But new evidence suggests that genes may be the most important factor in determining whether a girl will develop an eating disorder. Research presented at the New York City meeting this spring strongly suggests that environmental factors and genetics work together in diseases like anorexia and bulimia. Doctors have long known that eating disorders run in families, but it was usually assumed that family environment was the reason. Perhaps a lack of parental care contributed to the diseases, or maybe mothers passed their bad eating habits to their daughters. But when Bulik interviewed 2,000 twins, she found strong evidence for a genetic cause.
Identical twins are essentially clones with exactly the same genes.
But fraternal twins are no more related than siblings, and share only 50 percent of their genes. So if a characteristic is more common among identical twins than fraternal twins, it is almost surely genetic. That was the case in Bulik's study of eating disorders. Based on her research, Bulik found that bulimia is 83 percent genetically influenced. She later found that anorexia nervosa is 58 percent genetic.