The study’s authors analyzed data from more than 5,800 students from kindergarten through fifth grade and found that boys across all racial groups and in all major subject areas received lower grades than their test scores would have predicted.

The scholars attributed this “misalignment” to differences in “noncognitive skills”: attentiveness, persistence, eagerness to learn, the ability to sit still and work independently. As most parents know, girls tend to develop these skills earlier and more naturally than boys.

No previous study, to my knowledge, has demonstrated that the well-known gender gap in school grades begins so early and is almost entirely attributable to differences in behavior. The researchers found that teachers rated boys as less proficient even when the boys did just as well as the girls on tests of reading, math and science. (The teachers did not know the test scores in advance.) If the teachers had not accounted for classroom behavior, the boys’ grades, like the girls’, would have matched their test scores.