Findings have established that having a twin sister appears to be quite risky for men as they are more prone to develop the eating disorder anorexia nervosa than other males, including those who have a twin brother.
The revelation comes into a report that supports the hypothesis that the exposure to female sex hormones in the womb may be correlated with the risk for anorexia nervosa.
The authors of the report said: "Anorexia nervosa is approximately 10 times more common in females than in males,"
"The reasons for this difference are 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."
Data from a study of Swedish twins born between 1935 and 1958 was analyzed by Marco Procopio, M.D., M.R.C.Psych., of the University of Sussex, Brighton, England, and Paul Marriott, Ph.D., of the University of Waterloo, Ontario, Canada.
They used two sets of diagnostic criteria, one broader and one more narrow, to find out which twins had anorexia nervosa.
On the whole, female twins were more prone to develop anorexia nervosa than male twins. The only exception was among males who had a dizygotic (fraternal) twin sister.
"In fact, their risk is at a level that is not statistically significantly different from that of females from such a pair," the authors wrote.
Out of 4,478 dizygotic opposite-sex twins, 20 females and 16 males had anorexia nervosa using narrow criteria and 32 females and 27 males qualified under the broad criteria.
The risk for such female twins was not much different than that of other female twins.
"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 the authors.
They added: "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 results of our study are compatible with the hypothesis that intrauterine exposure to sex hormones might influence neurodevelopment, affecting the risk of developing anorexia nervosa in adult life."
"This might be a factor contributing to the higher risk of developing anorexia nervosa in females."
The report was published in the December issue of Archives of General Psychiatry, one of the JAMA/Archives journals.