Children who go to grammar schools in England achieve better grades than those of similar ability who are not in selective areas, researchers claim.
Poorer children benefit more from grammar schools, the study says
A Bristol University study suggested pupils from poorer backgrounds do particularly well.
But relatively few children from the poorest families go to grammar schools.
"If access could be widened then the case for keeping selective education would be greatly enhanced," it said.
The research examined the 19 education authorities - out of a total of 150 - that still have substantial academic selection for secondary schools.
This selection, at the age of 10, typically leads to 25% of pupils being chosen for the 164 remaining grammar schools.
The academics say both sides of the heated pro- and anti-selection argument have overstated their cases, but that the findings of their study are very clear.
Overall there is little or no impact on attainment, but the quarter of children educated in grammar schools do substantially better than their peers in similar non-selective areas.
By substantially, they mean 3.5 grade points - the equivalent of raising 3.5 GCSEs from grade C to B.
The other 75%, who did not go to grammar schools, were disadvantaged by 0.5 grade point.
"In part these effects stem from the substantive under-representation of poorer and special-needs children in grammar schools," said the academics, from Bristol's Centre for Market and Public Organisation.
The change in the social mix of schools raised attainment in the grammar schools but had "moderately adverse effects" for the rest.
"The paradox is that, for the minority of poor children who do gain a place in a grammar school, the advantage this bestows appears to be greater than for more affluent children," they reported.
"If access could be widened then the case for keeping selective education would be greatly enhanced."
The Labour government promised not to extend selection but did not abolish the existing grammars, where pupil numbers have in fact increased.
Instead it set up a system of parental ballots. Only one was held, in Ripon, where parents decided to keep the selective system.
Pro-comprehensive campaigners say the ballot process is biased against those who want to get rid of grammars.
The Result of 11 Plus Selection: An Investigation into Opportunities and Outcomes for Pupils in Selective LEAs by Adele Atkinson, Paul Gregg and Brendon McConnell.