The big newspaper ad for one of America's oldest, most prestigious girls' schools made its point most succinctly in the photo caption:
"At single-sex schools, girls are the heroes, the problem-solvers, the innovators. All classroom voices are theirs, as are all honors, gavels, and leading roles."
The ad, featuring a picture of a confidently smiling young woman, was in a series run in the fall by the Emma Willard School in Troy, N.Y. The series caused a stir in the usually placid world of private institutions because it tweaked some of the school's competitors -- the big-name, one-time boys' schools that have gone coeducational.
But a growing body of research, including recent national surveys of public school students and private girls' school graduates, seems to bolster the ad campaign claims that adolescent girls do better when they don't have to share classrooms with boys.
The findings have given some hope to advocates of single-sex education who have watched in consternation as the numbers of all-boys' and all-girls' schools plummeted during the 1960s and 1970s, merging with opposite-sex schools or going coed in the face of declining interest in single-sex schools.
The National Association of Independent Schools, which represents non-parochial, college-preparatory schools, reported that during the 1963-64 school year, only 38 percent of its approximately 900 institutions were coeducational. By the 1990-91 school year, that proportion had risen to 79 percent.
Roman Catholic schools, the largest segment of the non-public school spectrum, experienced a similar phenomenon. Today, there are only about 300 girls' schools and 200 boys' schools; together they represent about 38 percent of the 1,300 Catholic secondary schools in the United States.
"It seems unfortunate in a sense that, just as research is showing there may be some benefits [to single-sex education], especially for girls, the trend is only one direction -- down and out," said University of Michigan education professor Valerie E. Lee, who has participated in some of the recent research on single-sex schools.
"I can't think of a single single-sex school that was opened in recent years. There is a strong group of supporters of single-sex education, but it is small, and it has not grown," Ms. Lee said.
Studies comparing Catholic school students at coed campuses with those in single-sex schools found students at all-boys' or all-girls' schools tend to have higher educational aspirations, do more homework and have more positive views of their own school. For the most part, the differences were strongest for girls.
Other research suggests boys tend to dominate discussions in coeducational classrooms and are more often elected to school leadership posts, thus helping perpetuate sex-stereotyping.
A survey released last year by the American Association of University Women found that large numbers of once-confident little girls emerged from adolescence with poor self-images and lower expectations.
By contrast, a survey of about 1,200 women, commissioned by a coalition of girls' schools and conducted by Yankelovich Clancy Shulman, a Washington-based public opinion company, found most alumnae of single-sex schools to be successful, independent, confident and involved in their communities. Girls' school graduates said their educations gave them greater self-confidence and more ambition.
Those most intimately involved with girls' schools say the benefits are real.
Barbara E. Wagner, head of Marlborough School in Los Angeles, which has been educating young women for more than 100 years, said that when she first arrived three years ago she immediately was struck by the Marlborough girls' "ability to be articulate, to ask probing questions. It just jumped out at me."