Forty-five years ago, members of the American Psychiatric Association decided, by a slim 58 percent majority, to remove “homosexuality” from the list of mental disorders in its Diagnostic and Statistical Manual of Mental Disorders. In his old age, the great gay rights activist Frank Kameny recalled Dec. 15, 1973, as the day “when we were cured en masse by the psychiatrists.”

In a single stroke, the A.P.A. helped transform homosexuality from a medical condition to a social identity. It would take another 27 years for the World Health Organization to eliminate homosexuality from its own classification of mental disorders in the International Classification of Diseases, the comprehensive manual of some 55,000 diagnostic codes that doctors everywhere use for diagnosis and insurance reimbursement. But this summer, the W.H.O. beat the A.P.A. to the punch on another issue — transgender rights — by moving “gender incongruence” from its chapter on mental health to its chapter on sexual health. On its website, under the heading “Small Code, Big Impact,” the W.H.O. says that gender incongruence is a sexual health condition for which people may seek medical services, but that “the evidence is now clear that it is not a mental disorder.”

The A.P.A. should now do the same by eliminating its category of gender dysphoria, a technical term for people unhappy because of their gender incongruence. It would be an important step in advancing transgender rights and reducing the stigma and prejudice that people experience when, because of nothing they or anyone else did wrong, they cannot abide the sex they were assigned at birth.

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The 1973 decision on homosexuality taught us that we shouldn’t expect too much too quickly. Indeed, Frank Kameny overstated the A.P.A.’s power for sarcastic effect. Most of the 42 percent who objected clung to the psychoanalytic view articulated by Sigmund Freud in 1914 that homosexuality was a developmental problem. Nor did the A.P.A. immediately excise homosexuality from the D.S.M. As a compromise, the organization retained diagnoses in subsequent editions to denote people unhappy about being homosexual — ego dystonic homosexuality, for example — and eliminated homosexuality completely only in the 1987 revision.