Experts say increasing numbers of little kids are showing up in doctors offices insisting they were born into the body of the wrong sex — kids like Coy Mathis, the 6-year-old child banned from using the girls’ bathroom at her Fountain school because she is biologically male.

“We’re seeing it all across the state,” said Dr. Daniel Reirden, a Children’s Hospital Colorado physician who said his referrals “went up 200-fold” after he spoke on the issue of children and gender-identity disorder at a recent conference.

Described as “gender-variant,” these children typically begin to demonstrate cross-gender behaviors between the ages of 2 and 4. They express a strong desire to live as the opposite gender — clothes, toys, pronouns.

Sometimes, it’s typical preschool curiosity. But for other kids, it’s the beginning of a life out of the ordinary, challenging societal expectations of what defines their gender.

“In many rural communities, or places with less metropolitan areas, a lot of times kids will know something is different about them, and they identify this way, but there is social pressure to be your birth gender,” Reirden said.

Some adults who grew up in the ’50s and ’60s felt this way but never talked about it, said Karen Scarpella, program director for the Gender Identity Center of Colorado.

“There all these adults who say, ‘I knew when I was 3 or 4, but I couldn’t tell anyone. That was made very clear to me.’ But now we’re in a society that allows more gender-bending, and we can express ourselves more,” Scarpella said.

Sex is assigned at birth according to biological status, but gender deals with the roles, behaviors, activities and expressions that society deems appropriate for males and females.

Children who are gender nonconforming are not considered to have mental disorders unless they experience great distress and discomfort at being unable to express who they believe they are.

As the general public becomes more aware of the issue through stories about kids such as Coy, the scientific community is also starting to expand its research into life cycles of transgender people to include children.

In 2009, the Endocrine Society released evidence-based medical guidelines for endocrine treatment of people with gender-identity disorder, including children.

In 2010, the World Professional Association for Transgender Health released its standards of care that for the first time included children.

Because the field is so new, especially regarding children, “It’s not without controversy,” said Reirden. “Even within the psychological community, there are people on both sides of the camp.”

There are two basic models of treatment.

One advocates allowing the child to express cross-gender identity, which is what medical doctors and therapists recommended for Coy Mathis.

The other advises conditioning the child to fit the gender norm.

There are only a few clinics in the U.S. that specialize in gender-identity disorder in children, and the leader is the Gender Management Service Clinic at Boston Children’s Hospital, founded by pediatric endocrinologist Norman Spack.

Last year the medical journal Pediatrics published the first U.S. study of children and teens with gender-identity disorder, based on 97 patients of the clinic.

More than 44 percent described experiencing gender dysphoria — the feeling of a mismatch between biological sex and gender identity — or cross-gender behavior in preschool years.

The study found that gender-dysphoric children who did not receive counseling are at higher risk for behavioral and emotional problems.

In Denver, parents of gender- dysphoric children usually find their way to Reirden or clinical psychologist Sarah Burgamy, who specializes in working with gender-variant children and their families.

“It’s important for people like us to collaborate because this is a medical issue and an emotional-psychological issue,” Reirden said. “I don’t view it as psychopathology. That generally comes about because families refuse to offer support, or society doesn’t allow the child to express as the preferred gender.”

When a parent shows up in Burgamy’s office with a gender-dysphoric child, she usually advises a wait-and-see approach, because research shows that young children can be gender-fluid. In most children, gender dysphoria will disappear around the age of puberty or before.

“I tell the parents, let’s give the child some room to tell us who they are,” she said. “But that can be a tricky thing, because in practical life, it’s a gendered society — sports, bathrooms, changing rooms, even boys’ and girls’ birthday parties.”

Reirden knows of families who work with their children to transition in steps. They go to school as their biological sex, he said, “but come home to a safe and supportive environment where they can express their perceived gender.”

Children who are insistent about being cross-gender for a sustained period of time, he said, “are probably not likely to change by the time of late childhood and pre-adolescence.”

Coy Mathis seems to be one of those children.

Her mother, Kathryn Mathis, recalls that as early as 18 months, Coy loved dresses and tutus. By age 2, she refused to leave the house dressed as a boy.

“Ever since she was able to talk, she expressed to us that she was a girl, not a boy,” said Kathryn Mathis.

As a triplet, Coy has a brother and a sister, so the Mathis parents were able to watch one boy develop a male gender identity, while the other became ever more insistent about being female.

Coy started kindergarten as a boy but soon became unhappy when classmates treated her like a boy. Being made to stand in the boys’ line made her cry.

Doctors diagnosed her with gender-identity disorder. After a few months in kindergarten, Coy’s parents consulted with therapists and began to help her make the social transition from boy to girl.

“When the transition happened, some kids asked what happened, that Coy was a boy and is now a girl,” said Kathryn Mathis. “The teacher said, ‘Coy is a girl, and that’s it.’ Kids at that age are very accepting.”

In December, officials in Fountain-Fort Carson School District 8 ruled Coy would have to use the boys’ bathroom at Eagleside Elementary, or the staff or health-room restrooms. Her parents on Feb. 15 filed a complaint with the state Civil Rights Division.

The Mathises have no second thoughts about their decision to allow Coy to express herself.

“When she was able to be a girl,” said Kathryn Mathis, “she blossomed.”

Colleen O’Connor: 303-954-1083, coconnor@denverpost.com or twitter.com/coconnordp