- Boys find girls more distracting in class than they find other boys. Likewise, girls find boys more distracting.
- Sex differences in math and science achievement are a product of social influence. Those influences will be reduced or eliminated if girls are in classrooms only with girls.
- Boys dominate classroom discussion, and so girls are denied practice in articulating and defending their views.
- Boys and girls have different brains, and therefore learn differently. If they are taught separately, teachers can tune their instruction to the way each sex learns.
A 2005 report written for the Department of Education (Mael et al, 2005) reported mixed effects, but generally a positive conclusion for single-sex classrooms in short-run academic outcomes. There was no indication of a boost to longer-term outcomes.
A new study (Pahlke, Hyde, & Allison, 2014) reports a meta-analysis of 184 studies representing 1.6 million students in K-12 across 21 nations. The authors place considerable emphasis on the problem of control in this research. They end up concluding that, with proper controls, analyses show that single-sex classrooms don't help students much.
In controlled studies, there were statistically reliably, but numerically quite modest positive effects of single-sex classrooms for both boys and girls in mathematics achievement, science achievement, and verbal achievement (Hedges g in all cases less than 0.10). Girls showed an edge in single-sex classes for math attitude, science achievement, and overall academic achievement, but again, the gains were modest. If one restricts the analysis to U.S. students, virtually all of these small effects disappear.
There was no effect for attitudes towards school, gender stereotyping, educational aspirations, self-concept, interpersonal relationships, or body image.
It's also notable that there was no dosage effect: the advantage was no larger when all classes within a school were single-sex classes, compared to when a single class was.
The authors were also interested in evaluating whether single sex classes were effective for boys of color. They reported that there were not enough controlled studies to answer this question, but even restricting the analysis to uncontrolled studies, the effects were minimal.
When you consider the factors that we know contribute substantially to academic attitudes and performance--the student's prior academic achievement, the curriculum, the home environment, the teacher's skill--it's easy to believe that the sex of the other students would have a modest effect, if any.
That said, it could be that a single sex school has a profound influence on a few students. A few years ago, friends of mine moved their 15 year old daughter to an all-girls school because she was "boy crazy." According to my friends, she didn't become any less interested in boys, but she did focus on work better during school hours. But then again it's possible my friends were kidding themselves.
Mael, F., Alonso, A., Gibson, D., Rogers, K., & Smith, M. (2005).
Single-sex versus coeducational schooling: A systematic review. Washington, DC: American Institutes for Research
Pahlke, E.,Hhyude, J. S, & Allison, C. M. (2014). The effects of single-sex cmopared with coeducational schooling on students' performance and attitudes: A meta-analysis. Psychological Bulletin, Advance online publication, http://dx.doi.org/10.1037/a0035740