female twins, age 12 to 22, using a standard scale to measure their thin-ideal internalization. They compared the results from identical twins, who have the same genes, to fraternal twins, who share only half of theirs. The identical twins were more closely matched in their internalization of the thin-ideal than the fraternal twins, suggesting that genetics play a key role in women's susceptibility. Indeed, the authors concluded that approximately 40 percent of the desire to be thin may be explained by certain genes.

What is unclear at this point is what, exactly, those underlying genetics are.

"This tells us nothing about specific genes," said Suisman. "All it tells us is that there's a genetic component."

"There's probably a small effect from several different genes," she continued.

Research has suggested that genetics influence certain personality factors, like whether a person is a perfectionist. Those same genes might also affect a woman's susceptibility to the thin-ideal.

Bulik cautioned that the new study does not distinguish if the genetic factors that may influence the internalization of the thin-ideal are the same factors that also influence the risk for eating disorders.

"What it does hint at is that some of what we might be talking about is 'genetically determined differences in sensitivity to the environment,'" she said. "Basically, depending on our genetic wiring, we might be more or less sensitive to environmental insults like the thin-ideal."

The researchers also looked at the extent to which environmental or outside factors affect women's susceptibility to the thin-ideal, finding that shared factors -- such as having the same parents, going to the same school or growing up with the same amount of money -- are less important than influences each sibling encountered on their own.

"This means that the influences that are specific and individual are the most important," Suisman said. "It might be one sibling was involved in a sport that really focused on body image, or having a peer group that focused on the media."

Although both experts cautioned that the new findings are highly preliminary, they said they do offer some insight into how we might understand and treat both thin-ideal internalization and eating problems down the road.

"A good take-home message," Bulik said, "is that it pays to be aware early on [of] how sensitive your children might be to these messages."