Girls who dress or act like boys, and boys who act more feminine, may be more likely to be abused and end up with post-traumatic stress disorder, according to a U.S. study.

Researchers whose findings appeared in Pediatrics said that parents or other adults who are uncomfortable with so-called gender non-conformity may treat children differently, sometimes violently, or be convinced they can change their feelings and behavior.

"In some cases, they believe they're helping the child, that gender non-conforming won't be accepted by other people," said Andrea Roberts, from the Harvard School of Public Health in Boston, who worked on the study.

"But of course, abuse is never protective."

Roberts and her colleagues analyzed data from a long-term study on children and teens that looked at more than 16,000 children, who recalled their favorite toys, roles they took on during play, and feelings of femininity or masculinity at age 11.

The participants were also asked about instances of abuse -- from kicking and grabbing, to threatening, to forced sexual contact -- that happened either before that time or during their adolescent years.

The researchers found that children who were the most gender non-conforming were between 40 percent and more than twice as likely to report any kind of childhood abuse as those who did confirm to typical gender roles.

They also reported more symptoms of PTSD, which include jumpiness, trouble sleeping and flashbacks.

Roberts said that while the findings can't prove that parents abused boys because they acted like girls, and vice versa, the study did hint that gender non-conformity in younger children predicted abuse during the teenage years.

A separate study that also appeared in Pediatrics reported on the experience of doctors from Children's Hospital Boston in treating 97 children and teens with gender identity disorder. This disorder goes beyond not conforming to gender norms and includes children who are very bothered by their physical gender and identify as the opposite sex.

Forty-three of those treated at the Children's Hospital clinic had a history of psychiatric problems, including 20 who had self-harmed and nine that had attempted suicide.

Researchers pointed out that in children who are already going through puberty and are serious about treatment to switch to the opposite gender, intervening at an early stage can keep them from developing secondary sex characteristics like facial hair and breasts, which may head off some of their distress.

"If the kid is unhappy, depressed, troubled about their own body, that's probably (a sign) the parents could use some help," said Roberts.

She added that while adopting some opposite-gender behavior is relatively common, far fewer children will be seriously bothered by their gender -- possibly about 1 in 1,000, though researchers don't have a full grasp of the extent of gender identity disorder in children.

Those are the children who may be at the highest risk of abuse and psychological problems, researchers said, adding that the most important thing for non-conforming children, including those who are seriously questioning their own gender, is to get support from their families and schools.