Girls aid cognitive outcomes....

....because they are civilizing.

NBER working paper by Victor Lavy and Analía Schlosser:

As important mechanisms, we find that a higher proportion of female peers lowers the level of classroom disruption and violence, improves inter-student and student-teacher relationships as well as students' overall satisfaction in school, and lessens teachers' fatigue. We find, however, no effect on individual behavior of boys or girls, which suggests that the positive peer effects of girls on classroom environment are due mostly to compositional change, namely due to having more girls in the classroom and not due to improved behavior of peers.

Via Marginal Revolution.

UPDATE: The Undercover Economist has a nice essay on these results.