When a boy struts in a tutu or a girl dons boxer shorts, it makes grown-ups nervous. It's one of the first lessons kids who are gender nonconforming learn.

Mich is biologically female, but didn't identify as a girl. As a child, Mich insisted on having boy-cut short hair, shunned all things pink and refused to play with dolls or wear dresses.

At age 3, "I told my mom I wanted to be a boy," said Mich, who requested to be identified by first name only. "And, throughout the years, I learned that saying that was not right ... and so, you hide this part of yourself. But you still know something's up. The problem with kids is that they don't have the language to say it, but they know."

The pressure to be more girly came from Mich's parents and other adults, rather than school bullies, said Mich, who now is 25 and lives in the Bay Area.

"The messages from adults, especially my parents, were this was not how it was supposed to be," Mich said. "I don't think it was subtle. I would cut my hair really short and my mom would say, 'Why do you look like a boy? You can't be a boy.' An adult would say, 'Why aren't you in a dress?' They're pushing this message on you."

When teenagers and children reject conforming to their biological gender roles, they are often teased, misunderstood or scorned by both peers and adults.

A study published in Pediatrics this month showed that children who do not conform to gender roles are more likely to be abused, increasing the likelihood they will have post-traumatic stress disorder by the time they're in their 20s.

Gender nonconformity means that an individual tends to associate with roles, behaviors and activities of the opposite gender, rather than those of his or her biological sex. This could be a boy who grows his hair long or paints his nails, or a girl who only wears male clothing. These issues are often confused with transgender identity, but they are not the same thing.

Gender nonconforming behavior occurs in one out of 10 children, according to the study. A vast majority of these kids do not need medical interventions, because the behavior tends to fade as they grow older.

In the study published Monday, nearly 9,000 respondents were asked to recall their childhood experiences before age 11, including favorite toys, games, roles they took while playing, media characters they imitated or admired, and feelings of femininity and masculinity. When they reached adulthood, the participants were surveyed again -- this time about whether they experienced physical, sexual, or emotional abuse and were screened for PTSD.

The results showed "very clear patterns," said S. Bryn Austin, one of the study's authors. "The young people who as children were most nonconforming were much more likely to report mistreatment or abuse, within the family, by people outside the family. They were targeted for abuse."

There should be extra precautions taken to protect them, she said.

"We are concerned about the health and risk of abuse and harassment targeting children who behave in a way, or express their gender in a way that's not typical," said Austin, an associate professor of pediatrics at the Children's Hospital Boston and Harvard School of Public Health. "We know there's a lot of bias about how girls and boys are supposed to behave."

Gender nonconformity tends to diminish as kids get older. And in many cases, kids with these tendencies grow up to be gay or lesbian, experts say.

"A lot of children seem to be experimenting with cross-gender behavior, but very few are following through to request gender change as they mature," wrote Dr. Walter Meyer III, a pediatric psychiatrist at the University of Texas Medical Branch in Galveston, Texas, in a separate commentary published in Pediatrics.

Mich, who wanted to be a boy during childhood, does not want to become a man, having reached adulthood. Mich now identifies as gender neutral -- meaning neither female nor male.

When children cross-dress, they toe the gender divide and challenge conventions, making their parents and adults very anxious. It's an issue that Diane Ehrensaft, director of mental health and founding member of the Child and Adolescent Gender Center and a clinical psychologist in the Bay Area, has dealt with for years.

Ehrensaft raised a gender nonconforming son and wrote a book, "Gender Born, Gender Made," in which the cover shows a young boy with curly hair in a tutu. Her son, now an adult, identifies as gay.

"I started seeing more and more children and families who just came to me around their child's gender nonconforming behaviors," Ehrensaft said in an interview with CNN.com at a health conference last year. "In the last five years, there has been an explosion in the number of children who are saying you guys have got it wrong. I'm not the gender you think I am."

There are "princess boys" and girls who only wear boy clothes, and many others who express their gender identity in unconventional ways.