As of August, many more younger children in Scotland will be spending roughly the same amount of time in preschool as older children do in school.

That’s because of the Scottish government’s commitment to almost double the number of free hours of early learning and childcare families are entitled, from 600 to 1,140 hours a year. In some areas, the new entitlement is already available as councils gear up for full rollout after the summer holidays.

But what can we expect the impact to be on children’s educational outcomes, given the government has made it clear this policy is not just about making it easier for parents to work, but also part and parcel of its flagship efforts to close the attainment gap?

Background: Extra nursery hours may not improve attainment

Audit: ‘Significant risks’ to nursery expansion remain

First audit: Watchdog warns of £160 million ‘black hole’ in Scottish nursery plan

Related: Progress on nursery hours - but staffing ‘crisis’ looms

News: Fresh fears over doubling of free nursery hours

If the Growing Up in Scotland (GUS) survey - longitudinal research following thousands of Scottish children from birth - is to be believed, this huge investment could have minimal impact on outcomes, and possibly even “detrimental effects”.

A 2018 report by GUS looked at whether there was a relationship between the average number of hours a child attended nursery and five-year-olds’ development, using data collected from more than 6,000 children and their families.

Its analysis did not find “any evidence to suggest” the increase in hours was “likely to have any notable impact on children’s outcomes by the time they enter school, either positive or negative.” It added that, if the expansion of free hours resulted in a dip in the quality of nursery provision, extra time in preschool “may well have more detrimental effects”.

The Effective Pre-School, Primary and Secondary Education study - another piece of longitudinal research, this time from England - found that, when it came to children’s outcomes, half days in nursery were as good as full days, as long as the quality of the nursery was good.

The co-director of that research, Iram Siraj, a professor of child development and education at the University of Oxford, has said that the universal expansion of free nursery could widen the attainment gap in Scotland, because middle-class parents are more likely to get their children into the best-quality provision.

Then there is the Education Endowment Foundation, which through its “teaching and learning toolkit” attempts to weigh up available research evidence for different education interventions in Scotland - from homework to class sizes - and come to a conclusion about how effective they are.

On early-years interventions, the EEF says there is “moderate impact” for “very high cost”. It adds that once early years provision is in place, “improving the quality of provision, for example by training staff to improve the interaction between staff and children, appears to be more promising than increasing the quantity of provision (by providing extra hours in the day)”.

In a bid to ensure the quality of preschool provision offered via the expansion is at least “good”, the Scottish government has told councils they must only take on partner providers to help provide the hours that are rated “good” or better by the Care Inspectorate. The government has also said that all staff providing early learning and childcare should be paid the living wage.

However, an Audit Scotland report published last week - which provided an update on how the expansion was progressing - pointed out that “EU procurement rules prohibit councils from specifically requiring providers to pay a wage that is higher than the national minimum wage”.

Given this, and that the research is far from a ringing endorsement of the government’s policy - at least from the point of view of educational outcomes - the one thing the government should be ready to do is monitor impact of the expansion.

Reassuringly, the Audit Scotland report said the government was making “good progress with evaluation plans”. The Scottish Study of Early Learning and Childcare (SSELC) will measure children’s social, emotional, behavioural and cognitive development when they leave funded early learning and childcare to start school, after having experienced 1,140 hours a year. These children will be compared to baseline samples of children receiving the current 600 hours.

So, we should have conclusive evidence in a few years’ time. Until then, the policy is something of an article of faith - and one which, when fully up and running, will cost the government an extra £567 million a year.