PORTLAND, Maine — The president of the Maine Chapter of the American Academy of Pediatrics said Friday that “a transgender girl needs to be treated like any other girl” and should be allowed to use the girls bathroom at school.

Dr. Steve Feder, a pediatrician in Boothbay Harbor and Damariscotta, said in a telephone interview that the Maine Supreme Judicial Court should recognize that gender identity is internalized and not simply a matter of physical characteristics, especially for transgender children.

The physician said that his organization has joined a “friend of the court” brief in a case pending before the state supreme court. Other groups who signed onto the amicus brief are the Maine Psychological Association, the National Association of Social Workers-Maine Chapter, Trans Youth Equality Foundation, the Maine Women’s Lobby and the Downeast and Southern Maine chapters of the Gay, Lesbian & Straight Education Network.

The case concerns a transgender student’s use of the girls bathroom at schools in Orono.

“We would like [the justices] to recognize that from a medical and mental health perspective gender identity is internalized,” Feder said. “It involves more than hormones and external genitalia. Gender identity is a mixture of those things and that is well recognized in the literature.”

About 200 of Maine’s 220 pediatricians belong to the Maine chapter of the national group, he said. It is the first time the organization has signed onto a brief before the state’s high court.

Feder said that members of the organization had “an active dialogue” before its board made the unanimous decision to sign on to the brief.

“This case is a good example of how that is playing out in Maine,” Feder said. “We support this young woman being recognized as a young woman. A transgender girl needs to be treated like any other girl.”

Feder said the condition of gender identity dysphoria is very rare. The American Psychological Association has estimated that 1 in 10,000 biological males and 1 in 30,000 biological females in Western society are transgender, according to a previously published report.

The incident that sparked the court case began in 2007 when a child, who was born male but identifies as female, was forced to stop using the girls bathroom in a Penobscot County school and was told to use a staff bathroom after the grandfather of a male student complained. The girl’s parents and the Maine Human Rights Commission sued the district after the commission ruled in her favor.

Both have appealed the November decision of a Superior Court judge to the Maine Supreme Judicial Court. Justice William Anderson’s decision sided with Riverside RSU 26, formerly the Orono School District, over the girl’s use of the student bathroom.

The family and their child are not identified in the lawsuit and are referred to as the Does. Subsequent news stories, however, have identified the parents as Wayne and Kelly Maines and their twin children as Nicole and Jonas, both now 15. In court documents and in Anderson’s decision, family members were not named but referred to as the Does, with Nicole Maines being called “Susan Doe.” All the briefs filed in the appeal do the same.

The amicus brief, written by attorney Richard O’Meara of Portland, is focused on adolescent social and psychological development as they relate to gender identity.

“One of the core aspects of adolescent development is learning how to develop and consolidate one’s gender identity and gender role, including by building friendships with same-sex peers,” the 24-page brief said. “For girls, no small part of that experience takes place in girls’ bathrooms. Adolescent girls commonly go to the restroom together in groups or pairs and use restrooms as sites for a multitude of social activities. The socializations and bonding that take place in these settings are critical components of building confidence, self-esteem, and healthy relationships with same-sex peers.

“Excluding an adolescent girl from these social spaces is profoundly isolating and makes it much more difficult for her to develop peer relationships,” the brief continued. “Rather than having multiple opportunities to socialize, interact with other girls, and develop a healthy gendered self, she is singled out, isolated and marginalized. As Susan, the plaintiff in this case, explained after she was required to use a separate bathroom: ‘There’s no one to socialize with in there except yourself in a mirror.”

Oral arguments in the case have not been scheduled.