4 October 2012

Twin-study shows eating disorders influenced by genetics

by Will Parker

Genetics may make some women more vulnerable to the pressure of being thin, say Michigan State University researchers who also found media exposure to idealized body size plays a less significant role than previously thought.

For the study, the researchers focused on the psychological impact of women buying into the perceived ideal of thinness, which the researchers call thin-ideal internalization. "We're all bombarded daily with messages extoling the virtues of being thin, yet intriguingly only some women develop what we term thin-ideal internalization," said Jessica Suisman (pictured), lead author on the study. "This suggests that genetic factors may make some women more susceptible to this pressure than others."

To explore the role of genetic factors in whether women "buy in" to the pressure to be thin, the idealization of thinness was studied in sets of twins. More than 300 female twins aged between 12 and 22 took part in the study. The researchers measured how much participants wanted to look like people from movies, TV and magazines. Once the levels of thin idealization were assessed, identical twins who share 100 percent of their genes were compared with fraternal twins who share 50 percent.

The results show that identical twins have closer levels of thin idealization than fraternal twins, which suggests a significant role for genetics. Further analysis shows that the heritability of thin idealization is 43 percent, meaning that almost half of the reason women differ in their idealization of thinness can be explained by differences in their genetic makeup.

In addition to the role of genes, findings showed that influences of the environment are also important. The results showed that differences between twins' environments have a greater role in the development of thin ideal internalization than wider cultural attitudes, which women throughout Western societies are exposed to.

"We were surprised to find that shared environmental factors, such as exposure to the same media, did not have as big an impact as expected," Suisman said. "Instead, non-shared factors that make co-twins different from each other had the greatest impact." She speculated that non-shared environmental influences might include involvement by one twin in a weight-focused sport like dance or one of the twins having a friendship group that places importance on weight.

"The broad cultural risk factors that we thought were most influential in the development of thin-ideal internalization are not as important as genetic risk and environmental risk factors that are specific and unique to each twin," she concluded.

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Source: Michigan State University