Most people think of eating disorders, such as bulimia and anorexia, as the result of contemporary beauty ideals fabricated by the media and the fashion industry. The underlying argument is quite simple: teenage girls (and increasingly boys) grow up admiring celebrities, which appear to come in smaller sizes every year. Thus there is growing pressure on young people to maintain a petit body structure; somewhat paradoxically, this pressure co-exists with unprecedentedly high rates of obesity in most Western societies. Are size-zero models really to blame for this? And why are we not getting thinner then?
Although over-eating and under-eating rates are both increasing, it is important to examine not only their social determinants, but also how these interact with personal factor. In the past five to seven years, there have been robust psychological studies highlighting important genetic factors underlying the etiology of eating disorders. These factors also suggest that the degree to which the social context (including media-endorsed stereotypes) disrupts an individual’s nutritional habits is by and large dependent on his or her personality.
One of the most compelling sources of evidence for the heritability of eating disorders is a study published last year by Stephanie Zerwas and Cynthia Bulik. The authors review several decades of genetic research, including family, twin and adoption studies. Of these studies, the most powerful methodology for disentangling the effects of nature and nurture is no doubt twin designs. Given that identical twins are twice as similar to each other in their genetic makeup as non-identical or fraternal twins are, a comparison of eating disorders rate between identical and non-identical twins should enable us to estimate the degree to which nature (shared genes) influences eating disorders. The authors found that, among white Caucasians, eating disorders have a substantial hereditary basis. For example, in a study conducted with a US-representative sample of Minnesota twins, Klump (2009) reports that 50 percent of the variability of individual differences in eating disorders can be attributed to genes. That said, until puberty genes contribute 0 percent risk, which means that a potential vulnerability towards eating disorders is only “activated” when individuals reach puberty (no doubt because of biological changes as well as changes in environments and interests, such as sexual relationships).