The simple fact was that he had done something wrong, and at the end of a long and revolutionary career it didn’t matter how often he’d been right, how powerful he once was, or what it would mean for his legacy.

Dr. Robert L. Spitzer, considered by some to be the father of modern psychiatry, who turns 80 next week, lay awake at 4 o’clock on a recent morning knowing he had to do the one thing that comes least naturally to him.

He pushed himself up and staggered into the dark. His desk seemed impossibly far away; Dr. Spitzer suffers from Parkinson’s disease and has trouble walking, sitting, even holding his head upright.

The word he sometimes uses to describe these limitations — pathetic — is the same one that for decades he wielded like an ax to strike down dumb ideas, empty theorizing, and junk studies.

Now here he was at his computer, ready to recant a study he had done himself, a poorly conceived 2003 investigation that supported the use of so-called reparative therapy to “cure” homosexuality for people strongly motivated to change.

What to say? The issue of gay marriage was rocking national politics yet again. The California State Legislature was debating a bill to ban the therapy outright as being dangerous. A magazine writer who had been through the therapy as a teenager recently visited his house, to explain how miserably disorienting the experience was.

And he would learn later that a World Health Organization report, released on Thursday, calls the therapy “a serious threat to the health and well-being — even the lives — of affected people.”

Dr. Spitzer’s fingers jerked over the keys, unreliably, as if choking on the words. And then it was done: a short letter to be published this month, in the same journal where the original study appeared.

“I believe,” it concludes, “I owe the gay community an apology.”