For years, Dr Kinnon MacKinnon, like many people in the transgender community, considered the word “regret” to be taboo. MacKinnon, a 37-year-old transgender man and assistant professor of social work at York University here, thought it was offensive to talk about people who transitioned, later regretted their decision, and detransitioned. They were too few in number, he figured, and any attention they got reinforced to the public the false impression that transgender people were incapable of making sound decisions about their treatment. “This doesn’t even really happen,” MacKinnon recalled thinking as he listened to an academic presentation on detransitioners in...