New research is suggesting that a male twin who shares the womb with a female twin is almost as likely as twin girls to be diagnosed with anorexia.
The researchers say the suggestion is that prenatal conditions influence the likelihood of developing the eating disorder.
Researchers Dr. Marco Procopio of the University of Sussex in Brighton, England and Dr. Paul Marriott of the University of Waterloo in Ontario, Canada examined data from a Swedish study on thousands of twin pairs born between 1935 and 1958.
Some experts believe that the increased level of estrogen is the reason for the increased risk of anorexia in males and the researchers wanted to determine whether such prenatal exposure to sex hormones might be a factor in eating disorders.
The eating disorder anorexia nervosa is ten times as common in females as in males but the reason for this sharp gender difference is unclear.
Their analysis revealed that men with a female twin were much more likely to develop anorexia than men from same-sex twin pairs and the anorexia risk for these male twins was not significantly different from that of their female twin.
However it was also seen that females with a male twin were no less prone to anorexia than females from same-sex twin pairs, which say the researchers, suggests that exposure to male hormones does not protect against the eating disorder.
Dr. Procopio says even though there is a genetic disposition to anorexia, that does not fully explain the case, as on a genetic basis alone an identical twin of an anorexia patient would also have the disease, which is not so.
Other research has suggested that upbringing may be a factor in the gender difference in rates of occurrence of the disorder but that has yet to be proven.
Dr. Procopio says their findings may lead to ways to treat and even prevent anorexia, which has become one of the more common psychiatric problems.
The research appears in the Archives of General Psychiatry, December 2007.