Male babies surgically turned into girls at birth because of a rare birth defect frequently continue to feel like boys and may eventually switch their gender back to male, even with no knowledge of their history, according to a new study.

The finding, reported in the current issue of the New England Journal of Medicine today, suggests there is an intrinsic, biologically determined feeling of sexual identity that is hard to override through rearing, experts said.

The study also suggests that surgeons should re-examine the practice of altering the gender of babies born with this condition.

"This paper says a very important thing — that it’s difficult for nurture to overcome nature," said Dr. Eric Vilain, associate professor of human genetics, pediatrics and urology at the University of California, Los Angeles. "It shows that the theory of gender neutrality at birth has holes in it."

The study, conducted by Dr. William Reiner, a psychiatrist at the University of Oklahoma and Dr. John Gearhart, chief of pediatric urology at the Johns Hopkins Children’s Center, examined the progress of 16 genetically male children who had been born with a rare birth defect known as cloacal exstrophy.

Babies with this condition, which affects one in 400,000 births, have severe abdominal abnormalities in such organs as the bladder, intestine and genitals.

Because male babies with cloacal exstrophy lack a penis, the decision in these cases traditionally has been to remove the testes, construct a vagina and raise the children legally and socially as girls.

This decision is in line with theories espoused from the 1950s by American psychologist John Money that babies are blank slates at birth, adopting gender identities through the influences of their environment.

For decades, Money’s theory, coupled with the greater ease of surgically turning boys into girls, influenced doctors and parents in deciding the gender fate of children born with malformed or ambiguous genitals.

Until the late 1970s proper studies weren’t performed on children who’d been assigned a sex at birth to assess whether they had adjusted well, said Sheri Berenbaum, professor of psychology and pediatrics at Pennsylvania State University, University Park.

The studies were hard to do, and "people just believed John Money, they just believed we were neutral at birth," she said.

In the current study, Reiner and Gearhart reported on 16 genetically male children age 5 to 16 with cloacal exstrophy, 14 of whom were raised as girls. Children and parents were asked detailed questions about the children’s play patterns, levels of aggression, career goals and attitudes toward sex roles.

"We challenge the wisdom of routine gender conversion of these people," Gearhart said. "The science is telling us that the majority of these children should be left as the male sex."

The families were followed for between 34 and 98 months.

At the last assessment, eight of the 14 individuals raised as girls had declared themselves boys, including four who had not been told of their surgical transformation. Reiner said that one child refused at age 12 to accept estrogen injections for induction of puberty because he felt he was a boy. Another five subjects were living as females, apparently without complaint. One was too angry to discuss the issue.