PHOENIX — Family courts have no right to interfere with the decisions of a parent given sole legal custody of a child after a divorce, absent some showing of specific harm, the Arizona Supreme Court ruled Thursday.

This holds true even when the issue involves the child’s sexual identification, the justices said.

Justice Ann Scott Timmer, writing for the unanimous court, told family court judges to shelve their own views.

“The court must be mindful not to unnecessarily intrude on the sole legal decision-maker’s unshared authority to make major decisions concerning the child’s upbringing, even if those decisions conflict with expert opinion or the court’s own views on child rearing,” Timmer wrote.

What makes Thursday’s ruling particularly significant is that the underlying dispute involves gender-identification questions of the child in the case, who is biologically male.

Divorced three years after the birth, the child’s father and mother were granted joint legal decision-making authority with equal parenting time. But the father was given final authority about the child’s education, medical and dental care.

The dispute erupted when the father learned that the mother was attempting to socially transition the child to identifying as female, with what Timmer said were sometimes negative consequences.

The father agreed to counseling, including allowing the child to explore wearing clothes and playing with toys typically associated with girls while at the mother’s house.

But the mother went beyond that, including allowing the child to appear in public in clothes associated with girls and even speaking with the child, 5 at the time, about sex reassignment surgery and hormone therapy.

Medical professionals diagnosed the child with gender dysphoria of childhood, meaning a marked difference between biological and experienced or expressed gender.

Ultimately the family court judge appointed a specific treating therapist for the child and a consulting expert for the court, moves that Timmer said restrained the father’s authority.

That, she wrote, was beyond the court’s power — and contrary to state law.

“(The law) authorized the family court to impose a specific limitation on the sole legal decision-maker’s authority only when the other parent demonstrates that absent that limitation, the child would be physically endangered or the child’s emotional development would be significantly impaired,” the justice said.

Timmer made it clear that the issues here — including “the complexity of the child’s situation” — did not rise to that level.

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“Fit parents, like Father, frequently guide their children through complex situations without court interference,” she wrote.

Timmer said there is no suggestion the father will exercise his authority in a way that endangers the child.

She acknowledged that the father has, in the past, been reluctant to accept the diagnosis of gender dysphoria.

But Timmer said that “does not demonstrate he would fail to appropriately address that diagnosis in the future.”

Timmer said the father has been willing to allow the child to explore gender issues in his home and to learn about gender issues himself.

There is no evidence that even if the father mismanaged the gender dysphoria diagnosis by acting on his own, without court oversight, that would put the child at risk for harm, the justice wrote.

Family court judges can impose specific limits on the power of the parent who is the sole decision-maker, Timmer said. But any limitation must be carefully tailored to prevent harm.