Women in their 40s who went to single-sex schools subsequently had higher incomes than those in mixed classes, a long-term study shows.
Researchers think greater self-confidence might be a factor
Researchers at London University's Institute of Education are tracking almost 13,000 people born in 1958.
They said boys and girls in single-sex schools were less likely to have studied "gender stereotyped" subjects.
This might have accounted for the girls' higher earnings, because actual exam results were no better, they said.
Girls in girls' schools were more likely to have taken sciences and maths at A-level than those in co-educational establishments.
Boys in boys' schools were more likely to have chosen modern languages and English at A-level.
Researcher Dr Alice Sullivan said: "Single-sex schools seemed more likely to encourage students to pursue academic paths according to their talents rather than their gender.
"This suggests that co-educational schools need to examine the ways in which they have, probably unwittingly, enforced powerful gender stereotypes on both girls and boys."
Another member of the research team, Prof Diana Leonard, said the single-sex schoolgirls' higher earnings might have been because they were filling more technical or scientific roles even within female-dominated jobs.
"For example, becoming science teachers rather than French teachers, or because they have learned to be more self-confident in negotiating their wages and salaries."