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Why Single-Sex Schools Are More Successful

According to some studies, students from single-sex schools perform better than their peers in mixed-gender schools. By using a government policy in South Korea that caused some single-sex schools to convert to co-ed one grade at a time, this article aims to separate the causes from the effects. Even if their class remained single-sex, academic performance fell for boys when their schools went co-ed, but only for girls whose classes went mixed. These findings point to various mechanisms by which mixed-gender schools affect both boys’ and girls’ academic performance.

Single-sex education has drawn a lot of policy attention recently as a potential tool for boosting students’ academic achievements and eliminating various gender gaps. Single-sex education proponents contend that the environment is better for academic success because students avoid the attraction to the other sex and are less likely to experience gender stereotypes. On the other hand, proponents of mixed education contend that boys who attend coed schools perform better because they are surrounded by better-behaved and more intelligent female peers. The mixed-gender environment is also said to be better for both boys’ and girls’ social skill development, better preparing them for the “real world.”

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Therefore, whether or not a single-sex environment raises students’ achievement levels and whether or not the effects are felt by both genders is a crucial question. To advance in this, it is necessary to calculate the causal impact of students’ exposure to single-sex peers as opposed to mixed-gender peers. It is difficult to calculate this effect, especially given the endogenous procedures used to assign students to specific schools.

research objectives and plan
In a recent paper, we specifically address this issue. In Seoul, South Korea, students were randomly assigned to academic high schools within districts. Park et al. (2013) used this assignment policy. These authors demonstrate that boys (girls) who were arbitrarily assigned to all-boys (all-girls) schools outperform their counterparts at coed schools using data for a single cross-section (for the year 1999). However, the existing single-sex and co-ed schools in Seoul may differ not just in their pupil gender type but also in various observable and unobservable inputs that can affect student achievement. Therefore, even if pupils are randomly allocated to a single-sex versus a co-ed school, any differences in pupils’ outcomes between schools could be due to pupil gender type (the direct effect of pupil gender composition), or other observable and unobservable differences in school inputs (school effects). In turn, the direct effect of pupil gender composition could be due to exposure to mixed-gender peers at the school level, or the classroom level. For policymakers who are considering creating a new single-sex school or a single-sex classroom, the key question is whether there are any direct benefits to being exposed to a single-sex versus mixed-gender learning environment on either the school or classroom level.

The total effect of single-sex versus co-ed schools
We identify three distinct parameters, using administrative data on the national college entrance exam, taken by South Korean 12th graders from 1996-2009. To address the sorting problem, we use – similar to Park et al. (2013), though based on multiple years of data – the random assignment of pupils to schools within school districts at each cohort. This allows us to address our first question:

While causal, this shows the combination of the direct effect (driven by pupil gender composition) and the school effect (driven by other differences between single-sex and co-ed schools).

We find robust evidence that pupils in single-sex schools outperform their counterparts in co-ed schools, by 5–10% of a standard deviation for boys and 4–7% for girls, with similar estimates across subjects (which include Korean, English, and maths). This is consistent with the findings reported by Park et al. (2013). While these effects measure the causal effects of attending a single-sex (versus co-ed) school on test scores in Seoul, this total effect is specific to that context, as it combines the direct effect of exposure to single-sex versus mixed-gender peers and context-specific differences in school inputs between single-sex and co-ed schools. Therefore, this parameter is of interest to students and parents in Seoul, but not generalizable to other settings.

Direct effect
To isolate the direct effect, we exploit the fact that some of the existing single-sex schools in Seoul converted to the co-ed type in the 1990s and 2000s due to a government policy that favored co-education. This allows us to eliminate the effect of unobserved, time-invariant (and observed time-variant) school characteristics. That is, we can address the question:

Here, we exploit school-type changes (in conjunction with the random school assignment) and compare cohorts that were exposed to either a single-sex or co-ed environment at both the school and classroom levels. We find that the conversion of school type from single-sex to co-ed leads to worse academic outcomes for both boys and girls, conditional on school-fixed effects, suggesting that the combined effects of school- and classroom-level exposure to a single-sex (versus mixed-gender) environment are likely positive.

Classroom vs school-level exposure
Finally, we make advantage of the fact that academic high schools in South Korea consist of three grades (grades 10, 11, and 12) and that the school-type conversion was undertaken at the cohort level. Specifically, the first cohort accepted under the co-ed system was exposed to mixed-gender classmates at the classroom and school levels for three years. The prior cohort, although not exposed to mixed-gender classmates at the classroom level, were exposed to a school-level co-ed setting (and any school-wide changes owing to the move) for the final two of their three years of high school. To the extent that school-level exposure to the newly-co-ed environment influences these two cohorts equally, the difference in achievement between the two cohorts (and the associated difference in non-switching schools) permits us to answer our last question:

We make advantage of the multi-grade aspect of South Korean high schools, and the cohort-level conversion of child gender type. We discover that class-level exposure to same-sex (vs mixed-gender) peers has a substantial favorable influence on the achievement of females. Specifically, when we exogenously modify the percentage of females in the same cohort from 100% to roughly 50%, the accomplishment of girls in languages reduces by 8–15% of a standard deviation in the score distribution. For males, however, the advantages of having same-sex (vs mixed-gender) classmates in their cohort are minor and statistically inconsequential. Given our results concerning the direct impact above, this study shows that the presence of females in the same school (even if not in the same cohort) distracts boys from academic pursuits. In contrast, girls are less likely to experience these distractions, but they still experience disadvantages when they interact with boys in the classroom.

Conclusion and discussion
Our study’s main finding is that both boys and girls are likely to benefit from exposure to same-sex peers in the classroom and at school, as opposed to peers of different genders (what we referred to as the direct effect above). The underlying mechanisms, however, differ. The disadvantage of co-ed education for boys is primarily a result of their exposure to a co-ed environment at the school level. For girls, however, it is classroom-level exposure to mixed-gender (versus same-sex) peers that explain the disadvantage of co-ed schooling.

Teenage boys may be more likely than girls to be distracted in a mixed-gender classroom (Coleman 1961; Hill 2015), but girls may suffer more as a result of, for example, an increase in disruptive behavior (as discussed by Figlio 2007) or a teacher’s focus being diverted to less capable students (as suggested by Lavy et al. 2012).


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