Katherine K. Dahlsgaard, Ph.D., lead psychologist of the Anxiety Behaviors Clinic at Children's Hospital of Philadelphia, wrote this for the "Healthy Kids" blog at Philly.com.

Girls consistently earn better grades than boys, not because they are more motivated to do so, but because they show greater self-discipline, according to a recently published study in the journal Learning and Individual Differences.

It's long been known - by classroom teachers and researchers - that the typical female student outperforms the typical male when it comes to report cards. That starts early in elementary school and continues through college. It is also consistent across academic subjects, including math and science.

But the reasons girls earn better grades than boys aren't well understood. It is not that girls are smarter or more academically able. Girls and boys perform about equally on IQ tests. They also perform similarly on tests of academic abilities.

Many have guessed girls are simply more motivated to earn better grades, perhaps because they are eager to prove themselves to be good girls who please their teachers and parents.

The study's lead author, Angela Duckworth, a psychologist at the University of Pennsylvania, tested this motivation theory along with another potential explanation - self-discipline.

Duckworth showed in 2006 that a group of middle school girls earned better grades because they were more self-disciplined than boys. The new study delved deeper by specifically testing the motivation versus self-control question in a series of smaller sub-studies.

First, middle schoolers were surveyed specifically about motivation, answering questions about how interesting school was or how important it was to them. Interestingly, the boys and girls reported no differences there.

Then their teachers or parents were surveyed about the students' self-control abilities, such as developing good work habits or staying on task even when another activity would be more fun.

Girls were not only rated higher than boys, but this better self-control accounted for the girls' outperformance on grades. In other words, girls outperformed boys because they were more self-disciplined.

The results are of interest for two more reasons. The first is that the reports of motivation by the kids themselves differed from those of their teachers. In another part of the study, Duckworth and colleagues asked a different set of middle school teachers about girls' and boys' motivation and self-control.

Teachers rated girls as having more of both. That suggests boys get less credit for school motivation than they themselves report.

Another important aspect of these studies is that the researchers surveyed schools with almost entirely African American or Hispanic student populations.

Too often, scientific studies rely on mostly white participants, which limits how much the results can be applied to our increasingly diverse population.

The results here are similar to previous research on this topic, suggesting the advantage of self-control applies to everyone.

The takeaway from the study? When it comes to school grades, motivation isn't enough. Let us follow the lead of girls. Self-control is queen.