If you’re unfamiliar with Jazz Jennings, you’re missing one of the paradigmatic characters of early-twenty-first-century America. Jazz—this isn’t her birth name, for reasons that will become clear—was a boy born to a southern Florida couple in 2000. Nothing was unusual about his infancy; but by the time he was two, he was showing a marked preference for clothing and toys ordinarily chosen by girls: mermaids, princesses, all things pink—the whole shebang. Soon, he began insisting that he was a girl. His mother remembers him asking when the good fairy would come and turn his penis into a vagina. His “gender dysphoria,” as the medical world calls an emotional alienation from one’s sexed body, continued to trouble him until his parents decided to let him adopt a new name and pronoun, wear girl’s clothes, and present himself as a girl, a process known as social transition. At his fifth birthday, he debuted his new identity: “I got to wear the sparkly bathing suit for my party. I was a girl,” Jazz recalled later.