A study in twins reveals how genetic factors make some girls more prone to buying into society's fascination with thinness -- and therefore more at risk of developing eating disorders.

PROBLEM: We often (justifiably) blame media and general societal attitudes for the prevalence of eating disorders in Western culture. But why is it that some women strongly subscribe to the ideal of thinness endorsed by airbrushed photographs and stick-thin celebrities -- the first step down the path to body dissatisfaction and the ultimate possibility of developing anorexia, bulimia, or just an unhealthily preoccupation with weight -- while others seem immune?

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METHODOLOGY: In order to parse out the roles of genetic and environmental factors in the internalization of these attitudes, researchers interviewed over 300 female twins, aged 12 to 22, and used a standardized scale to determine the degree to which each girl desired to look like people from magazines, movies, and television. Then, controlling for age and BMI, they compared the genetically identical, monozygotic twins to the dizygotic twins, who only share half of their genes. 

RESULTS: The identical twins were more similar to one another in their internalization of the thin ideal than were the fraternal twins, but all pairs of siblings, despite growing up in similar environments, differed from one another in the extent to which they desired thinness.

CONCLUSION: Over 40 percent of the desire to be thin can be attributed to genetics. General environmental factors have no measurable effect on the internalization of the thin ideal, but nonshared environmental factors -- those that each sibling experiences individually -- significantly influence it.   

IMPLICATIONS: For general cultural messages about thinness being desirable to translate into eating disorders, they must be first be internalized. These findings demonstrate that genetic predispositions, along with nonshared environmental factors, contribute to this crucial middle step, and are in fact risk factors that make some women more susceptible to this progression than others. 

Although the researchers' methodology didn't allow them to study specific risk factors, they hypothesize that the genetic predispositions to thin idealization may take the form of personality traits, such as perfectionism, which are known to be heritable. Nonshared environmental factors, for their part, could be anything from parents encouraging only a heavier twin to lose weight to the influence of differing activities and friend groups.

Even though these factors contribute to thin idealization more than shared environmental factors, the broad culture climate is of course still important. It's what established the ideal of thinness in the first place. As the authors explain, "in cultures where thin-ideal exposure is rare, even individuals who have genetic predispositions for thin-ideal internalization may never internalize the thin ideal because they are never exposed to it." Better understanding the risk factors for eating disorders is one part of the solution; changing the messages that all girls, regardless of susceptibility, are exposed to, is just as crucial.

The full study, "Genetic and Environmental Influence on Thin-Ideal Internalization," is published in the International Journal of Eating Disorders.