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.
His weight would ultimately bottom out at 102.75 pounds -- dangerously light for his size but still heavier than the 95-pound goal he had set for himself.
"I started to engage in self-harm.I started to cut myself. I became suicidal," he said.
Grahl does not have a twin sister. But there was no doubt he had anorexia. And he said his tendency toward perfectionism, combined with his overly sensitive nature and a difficulty communicating with others, drove him further into the anorexia trap.
Part of the battle for men, Grahl said, is confronting an illness that normally is associated with women.
"I think it has been considered to be traditionally a female-dominated illness" Grahl said. "But I'm proof that there are men who are struggling with it."
Now that he has recovered from his disorder, Grahl works frequently with the National Association of Anorexia Nervosa and Associated Disorders, sharing his experiences with others who may be at risk. He also wrote a book on his experiences, titled "Skinny Boy: A Young Man's Battle and Triumph Over Anorexia."
"I needed to get the message out to men that, you know, it's OK to talk about this," he said. "A lot of men still feel there is a stigma attached to this condition."
Fortunately for these men, help is available. And the first step is admitting that there is a problem.
"You need professional help, and for men, especially, one of the hardest steps is to accept that you have an eating disorder," Grahl says.