In Slate, by Lise Eliot and Diane Halpern.
An excerpt:
The biggest problem with the survey is that the state Single-Gender Initiatives program that administered it--and which fervently advocates single-sex education--did not give students the option of answering "no change" when asking how their current attitudes compared with their feelings before they moved to single-sex classrooms. Forced to choose between "increase" and "decrease" for questions about their motivation and confidence since switching from coeducation, the students were more likely to pick the positive option. The younger students' glowing answers to every question in particular reflect the instinctive desire to please adults in many kids of their age group. By contrast, about 42 percent of middle-school-aged boys responded that their motivation had decreased since switching to single-gender classrooms--a finding that is easily lost in the way the data are presented.