Single-sex education seems to make no difference to students' achievements, a review of available research concludes.
Fewer parents are choosing single-sex schools
Studies from Australia, North America, New Zealand, Ireland and the UK were reviewed by Alan Smithers and Pamela Robertson of Buckingham University.
The findings show that gender is not necessarily important, they will tell a co-education conference on Wednesday.
"The main determinants of a school's performance are the ability and social background of the pupils," they say.
In the UK the girls' schools which topped the league tables tended to be independent, grammars or former grammars.
"While both single-sex and co-education have passionate advocates, half a century of research has so far revealed no striking or consistent differences one way or the other," they said.
"While there are some very good girls' schools and boys' schools, it does not look as though they are good because they are single sex."
The researchers note that attention recently has shifted to single-sex classes within schools.
They say there has been little research and no consistent findings, but "a number of enthusiastic media reports".
"These could be examples of the Hawthorne effect [a kind of placebo effect] whereby the extra attention associated with any change seems to bring about a short-term improvement."
Overall, they say, what evidence there is would suggest that the gender mix is only one of the factors in a school's success and its effects, if any, are usually not strong enough to be detected by the methods of educational research.
Parents may choose single-sex schooling for other reasons - perhaps cultural or religious.
But in the independent sector, which has to respond directly to parental choices, single-sex education is in decline.
It remains to be seen what will happen in the state sector, they say, as the government attempts to create a market there.
"Most parents seem to be looking for a good school regardless of its gender mix."
The report, The paradox of single-sex and coeducational schooling, was commissioned by the Headmasters' and Headmistresses' Conference of independent schools.