What do these studies tell us about the relationship between single-sex education and achievement? Virtually nothing. Tidball made the common mistake of confusing correlation with causation. As Faye Crosby, a professor of psychology at Smith College, and other critics have observed, Tidball did not control for characteristics of women's colleges, apart from sexual homogeneity, that might well account for the success of their graduates. She did not allow for the socio-economic privileges shared by many graduates of elite women's colleges or for the selectivity of the schools.

Tidball's 1973 study focuses on women who graduated from women's colleges in the years before elite men's colleges were integrated; until the mid-1970s ambitious, high-achieving females gravitated to Seven Sister schools because they were among the most selective and prestigious institutions open to women. Students at these schools were self-selected for success, like their male counterparts in the Ivy League. They also tended to be well connected; many may have owed their success to the males present in their families more than to the absence of males from their classes.

Tidball's subsequent finding, that single-sex schools produce more female achievers in the sciences and medicine, also collapses under scrutiny. Faye Crosby and her Smith colleagues found in a comparison of similarly selective single-sex and coeducational schools that "women's colleges are not more productive." There are no definitive comparative data on the benefits of single-sex colleges for women. "Data are slim," Crosby writes, "but they indicate that coeducational schools are as likely to produce women scientists as are women's colleges."

Crosby's assertion challenges conventional wisdom about the risks of coeducation which is becoming increasingly fashionable in secondary schools. A recent, widely cited study commissioned by the American Association of University Women (prepared by the Wellesley College Center for Research on Women) decries the current system. Titled "How Schools Shortchange Girls," the AAUW report declares that "the educational system is not meeting girls' needs." The executive summary cites "gender bias as a major problem at all levels of schooling," asserting that girls are plagued by sexual harassment, even at the grade school level, and neglected by sexist teachers, who pay more attention to boys. As a consequence, it seems, girls fall behind their male classmates: "Girls and boys enter school roughly equal in measured ability. Twelve years later, girls have fallen behind their male classmates in key areas such as higher-level mathematics and measures of self-esteem."

These remarks might awaken the anxieties of many parents with daughters in coed schools. But the dire tone of the summary is belied by the underlying findings of the AAUW study. Reviewed in its entirety, it presents a more complex, nuanced view of the female educational experience, which varies with race and class. In fact, "socio-economic status," not sex, is said to be "the best predictor of both grades and test scores."