Dec. 4 (HealthDay News) -- Males with a twin sister are more likely than other males to develop the eating disorder anorexia nervosa, scientists say.

The finding supports the theory that exposure to female sex hormones in the womb may be associated with increased risk for anorexia nervosa, note British and Canadian researchers.

"Anorexia nervosa is approximately 10 times more common in females than in males. The reasons for this difference is not known, and it is likely that their unraveling will represent an important step forward in the understanding of the etiopathogenetic factors involved in the development of eating disorders," Marco Procopio, of the University of Sussex in England, and Paul Marriott, of the University of Waterloo in Canada, wrote as background information in their study.

They analyzed data from a study of Swedish twins born between 1935 and 1958 and found that, overall, female twins were more likely than male twins to develop anorexia nervosa.

The exception was among males with a fraternal (dizygotic) twin sister. The risk level of these males was not statistically different than that of females from such a twin pair.

Among 4,478 fraternal opposite-sex twins, 20 females and 16 males had anorexia nervosa as defined by narrow criteria, and 32 females and 27 males had anorexia nervosa as defined by broader criteria.

"A plausible explanation for this phenomenon is that in pregnancies bearing a female fetus, a substance is produced, probably hormonal, that increases the risk of having anorexia nervosa in adulthood," the study authors wrote. "Because the male half of an opposite-sex twin pair would also be exposed to this substance, it could account for the observed elevated risk in males with female twins. The most likely candidates are sex steroid hormones."

The study appears in the December issue of the Archives of General Psychiatry.

SOURCE: JAMA/Archives journals, news release, Dec. 3, 2007