“It was really heartbreaking,” she says.

The stereotype that brilliance and genius are male traits is common among adults. In various surveys, men rate their intelligence more favorably than women, and in a recent study of biology undergraduates, men overrated the abilities of male students above equally talented and outspoken women. But Bian’s study shows that the seeds of this pernicious bias are planted at a very early age. Even by the age of 6, boys and girls are already diverging in who they think is smart.

“It’s an excellent, important, and well-designed paper,” says Alison Gopnik from the University of California, Berkeley, who studies the minds of babies and children. “The pattern it reports is very consistent with other studies which show the emergence of gender stereotypes at around age 6.”

These differences can have lasting consequences. In 2015, Bian’s colleagues Sarah-Jane Leslie from Princeton University and Andrei Cimpian at New York University showed that in many academic fields, like physics, math, and philosophy, people believe that success depends on “raw, innate talent” that “just can’t be taught”. And as the team found, the greater the emphasis on this “brilliance required” message, the fewer female Ph.D.’s there were, and the bigger the gender gap. That’s not because of any actual differences in aptitude. Instead, the team argued, a double whammy of stereotypes—that men are more likely to be brilliant, and that brilliance is required in some fields—creates an atmosphere that makes women feel unwelcome, and pushes them away.

It begins early, Bian found. She offered 160 children a chance to play two new games—one for children “who are really, really smart” and another for those who “try really, really hard”. At the age of 5, girls and boys were equally attracted to both games. But among those aged 6 or older, the girls were less interested than the boys in the game for smart kids (but not the one for hard-working ones). “They’d go from being really enthusiastic to saying: ‘Oh I don’t want to play it, this isn’t a game for me,’” says Bian. And those who had most strongly assimilated the stereotype of male brilliance showed the lowest interest in the smart game. They had already mentally assigned themselves to Hufflepuff instead of Ravenclaw.

“The stereotype has an immediate impact,” she adds. “In the long-term it will steer away many young women from careers that are thought to require brilliance.”

“Unfortunately, this reveals another hurdle for efforts to recruit more women and girls into STEM,” says Sarah Eddy from Florida International University. “Not only do we need to break down the ‘science is male’ stereotype, but now we need to break down a ‘brilliance is male’ stereotype, too.”

Why do these beliefs occur? It’s not to do with actual ability. At that age, girls tend to outclass their male peers—and the girls in Bian’s study knew it. When she showed them pictures of four children and asked them to guess who got the best grades, the older girls were actually more likely to pick girls than the older boys were to pick boys. “Everyone agreed that girls do better in school but that didn’t seem to matter,” says Bian.