Nearly all state schools are co-educational, but most independent schools are single sex. Three academic papers I came across in the last few months suggest that the education authorities might have something to learn from the private sector—all three find that randomly switching people to single-sex education leads to substantial improvements in their outcomes. The first, Lee et al. (2014) looks at random assignment of Korean youths to middle schools, comparing single sex classes in coed schools with coed classes in coed schools and single-sex classes in single-sex schools.
Male students attending single-sex schools outperform their counterparts in mixed-gender classes by 0.15 of a standard deviation. The significant impact of single-sex schools on male students’ achievement are not driven by classroom gender composition, but largely accounted for by increases in student effort and study-time. We find little evidence that classroom or school gender composition affect the outcomes of female students.
The second, Lu & Anderson (2015) looks more closely, at the effect in Chinese middle schools of being randomly assigned to sit next to others of the same gender. Contrary to the Korean paper, they find that girls do better if they sit near girls, but there is no effect on boys from who they sit near.
We exploit random seat assignment in a Chinese middle school to estimate how the gender of neighboring students affects a student’s academic achievement. We find that being surrounded by five females rather than five males increases a female’s test scores by 0.2–0.3 standard deviations but has no significant effects on a male’s test scores.
The third, Booth et al. (2013) looks at universities in the US, and does a true random experiment, again finding an effect on women but none on men.
We examine the effect of single‐sex classes on the pass rates, grades, and course choices of students in a coeducational university. We randomly assign students to all‐female, all‐male, and coed classes and, therefore, get around the selection issues present in other studies on single‐sex education. We find that one hour a week of single‐sex education benefits females: females are 7.5% more likely to pass their first year courses and score 10% higher in their required second year classes than their peers attending coeducational classes. We find no effect of single‐sex education on the subsequent probability that a female will take technical classes and there is no effect of single‐sex education for males.
Now these three studies are hardly conclusive—there's a big literature out there. And one of the things we really know well is that improving education is hard. Most of the quality of private schools is in their students, teachers, facilities and parents. But perhaps it's no coincidence that private schools are mostly single sex, and perhaps a system with more parental choice would tend toward a system with different genders in different classes or different schools.