For boys, having a twin sister might actually increase the risk for developing anorexia nervosa -- commonly referred to simply as anorexia.
Researchers Dr. Marco Procopio and Paul Marriott analyzed more than 4,000 Swedish twins born between 1935 and 1958. In their research, they found that overall, women are still more likely than men to develop anorexia, an eating disorder in which people literally starve themselves due to a fear of gaining weight.
But they also found that for men having a twin sister imparts a higher risk of anorexia compared to their peers -- a risk "that is not significantly different from the females in the pair" reported the study authors.
Why might these men be at greater risk? Procopio and Marriott suggested that hormones could be a major factor.
Specifically, they wrote in the study, the male of the opposite-sex twin pair may be exposed to more estrogen in the uterus from the development of their female twin.
Experts not affiliated with the study agree. The increased estrogen "seems to be the only probable mechanism causing this difference," said Dr. Arnold Andersen, professor of psychiatry at the University of Iowa's School of Medicine in Iowa City.
This hormone mix affects the women of the twin pair as well. Procopio and Marriott mention in their study that the female of the male-female twin pair may have less risk of developing anorexia than a typical woman.
It appears that "the female increases vulnerability in the male, while the male decreases vulnerability in the female," Andersen says.
However, the study authors added that the reason for the increased risk for anorexia in the male twin does not have just one simple answer.
"[It] could be explained either by the effect on the prenatal environment of the presence of a female fetus in utero, or the effect of growing up with a female twin," the study authors noted -- adding that perhaps even a mix of these two factors could be to blame.
According to Andersen, it is a combination; genetics and the environment working together.
This study is "confirmation of the effect of what's called epigenetics" he said, adding that pegging something as complex as anorexia risk to genetics or the environment alone would yield only part of the story.
In general, women are more likely to develop anorexia; the study authors noted that the condition is "approximately 10 times more likely common in females than in males. But there are a growing number of men who are being diagnosed with this disorder -- and not just men with twin sisters.
Anorexia survivor Gary Grahl began dealing with the disorder during his sophomore year of high school. He played baseball for the high school team and was already catching the attention of Major League scouts when his anorexia manifested itself.
"In my sophomore year, I developed really weird exercise and diet patterns," Grahl said. "I wouldn't go out; I would just stay at home and exercise and diet."
Grahl now weighs 165 pounds -- a relatively normal weight for his 5-foot-8 frame. But he said that during his sophomore year in high school his weight dipped down to 115 pounds.
It was at this time that he was diagnosed with anorexia, and he began a battle for his life.
"From 1985 to 1989, I was in and out of the hospital six times," he said.