From Scientific American:
Scientific literature has been littered with studies over the past 40 years documenting the superior language skill of girls, but the biologic reason why has remained a mystery until now.
Researchers report in the journal Neuropsychologia that the answer lies in the way words are processed: Girls completing a linguistic abilities task showed greater activity in brain areas implicated specifically in language encoding, which decipher information abstractly. Boys, on the other hand, showed a lot of activity in regions tied to visual and auditory functions, depending on the way the words were presented during the exercise.
The finding suggests that although linguistic information goes directly to the seat of language processing in the female brain, males use sensory machinery to do a great deal of the work in untangling the data. In a classroom setting, it implies that boys need to be taught language both visually (with a textbook) and orally (through a lecture) to get a full grasp of the subject, whereas a girl may be able to pick up the concepts by either method.