Children as young as 3 years old have internalized harshly negative attitudes about body size, a landmark 1998 study found.

In the study, adults read preschool boys and girls stories in which an anonymous child wreaks havoc, insulting other kids and kicking over sandcastles. When researchers asked the preschoolers to choose the guilty party from a lineup featuring a thin character, a character of average weight, and a fat character, the children overwhelmingly picked the fatter character, selecting them up to 81% of the time, over 5 times more frequently than the thin character.

Little has changed in the two decades since the study was published. In 2017, the American Academy of Pediatrics noted that weight-based bullying among adolescents is “among the most frequent forms of peer harassment,” and as recently as last year, German researchers confirmed that weight bias worms its way into classrooms: “Teachers grade overweight students more severely than comparable normal weight students.”

Jokes about weight are common—but they aren’t funny. Bias around body size is rampant, harms children, and remains a potent threat to happiness and social belonging from age 3 onward. To nip it in the bud, we need to start as early as preschool, confront weight-based bullying when it happens, and build curricula that include all bodies.