"Over the past 18 months, we have conducted interviews and focus groups with around 100 American girls aged 12 to 19 and their mothers, most of them Midwestern and middle class."

I, too, would like to read about how young boys deal with modern life. But I would not expect to find such information in research with the stated goal of investigating how girls do the same.

Saying "group A experiences these problems" does not imply exclusivity or that group B experiences the opposite. It is merely a statement about group A.