And how did the researchers determine which teachers were better? By looking at twins assigned to different classrooms, according to this Associated Press story.

Researchers studied more than 550 first- and second-grade classrooms with at least one identical twin and more than 1,000 classes with at least one fraternal twin. Among the identical twins, 42 pairs out of 280 pairs showed significant differences in reading.

The FSU researchers said teacher quality was the biggest reason for the gap, though they conceded other factors may have played a role, such as the behavior of classmates.