Here’s some news that preschool boys don’t want to hear: Those who attend preschool classes with a majority of girls receive an intellectual boost by the end of the school year. Conversely, preschool boys who attend majority-boy classes fall increasingly behind girls on measures of learning skills and other developmental feats.

Yet the proportion of boys and girls in preschool classes has no effect on girls’ development.

These provocative but still preliminary findings come from the first