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Without meaning to, and without even knowing they’re doing it, elementary-school teachers may be grading their girl students differently from the boys when it comes to math tests, according to some interesting new research recently highlighted by NPR’s Shankar Vedantam.

The researchers, Edith Sand and Victor Lavy at Tel Aviv University, analyzed the math-test scores of about 3,000 Israeli sixth graders in two different ways: First, they looked at the way the kids’ teachers had graded the tests. But they also gave those tests to a separate group of teachers who did not know the children and didn’t know whether the test they were looking at had been taken by a girl or boy. The results: The kids’ actual teachers tended to give the girls lower scores on the math tests than the other group of teachers; they also gave the boys higher scores on average than did the teachers who didn’t know the kids. “Now, since the external teachers don’t know the gender of the students, this suggests the classroom teachers are biased,” Vedantam explains. “They’re giving the girls lower math scores than they deserve.”

It’s not very likely that they were giving their girl students lower scores in math on purpose — after all, many of the teachers were themselves women. Instead, it’s “more likely these biases are operating at an unconscious level,” Vedantam continues. It’s just one study, and in just one country, so let’s not go wild with the broader implications of this one piece of research. (Gender biases operate differently in different cultures, for one, and there’s always the question of whether the results will hold up if replicated.) Still, it’s a fascinating window into the potential harm that can come from biases, even (or perhaps especially) the ones people aren’t aware they have.