Babies who go on to develop anorexia may be programmed in the womb by their mother's hormones, evidence suggests.
Male twins may be influenced by their sister
Women are usually much more likely than men to have the eating disorder, but a University of Sussex study found men with a female twin were more at risk.
This suggests the hormones released to aid female development may be key.
Commenting on the Archives of General Psychiatry study, a UK expert said other factors in childhood and adolescence remained important.
It is estimated that up to 90,000 people will be receiving treatment for eating disorders in the UK at any one time, with many other cases going undiagnosed.
No-one is sure why women are more prone than men. Some experts suggest that the pressures of modern society are partly to blame while others look at brain changes much earlier in life.
Research into twins is a way to examine the factors involved, as the single most important period for brain development is during the months of pregnancy.
Dr Marco Procopio, from the University of Sussex, worked with Dr Paul Marriott from the University of Waterloo in Canada to look at information drawn from thousands of Swedish twins born between 1935 and 1958.
Overall, as expected, female twins were more likely to develop anorexia than male twins.
The only exception was among mixed-sex twins, where the male was as likely to develop anorexia as the female.
The researchers wrote that the most likely reason was because of sex steroid hormones released into the womb during pregnancy.
"A plausible explanation for this phenomenon is that in pregnancies bearing a female foetus, a substance is produced, probably hormonal, that increases the risk of having anorexia nervosa in adulthood.
"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."
Susan Ringwood, from the Eating Disorders Association, said: "There is a lot of research now into the effects of hormones on brain development.
"This is an interesting study, although it's important that we also look at other factors such as perception of body-image in childhood and adolescence."