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Researchers at the University of Colorado Boulder have identified a genetic component that could help explain why women are more likely to perceive themselves as overweight than similarly proportioned men.“Among two comparably sized individuals, women are more likely to consider themselves to be overweight,” said Jason Boardman, professor of sociology at ���������� Boulder and director of the Health and Society Program at the Institute of Behavioral Science (IBS).

“This, we believe, is the social context that enables genotype to explain weight identity among women but not men,” he said.

Jason Boardman

Boardman and colleagues from ���������� Boulder, the University of Illinois at Urbana-Champaign and Brown University published these findings in the September edition of the journal Social Science & Medicine.

Genotype, which refers to all of an individual’s DNA, is the stuff that makes your eyes a certain color and determines whether you can roll your tongue. The Boardman team’s work suggests that it even plays a role in some behavioral traits, such as how women view their bodies.

To say this definitively, Boardman and his colleagues compared the self-perceptions of twins. Twin studies are powerful tools, commonly used to determine if certain traits are due to nature (genetics) or to nurture (environment). If, for instance, two identical twins (sharing the same DNA) differ less in a trait than fraternal twins (sharing only one-half of their DNA), genes are said to significantly influence the trait.

On the other hand, if identical and fraternal twins differ within a trait not at all or at roughly the same levels, the trait is said to be influenced more by environmental factors.

In this twin study, researchers first collected objective weight data—body mass index (BMI)—from identical and fraternal twins. Participants were asked how they felt about their weight, with responses ranging from “very underweight” to “very overweight.” The investigators then compared individuals’ feelings about their weight to their objective BMI.

Researchers found that identical and fraternal twins show roughly the same level of variability with regard to subjective weight reporting. This suggests a more pronounced environmental impact and less of a genetic one.

Researchers went one step further:

“We then split models between men and women, and we again calculate the heritability, or the amount of variation in weight identity that’s due to genes. We find on average that those estimates for men are small and for women remain heritable and significant,” said Robbee Wedow, a graduate student in sociology and the study’s lead author.

“We explain this by saying that there is likely something about the gendered context that allows us to see genetic effects for women, but not for men. It’s something about the social construction of gender,” Wedow said.

So how do genetics figure into the picture?

The study reports that genes can affect people’s willingness to incorporate a given trait into their identity. Being politically liberal or conservative, for instance, may be important to one individual’s identity, but not to another’s, and that identity has a genetic component, previous research has found. Boardman and Wedow extend this logic to their findings:

“Individuals vary in the sensitivity of identify formation processes to objective facts,” the researchers write. “The extent to which genetic variance plays a role in weight identity depends on gender.” So the tendency to incorporate objective weight into one’s identity is heritable across genders, the researchers find, adding that the social context makes this more likely for women.

The findings have implications beyond the social realm. Subjective weight identity can predict future health outcomes; if the tendency to report the same subjective identity despite increases or decreases in weight is a heritable trait, individuals may be less affected by traditional health interventions.

“Individuals who have a mismatch between their subjective weight assessment and their physical weight assessment are less likely to engage in health protective behaviors related to exercise and nutrition. We see this with type-II diabetes and body mass,” said Boardman.

While Wedow emphasized that the extent to which genetic variance plays a role in weight identity depends on gender, “It is really important to remember that this amount of variation in the trait is due to genetic effects, but that it will always be changes in social institutions that drive outcomes more than genes.”

Boardman added: “Changes in the typical body size of people around them may have a strong effect on everyone, regardless of genotype. This is why we tend to emphasize these broad social-level interventions rather than seeing obesity as a personal illness or morbidity that needs medical attention.

The study was funded in part by the Eunice Kennedy Shriver National Institute of Child Health and Human Development, which is part of the National Institutes of Health, as well as the University of Colorado Population Center and the National Science Foundation’s Graduate Research Fellowship Program.