It wasn’t his high blood pressure or high cholesterol that caught Matthew Moore by surprise when he went to his new physician earlier this year. Moore, a Southern California man in his mid-40s, described those conditions as “normal for me.” Nor was Moore, who is openly gay, shocked to see that his doctor noted his sexual orientation on his medical paperwork — until he saw the way that she noted it.

“The sickness label was used to justify discrimination, especially in employment, and especially by our own government.”

Listed as a chronic condition, Moore noticed “homosexual behavior” on his paperwork, followed by the medical code 302.0. As unsettling as the notation already was, Moore decided to research what the code meant, and he was left wondering how the diagnosis could happen today: “When I look[ed] up code 302.0 [ I learned that it meant] sexual deviancy or mental illness, and that code has been removed or suggested heavily not to be used since 1973.”

“My jaw was on the floor,” Moore recounted. “At first, I kind of laughed, [and then] I thought, ‘Here’s another way that gay people are lessened and made to feel less-than,’ and then as I thought about it and as I dealt with it, it angered me,” he told a local news station.

Moore complained to his physician, and, dissatisfied with her response when she defended the diagnosis, he wrote a letter to the parent company of the Manhattan Beach office where his physician practiced medicine. Moore received a written apology and a refund of his co-pay.

Moore’s story made the news earlier this year because of how anomalous — and appalling — it was. But prior to 1973, Moore’s experience would have been almost inevitable, unless he took precautions to keep his sexual orientation as private and secret as possible.

Until a decision by the American Psychiatric Association (APA) changed the course of history 40 years ago this Sunday, on December 15, 1973, gay and lesbian people couldn’t escape the perception that their sexuality was a sickness. Months before her death in 2007, the gay rights activist Barbara Gittings recalled what life was like for gay and lesbian people before that decision: “The sickness label was an albatross around the neck of our early gay rights groups — it infected all our work on other issues. Anything we said on our behalf could be dismissed as ‘That’s just your sickness talking.’ The sickness label was used to justify discrimination, especially in employment, and especially by our own government.”

The breadth of human culture and recorded history has never provided a simple narrative about the way homosexuality has been regarded. Depending on time and place, it has been accepted, tolerated, ignored, or vilified. But in Western culture after the Middle Ages, hostility toward homosexuality took root, and that hostility was reflected in religion, law, and the practice of medicine. Following the examples of religion, which viewed homosexuality as a sin, and law, which viewed it as a crime, medicine and psychiatry from the 19th century onward viewed it as a sickness.

To be sure, there was not a consensus among scientists and mental-health professionals. Sigmund Freud, for example, wrote in an oft-quoted letter in 1935, “Homosexuality is assuredly no advantage, but it is nothing to be ashamed of, no vice, no degradation, [and] it cannot be classified as an illness.” But Freud’s view didn’t prevail until decades later.

Although now seen as flawed in many ways, the pioneering work of biologist and sexologist Alfred Kinsey put some of the first cracks in the foundation that once kept homophobia entrenched among medical and mental-health professionals. Kinsey’s research on sexual behavior among American adults revealed that same-sex sexual activity was far more common than previously assumed — enough so that it couldn’t be dismissed as a strange fixation of social deviants. Adding more cracks to the foundation was the work of psychologist Evelyn Hooker, whose 1957 study of the psychological health of heterosexual and homosexual men found that they were indistinguishable from each other in each of the three personality tests she administered.

By the early 1970s, the mounting evidence, together with pressure from activists, pushed the psychiatric community to reevaluate how homosexuality was classified. In 1971, a lesbian newspaper published a petition to the American Psychiatric Association demanding reconsideration of the classification, and when the APA met in Washington, D.C., that year, the conference included a panel discussion called “Lifestyles of Non-Patient Homosexuals.” As Barbara Gittings pointed out, it marked the first time the APA acknowledged that there were homosexuals who weren’t receiving psychiatric care — and who didn’t need to be.

When the APA met the following year in Dallas, the conference again included a panel discussion about homosexuality. Gittings and her partner Kay Lahusen were involved in organizing the panel, which included two psychiatrists and two gay panelists. Lahusen suggested that the panel also include someone who could speak as both a psychiatrist and a gay person. After many letters and phone calls, Gittings and Lahusen found multiple gay psychiatrists — none of whom would risk their careers to reveal their sexual identity to their colleagues.

One, however, stepped forward, agreeing to participate on the condition that he could wear a disguise. John Fryer knew the risk he would face if his identity were revealed. As he recounted in the Journal of Gay & Lesbian Psychotherapy, “In 1971 I was not feeling very secure. I was not [employed] full-time anywhere. I was only on the clinical faculty at Temple [University] and did not have tenure … I had been thrown out of a residency because I was gay; I had lost a job because I was gay.”

It was one of history’s lucky coincidences that Fryer’s partner at the time was a drama student, someone used to seeing people reinvent themselves for an audience. With a mask and an oversized outfit, he transformed Fryer into Dr. H. Anonymous so that he could share his story.

Behind his mask, Fryer introduced himself as a psychiatrist and a homosexual. He added that he “could be any one of more than a hundred psychiatrists registered at this convention,” but that “the curious among you should cease attempting to figure out who I am and listen to what I say.” He challenged the notion that being homosexual and healthy was “an impossible oxymoron,” arguing that he and other gay psychiatrists “are required to be more healthy than our heterosexual counterparts” — enough so that they can be therapists to others even as they maintain the resolve to work among colleagues who would view them as sick. To support Fryer’s testimony, Gittings and Lahusen read excerpts from the responses they’d received from the many other gay psychiatrists who declined to participate in the panel but nonetheless wanted to share their stories.

The panel was a game-changer, and the following February the APA’s Nomenclature Committee convened to put its classification of homosexuality under review. The committee heard arguments both for and against a change in classification, and members looked at the scientific evidence available to them, including the work of Hooker and many other psychologists, as well as many psychiatrists and psychoanalysts. Following that review, on December 15, 1973, the APA’s Board of Trustees voted to remove homosexuality from their Diagnostic and Statistical Manual of Mental Disorders (DSM).

Despite the scientific evidence that backed that decision, the board’s vote sparked an immediate backlash. Two opponents gathered supporters for an initiative that put the question up for vote by the entire APA membership. Their opposition was especially unwarranted in light of the fact that, at the suggestion of psychiatrist Robert Spitzer, homosexuality was removed from the DSM and replaced with “sexual orientation disturbance.” It was a new diagnostic label that still left room for psychiatrists to provide treatment for homosexuality, even if homosexuality was no longer considered a mental disorder. Spitzer suggested the compromise, thinking that at that time “there was no way that homosexuality was going to be removed entirely” from the DSM.

Ultimately, the opponents’ initiative failed to reverse the decision, and eventually the new diagnostic label Spitzer introduced was removed from the DSM as well.

The 1973 decision still has its detractors, who insist that it was a political one that debases science. Although spurred by activism and full of political implications, the decision was based on a scientific evaluation of the best available evidence. Its scientific soundness was independent of its political importance.

Since his role in the 1973 decision, Robert Spitzer has remained a name inseparable from the question of how the psychiatric community regards homosexuality. Although he was among those who wanted to end its classification as a mental disorder, he was nonetheless intrigued later in his career, in the late 1990s and early 2000s, by the idea that homosexuals could be converted to heterosexuals — so-called conversion therapy. Spitzer conducted a study of 200 men and women who claimed to have changed their sexual orientation from homosexual to heterosexual. Although his colleagues found methodological problems in his study and advised against publishing his findings, Spitzer did just that.

Years later, in his retirement, Spitzer admitted to journalist Gabriel Arana that his critics had been right about his study on conversion therapy. The therapy’s effectiveness was easily discredited, and many who had undergone the treatment could tell about the harm it did to them. One of those many was Arana himself, who had been driven to thoughts of suicide and struggled with self-acceptance.

In May of last year, Spitzer published a letter in the same journal that had published his study. He needed to get it off his chest: “I believe I owe the gay community an apology.” Social conservatives who had held up the study to support anti-gay agendas were now empty-handed.

Those who still insist that homosexuality is a curable sickness ignore the evidence against them, no matter how much ridicule that invites. In July of this year, a group purporting to represent ex-gays held a lobby day on Capitol Hill. The organizer predicted that “thousands of ex-gays” would arrive in Washington, convincing lawmakers to support therapy to treat homosexuality. When the headcount was in, they realized that fewer than 10 people showed up. The disparity must have been baffling.