There are three categories of evidence:

1. Major nationwide studies, involving tens or hundreds of thousands of students, in countries such as Australia or the United Kingdom where single-sex public education is widely available;
2. "Before and after" studies, examining a particular school or schools before and after the introduction of single-sex classrooms. Because these studies usually involve no change in resources -- the facilities and student-teacher ratios are the same before and after the switch -- the school serves as its own control;
3. Academic studies, in which investigators study coed and single-sex schools while attempting to control for extraneous variables

First category of evidence: Major nationwide studies: England, Australia, Jamaica

England, July 2002: The National Foundation for Educational Research was commissioned to study the effect of school size and school type (single-sex vs. coed) on academic performance. The Foundation studied 2,954 high schools throughout England, where single-sex public high schools are widely available. They released their report on July 8 2002. They found:

1. Even after controlling for students' academic ability and other background factors, both girls and boys did significantly better in single-sex schools than in coed schools. In this age group (senior high school), the benefits were larger and more consistent across the board for girls than for boys. Specifically, girls at all levels of academic ability did better in single-sex schools than in coed schools; whereas for boys, the beneficial effect of single-sex schools was significant only for boys at the lower end of the ability scale. For higher-achieving boys, there was no statistically significant effect of school type on performance, positive or negative. (Remember, though, that this study only examined students in grades 9 through 12; other evidence [see below] suggests that single-sex education is most effective for boys in kindergarten and elementary school.)
2. Girls at single-sex schools were more likely to take non-traditional courses -- courses which run against gender stereotypes -- such as advanced math and physics. The researchers concluded that girls' schools are "helping to counter rather than reinforce the distinctions between 'girls' subjects' such as English and foreign languages and 'boys' subjects' such as physics and computer science" (p. 43). No such effect was seen for boys: for example, boys at single-sex schools were no more likely (actually somewhat less likely) to take courses in cooking than were boys at coed schools.
3. Schools of medium size (about 180 students per grade) seemed to do best. At smaller schools, there was a lack of course offerings especially at the advanced levels. At much larger schools, student performance appeared to suffer.

The Foundation concluded: "It would be possible to infer from the findings that, in order to maximise performance, [public] schools should [have] about 180 pupils per cohort, or year, and be single-sex." You can read the Foundation's press release (with links to obtain the complete study) here.

A large Australian study, 2001:The Australian Council for Educational Research released a study comparing single-sex and coeducational schools. Their analysis, which was based on six years of study of over 270,000 students, in 53 academic subjects, demonstrated that both boys and girls who were educated in single-sex classrooms scored on average 15 to 22 percentile ranks higher than did boys and girls in coeducational settings. The report also documented that "boys and girls in single-sex schools were more likely to be better behaved and to find learning more enjoyable and the curriculum more relevant." The report concludes: "Evidence suggests that coeducational settings are limited by their capacity to accommodate the large differences in cognitive, social and development growth rates of boys and girls aged between 12 and 16." The findings of the Australian commission were widely reported throughout the English-speaking world (including Australia, New Zealand, England, Scotland, Ireland etc.) -- but have never been mentioned in any American newspaper.

You can read an authoritative British news report on this study by going to the search page of the Times Educational Supplement, enter Geoff Maslen as the author, "Mixed classes fail both sexes" as the title, and September 2001 as the date of publication.
You can also read a summary of the ACER report at the ACER's own Web site.

Some critics used to argue that single-sex public schools attract children from more affluent families. These critics suggested that the superior performance of students in single-sex schools may be due to the higher socioeconomic class from which such students are purportedly recruited, rather than the single-sex character of the school itself. However, both the ACER study in Australia just mentioned, and the Foundation study mentioned at the top of the page, both found no evidence to support that hypothesis. In the United States, Cornelius Riordan has shown that girls who attend single-sex Catholic schools typically come from a lower socioeconomic background than girls who attend coed Catholic schools. Among boys, Professor Riordan found no difference in socioeconomic status. In 1998, the British Office for Standards in Education (OFSTED) tested whether socioeconomic variables might account for the superior performance of students in single-sex schools. They examined test results from 800 public schools, single-sex and coeducational. OFSTED found that the superior performance of students in single-sex schools cannot be accounted for by socioeconomic factors, but appears instead to be a direct result of single-sex education. They also found that students in single-sex schools have a significantly more positive attitude toward learning.¹

The Foundation study, which suggests that single-sex education is more beneficial for girls than for boys, is somewhat at variance with an earlier study which suggested that single-sex education was more beneficial for boys than for girls. Educator Graham Able published a study of student performance in 30 coeducational and single-sex schools in England. Dr. Able's study documented superior academic performance of students in single-sex schools, after controlling for socioeconomic class and other variables. "The most significant finding was that the advantage of single-sex schooling is even greater for boys in terms of academic results than for girls," Able said. "The unsubstantiated mythology of the educational establishment has been that girls do better in single sex schools but that boys are 'brought on' by the more studious girls in a co-educational environment. This mythology has never been supported by any objective evidence, and any policy derived from it must presumably sacrifice the advantages to one sex in order to promote the cause of the other," he wrote. "[Our] results suggest that single sex schools give an even greater academic advantage to boys than for girls. This directly contradicts the popular educational myth that boys do better in the classroom if girls are present to set them a good example. One could reasonably conclude from this study that both boys and girls are academically disadvantaged in co-educational schools, but that the disadvantage is greater for the boys."²

The British have good reason to be impressed by single-sex schools. Single-sex schools routinely and overwhelmingly earn the highest scores on the required nationwide examinations (the GCSE exams). Last year, almost every one of the 50 top-ranked British high schools, including all the top 20, were single-sex schools. The best-performing coeducational school just managed to make 32nd place -- an improvement from the year before, when the best-performing coed school landed at #38.³

A classic study from Jamaica:Marlene Hamilton, studying students in Jamaica, found that students attending single-sex schools outperformed students in coed schools in almost every subject tested. At the time of the study, public single-sex schools were still widely available in Jamaica, so that there were few if any socioeconomic or academic variables which distinguished students at single-sex schools from students at coed schools. Hamilton noted the same pattern of results which has been found in most studies worldwide: Girls at single-sex schools attain the highest achievement; boys at single-sex schools are next; boys at coed schools are next; and girls at coed schools do worst of all.⁴

Second category of evidence: "Before and after" studies

Critics of single-sex education sometimes object that studies comparing students at single-sex schools with students at coed schools are intrinsically untrustworthy, because (they say) one can never control for all the confounding variables. "Before and after" studies are done at just one school, before and after its transformation to a single-sex school. Same students, same teachers, same facilities. These studies offer another compelling proof of the superiority of single-sex education.

In 2000, Benjamin Wright, principal of the Thurgood Marshall Elementary School in Seattle, Washington, led his school in a transformation from traditional coed classrooms to single-sex classrooms. . . with astonishing results. Mr. Wright was concerned about the high number of discipline referrals he was seeing: about 30 children every day were being sent to the principal's office because of discipline problems (about 80% were boys). He decided to make the switch to single-sex classrooms in hopes of decreasing the discipline problem.

The results exceeded his hopes. Discipline referrals dropped from about 30 per day to just one or two per day. "Overnight. The change in the atmosphere happened overnight." Same kids, same teachers. Switching to single-sex classrooms had a dramatic effect, instantly.

But improved discipline wasn't the only benefit of the change. "We were just doing it to make sure that the discipline was taken care of. But once we made the switch, the boys were able to focus on academics, and so were the girls. The boys, remarkably, shocked the state with what they did on the Washington Assessment of Student Learning. Our boys went from being in the 10 to 30 percent listing to 73 percent. They went from a reading average of about 20 percent to 66 percent. Our boys outperformed the entire state in writing. They went from being in a low percentile of 20-something to 53 percent in writing. The girls improved their performance too, everywhere but math." (Contrary to several news reports, Mr. Wright is not the first American principal to attempt this transformation.)⁹

Another elementary school at the other end of the country -- in Washington, DC -- has reported results remarkably similar to those obtained in Seattle. Moten Elementary School is located in the poorest section of Washington DC. Over 98 percent of the students qualify for subsidized lunches. In the fall of 2001, principal George Smitherman made a daring move: he decided to split the classrooms up by sex. Girls would go to all-girl classes, boys would go to all-boys classes. He also cut the lunch hour in half. But he didn't tell anybody what he was doing. He didn't consult with or inform the superintendent. He just went ahead and did it.

The results exceeded his wildest dreams. In mid-June 2002, Smitherman learned that the percentage of students scoring in the two highest categories of the math portion of the Stanford 9 test had jumped in just that one year, from 49 percent to 88 percent. On the reading portion, the percentage of students in the top two categories had shot up from 50 percent to over 91 percent.

And that wasn't all. "As a result of the single-sex classes, discipline problems have decreased by 99 percent," Mr. Smitherman recently told CNN. In previous years, such an experiment would likely have faced termination by the district and/or a legal challenge on the grounds that the single-sex arrangement violated Title IX, regardless of its proven success. No more. This year, the achievements of Moten Elementary School made the front page of the Washington Post, which reported that "D.C. school officials said they will study the change . . . to see whether other schools should follow a similar course."

These results aren't confined to elementary schools. An inner-city high school in Montreal made the switch from coed classrooms to single-sex classrooms five years ago. Since making that switch, absenteeism has dropped from 20 percent before the switch to 7 percent now. About 80 percent of students pass their final exams, compared with 65 percent before the switch. And, the rate of students going on to college has nearly doubled. You can read more about this Montreal high school here.

The "before and after" experience of schools undertaking this transformation has been so consistent, and so impressive, that the British Secretary of Education two years ago asked the Office for Standards in Education (OFSTED) to investigate whether this model should be applied widely throughout Britain, in a wholesale conversion of coed schools to single-sex academies.⁷

Researchers at Manchester University in England recently tested this approach more formally. They assigned students at five public schools either to single-sex or to coed classrooms. 68 percent of boys who were assigned to single-sex classes subsequently passed a standardized test of language skills, vs. 33 percent of boys assigned to coed classes. Among the girls, 89 percent assigned to single-sex classes passed the test, vs. 48 percent of girls assigned to coed classes.⁸

Third category of evidence: academic studies comparing single-sex schools with coed schools

Cornelius Riordan, professor of sociology at Providence University in Rhode Island, has published a series of studies comparing short- and long-term outcomes of graduates of single-sex Catholic schools in the United States with graduates of coed Catholic schools in the United States. Riordan's research is notable because of the great care he takes to control for all relevant variables, including socioeconomic status, race, and ability at time of school entry. On a variety of measures, Riordan has found that girls in single-sex schools consistently outperform girls at coed schools. In Riordan's studies, the beneficial effect for boys is smaller than it is for girls (contrast this finding with Graham Able's report (see above) that the benefits of single-sex schooling are greater for boys than for girls). Riordan has done the most systematic work demonstrating that the beneficial effects of single-sex schooling are most impressive for children from underprivileged backgrounds.¹⁰ As Riordan observes, that finding is a corollary of one of the most basic and robust facts in educational research: namely, that school variables always impact performance of children from underprivileged backgrounds. Consider a child from an affluent family with a mother and father who are both college graduates and who care about the child's schooling. That child is likely to do well, almost regardless of what school they attend -- at least that's what many studies have shown. If the home atmosphere favors learning, that outweighs almost anything the school does. But now consider a child whose only parent is a single parent trying to work three jobs to make ends meet, and there are few college graduates in that child's neighborhood, and no library. That child is at high risk. A single-sex school allows that child to focus on academics. The beneficial effects are huge and well-documented.

Researchers at the University of Michigan compared graduates of Catholic single-sex high schools with graduates of Catholic coeducational private schools. Boys in the single-sex high schools scored better in reading, writing, and math than did boys at coed high schools. Girls at the single-sex schools did better in science and reading than girls in coed schools. In fact, these researchers found that students at single-sex schools had not only superior academic achievement, but also had higher educational aspirations, more confidence in their abilities, and a more positive attitude toward academics, than did students at coed high schools. And, girls at the single-sex schools had less stereotyped ideas about what women can and cannot do.¹¹ The same University of Michigan team later reported that the beneficial effects of single-sex education don't end after students leave the school. They found that graduates of single-sex schools were more likely to go to a prestigious college, and more likely to aspire to graduate school or professional school, than were graduates of coed schools. That finding held for both girls and boys.¹²

In one remarkable study of 2,777 English high school students, girls at coed schools were found to lose ground to boys in science and vocabulary as they progressed through high school. Exactly the opposite occurred at single-sex schools: the girls at single-sex schools outperformed both the boys at single-sex schools and the boys at coed schools. Again, this study reported the familiar pattern: girls at single-sex schools on top, followed by boys at single-sex schools, then boys at coed schools, with girls at coed schools doing the worst.¹³

Not just better students; more well-rounded people
The benefits of single-sex schools are not only academic. Just as importantly, single-sex education has been shown to broaden students' horizons, to allow them to feel free to explore the own strengths and interests, not constrained by gender stereotypes. A British researcher compared the attitudes of 13 and 14 year-old pupils toward different subjects. Students at coed schools tended to have gender-typical subject preferences: boys at coed schools liked math and science and did NOT like drama or languages, whereas boys at single-sex schools were more interested in drama, biology and languages. Likewise, girls at girls-only schools were more interested in math and science than were girls at coed schools.¹⁴

Andrew Hunter, now the principal of Merchiston Castle School in Edinburgh (Scotland) agrees. Having taught in both coed schools and single-sex schools, Mr. Hunter observes that there is "a subtle and invidious pressure towards gender stereotyping in mixed [= coed] schools. Girls tend to be cautious about going into subjects or activities which are thought of as essentially boys' things, but in boys' schools boys feel free to be themselves and develop, to follow their interests and talents in what might be regarded as non-macho pursuits: music, arts, drama."¹⁵ Brian Walsh, who has been a principal at both boys' schools and coed schools, made this observation: "Boys ordinarily do not even try to sing in a coed school, whereas they love choral singing in a boys' school; in the coed setting they make fun of French pronunciation, whereas in the single-sex setting they enjoy becoming fluent in French; in drama, they muck up or clown around to avoid seeming imperfect in a coed setting, whereas they excel at drama when by themselves."¹⁶

At many coed schools, it's not "cool" for kids to be excited about school. The game of who likes who, who's going out with who, who's cool and who's not, is what's really important at most coed schools. That's seldom the case at single-sex schools. Edison Trickett and Penelope Trickett, comparing students at private single-sex schools in the United States with students at private coed schools in the United States, found that students in the single-sex schools had a far more positive attitude toward academics than did students in coed schools. This finding held for both boys and girls. The students at the single-sex schools also developed better organizational skills, and were more involved in classroom activities.¹⁷

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