Part of Does single-sex schooling improve academic performance?

Does single-sex schooling improve boys' academic performance compared with coeducation, after accounting for which boys and families choose single-sex schools?

No reliable evidence

Summary:

No reliable study finds that single-sex schooling improves boys' academic performance once you account for which boys end up in those schools. The one study claiming large benefits for boys (Park 2012) cannot separate single-sex classrooms from the fact that 80% of the single-sex schools were privately run, and a meta-analysis of 184 studies finds near-zero controlled effects for boys across math, science, and reading.

Answer in brief

No credible study finds that boys do better academically in single-sex settings after accounting for selection — the fact that families choosing single-sex schools already differ in ways that predict better outcomes. The one randomized study claiming otherwise (Park 2012) cannot separate the single-sex effect from private-school governance, and its results shrink by more than half with even crude controls. This is a reasonably settled null: not proven impossible, but tested repeatedly without a positive result surviving scrutiny.

What the evidence shows

The meta-analysis finds nothing. Pahlke 2014, pooling 184 studies across 21 countries, reports controlled effects near zero for boys in math, science, and reading. Uncontrolled studies show modest advantages that shrink once you adjust for family background and prior achievement — the standard pattern of selection bias inflating raw comparisons.

The best quasi-experiment finds nothing for most boys. Jackson 2012 exploits Trinidad and Tobago's centralized school-assignment algorithm, which partially randomizes who attends single-sex schools. After removing selection bias, the apparent single-sex advantage shrank by roughly two-thirds. For boys without a strong stated preference for single-sex schools — the large majority — the effect was indistinguishable from zero. The paper cannot cleanly compare boys' and girls' responses because all-boys and all-girls schools are entirely different institutions with different staff, funding, and governance.

The Seoul lottery study cannot deliver what it promises. Park 2012 uses Seoul's school-assignment lottery — a strong design in principle. But 80% of single-sex schools in Seoul are private while only 30% of coeducational schools are. When the study adds a control for private status, the boys' college-attendance advantage drops 57%, and private-school status becomes the dominant predictor. The authors mention, without showing the data, that controlling for teacher gender composition "substantially reduced" the all-boys school effect. The boys' claim is rated not supported because the study cannot separate gender-segregated classrooms from the institutional advantages of private governance.

What is claimed but not shown

Park 2012 claims boys benefit as much as or more than girls from single-sex schools. The boys' advantage is largely explained by private-school status and teacher composition, and the key analysis is mentioned but never published.

Halpern 2011 states that no well-designed research shows single-sex education improves performance for either sex. This overstates its source — a U.S. Department of Education review that called results "equivocal," not absent. The paper also never tested the specific claim about boys.

Pahlke 2014 notes that claims about single-sex schooling being "particularly effective for minority boys in the United States" could not be evaluated — too few well-designed studies exist. The meta-analysis is explicit that this question remains open.

What would settle it

A randomized or regression-discontinuity study assigning boys to single-sex versus coeducational settings within the same institutional type (all public or all private), removing the governance confound that undermines Park 2012.
Publication of Park 2012's unpublished analysis of teacher gender composition, which the authors say substantially reduces the boys' effect.
Studies of single-sex programs for minority boys in U.S. public schools — the subgroup where advocacy is strongest and evidence thinnest.
A meta-analysis separating the 12 truly randomized studies from the weaker "controlled" designs pooled in Pahlke 2014.

Limitations of this evidence base

These papers span Ireland, Switzerland, Trinidad and Tobago, South Korea, and a cross-national meta-analysis — but none includes a large U.S. or U.K. public-school study with a credible causal design. The strongest designs (Seoul's lottery, Trinidad's assignment algorithm) come from institutional settings with no counterpart in the countries where single-sex schooling is most debated. The Swiss study (Eisenkopf 2011) examined only girls. Most data predate 2010; the recent expansion of single-sex programs in U.S. charter schools is unrepresented. No study examines whether specific types of single-sex programs (those emphasizing mentorship versus those emphasizing biological sex differences) produce different results for boys — 92% of studies in the meta-analysis did not report the program's philosophy. Whether particular subgroups of boys — by race, socioeconomic status, or prior achievement — might benefit differently is unanswerable with current evidence.

The papers behind it

Every paper this answer rests on, with the verdict from its Litmus review. The answer gives weight only to claims those reviews found supported.

Original studies

Academic Performance and Single-Sex Schooling: Evidence from a Natural Experiment in SwitzerlandEisenkopf et al. (2011)Dubious — probably doesn't hold up
This study claims that randomly assigning girls to all-female classes at a Swiss high school raised their math grades by about 10% of the usable range, especially with male teachers, and that the effect works through improved self-confidence. The core design idea — within-school assignment eliminating self-selection — is genuinely valuable, but because the only outcome is teacher-assigned grades with no standardized benchmark, the study cannot distinguish whether girls learned more math or were simply graded differently in single-sex classrooms, and the teacher-gender finding is almost certainly an artifact of having only eight math teachers.
Single-sex schools, student achievement, and course selection: Evidence from rule-based student assignments in Trinidad and TobagoJackson (2012). Journal of Public EconomicsCredible — likely right, with caveats
Uses Trinidad and Tobago's centralized school-assignment algorithm to estimate the causal effect of single-sex schooling — a genuinely clever design that convincingly shows naive comparisons are inflated by selection. The core finding that most students show no benefit is credible, but the more specific claims — that girls with strong preferences benefit enormously, that single-sex schools reduce girls' science-taking, and that universal expansion would be pointless — rest on small subgroups, untested assumptions about course offerings, and extrapolations beyond what the data support.
Causal Effects of Single-Sex Schools on College Entrance Exams and College Attendance: Random Assignment in Seoul High SchoolsPark et al. (2012). DemographyDubious — probably doesn't hold up
Claims that Seoul's lottery-based school assignment proves single-sex schools cause better exam scores and college attendance for both boys and girls. However, 80% of single-sex schools are private while only 30% of coeducational schools are, and the estimated effects shrink 40–57% with even limited controls — meaning the paper cannot separate the effect of gender-segregated classrooms from the effect of private-school governance, teacher hiring, and institutional culture.
Gender, single-sex schooling and maths achievementDoris et al. (2013). Economics of Education ReviewCredible — likely right, with caveats
This paper finds a gender gap in maths favouring boys among Irish nine-year-olds, concentrated at the top of the score distribution, and reports no evidence that single-sex schooling reduces it. The descriptive finding is solid, but the causal claims about single-sex schooling rest on a questionable assumption of random assignment and the study lacks the power to detect modest effects — the headline claim that the gap is actually larger in single-sex schools is not statistically distinguishable from zero.

Studies that pool other studies

The effects of single-sex compared with coeducational schooling on students’ performance and attitudes: A meta-analysis.Pahlke et al. (2014). Psychological BulletinCredible — likely right, with caveats
This meta-analysis of 184 studies finds that when studies control for selection effects, the average difference between single-sex and coeducational schooling is small across academic and social outcomes. The directional finding is probably right, but the strong conclusion that single-sex schooling provides 'no benefits' overshoots the evidence — the 'controlled' category mixes strong and weak designs, the 12 truly randomized studies are never analyzed separately, and pooling fundamentally different types of single-sex programs could mask real effects that cancel each other out.

Critiques

The Pseudoscience of Single-Sex SchoolingHalpern et al. (2011). ScienceDubious — probably doesn't hold up
This policy essay argues that single-sex schooling has no proven academic benefits and harms children by increasing gender stereotyping, calling for federal regulations to be rolled back. The argument demands rigorous experimental evidence before accepting that single-sex schools help, but accepts laboratory experiments and preschool correlational data as evidence that they harm — an asymmetry that aligns with all seven authors' roles as founders of an advocacy organization for coeducation, and that undermines the paper's claim to be following the evidence wherever it leads.