Kate Middleton has long been a magnet for unproven rumors: She pressured an art gallery to remove a royal portrait! She split from her husband! She changed her hairstyle to distract from pregnancy rumors! She did not give birth to her daughter!

This year, speculation kicked into overdrive. Ms. Middleton — now Catherine, Princess of Wales — has lain low since Christmas. Kensington Palace said she was recovering from “a planned abdominal surgery” and unlikely to resume royal duties until after Easter. Conspiracy theorists had other, more sinister ideas. The only explanation for the future queen’s long absence, they said, was that she was missing, dying or deceased, and that someone was trying to cover it up.

“KATE MIDDLETON IS PROBABLY DEAD,” read one post on X, with the text flanked by skulls and screaming emojis.

In her invented death, the princess joins a host of other celebrities and public figures — from President Biden to Elon Musk — who scores of online detectives have declared in recent months to be clones, body doubles, A.I.-generated avatars or otherwise not the living, breathing people they are.

For many of the people pushing the falsehoods, it is harmless fun: casual gumshoeing that lasts only a few clicks, a bonanza for meme generators. Others, however, spend “countless hours” on the pursuit, following other skeptics down rabbit holes and demanding that celebrities provide proof of life.

Whatever the motivation, what lingers is an urge to question reality, misinformation experts say. Lately, despite extensive and incontrovertible evidence to the contrary, the same sense of suspicion has contaminated conversations about elections, race, health care and climate.