From either perspective, the main issue is the role these schools play in ‘social mobility’. The concept has many definitions, but is generally about how easy it is for people from poor backgrounds to make the most of their abilities.
Grammar schools educate far fewer pupils than in their heyday
Grammar schools are still an important part of the education system in Northern Ireland, where they account for around one third of secondary schools.
But there are only 163 grammar schools in England, which we’re focusing on for the purposes of this article.
That’s a far cry from their peak: in 1964 there were as many as 1,300 across the country, accounting at the time for 25% of state secondary pupils. It’s now around 5%.
The few that remain are scattered across England, with concentrations in ‘selective’ counties such as Kent, Lincolnshire and Buckinghamshire.
As such, studies into whether grammar schools are aiding or hindering social mobility focus on examining how these areas perform and comparing them to everywhere else.
We've looked at research about grammar schools in the 1950s and 1960s in a separate article.
How to measure social mobility
To gauge 'performance', you need something to measure. There are two measures often used for social mobility in the education system.
The number of pupils claiming free school meals is one measure. That’s what the Department for Education uses for allocating the Pupil Premium to schools to spend on disadvantaged children.
Grammar schools and admissions
Less than 3% of grammar schools entrants are entitled to free school meals, according to research from the Sutton Trust. For non-grammar schools in the same area, it's 18%.
A much larger proportion of grammar school entrants—up to 13%—came from private schools.
The Institute for Fiscal Studies has found that “amongst high achievers, those who are eligible for free school meals or who live in poorer neighbourhoods are significantly less likely to go to a grammar school”.
In addition, grammar school enrolment was less likely if a pupil's primary school had more pupils from deprived backgrounds, and if their primary school had more high achievers at Key Stage 2 English and Maths.
Conservative MPs who support new grammar schools have reportedly suggested that they be “created in socially deprived areas to challenge the image they are targeted at the middle class”.
Grammar schools and attainment
“Pupils eligible for [free school meals] appear to suffer marginally less educational disadvantage if they attend grammar schools”, according to researchers at the Centre for Evaluation & Monitoring, who found in 2008 that “the difference is equivalent to about one-eighth of a GCSE grade”.
But while some pupils may benefit, experts also say that existing grammar schools generally widen the gap in attainment between rich and poor pupils.
“There is repeated evidence that any appearance of advantage for those attending selective schools is outweighed by the disadvantage for those who do not”, says Professor Stephen Gorard of Durham University. “More children lose out than gain, and the attainment gaps between highest and lowest and between richest and poorest are larger”.
Similarly, Luke Sibieta at the IFS says that grammars "seem to offer an opportunity to improve and stretch the brightest pupils, but seem likely to come at the cost of increasing inequality".
And the journalist Chris Cook wrote recently that “the minority of children streamed into the grammars do better. The remaining majority of children—who are not educated in grammars—do slightly worse”.
Mr Cook has previously established this point in an article for the Financial Times, using figures from the National Pupil Database to investigate differences in attainment among rich and poor pupils within selective areas.
His graph, below, demonstrates the effect. Performance is measured using 'FT points', where pupils are scored out of 40 for their five best GCSEs.
If everyone were to perform the same, regardless of deprivation background, the lines would be horizontal.
Instead, they slope, favouring the better off—and even more so in areas that retain selection.
Update 6 September 2016
To stress that the analysis is referring to the current grammar school system, we added the word "current" to the headline.
We also added in the word 'existing' to the sentence: "But while some pupils may benefit, experts also say that existing grammar schools generally widen the gap in attainment between rich and poor pupils."