CHICAGO, Dec 3 (Reuters) - The male half of opposite-sex twins shares a similar risk as his sister of developing the eating disorder anorexia later in life, a finding that researchers said on Monday suggests exposure in the womb to a female sex hormone may be responsible.

No one knows why girls and women are 10 times more likely than males to develop anorexia, which is a pathological fear of becoming fat that can lead to self-starvation and even death.

Many experts cite psychological factors behind the phenomenon such as low self-esteem and society's promotion of thinness, especially for women.

About 11 million Americans suffer from anorexia or bulimia nervosa, another eating disorder where binge eating is followed by purging either by vomiting, laxative abuse, or compulsive exercising.

In the study of 4,478 sets of fraternal, opposite-sex twins born between 1935 and 1958 in Sweden, a similar number of individuals of both genders were subsequently diagnosed with anorexia nervosa.

The chances that both opposite-sex twins would develop anorexia was about the same as the risk for women in the general population, according to the study published in the Archives of General Psychiatry.

Sets of male twins did not have any higher risk than men in the general population.

"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," wrote study authors Marco Procopio of the University of Sussex, Brighton, England, and Paul Marriott, of the University of Waterloo, Ontario, Canada.

"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," they wrote.

(Reporting by Andrew Stern; editing by Michael Conlon and Philip Barbara)

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