Third category of evidence: academic studies comparing single-sex schools with coed schools
Cornelius Riordan, professor of sociology at Providence University in Rhode Island, published a series of studies in the 1980's and early 1990's 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. On a variety of measures, Riordan found that girls in single-sex schools consistently outperformed 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 believes that the beneficial effects of single-sex schooling are most impressive for children from underprivileged backgrounds. However, this belief sets him apart from many other researchers in the field, particularly outside the United States.
Source: Cornelius Riordan. Girls and Boys in School: together or separate? New York: Teachers College Press, 1990.