It's preschool. Oxford University's Edward Melhuish and Rutger University's Steve Barnett are visiting Australia for a series of talks, calling on Australian governments to extend public preschool to 3 year olds as well as 4 year olds.

That's because the weight of international evidence shows that the achievement gap when 5 and 6 year olds start school carries all the way through (if they don't get targeted help) and impacts on social adjustment and earning capacity later in life, with broader costs for society.

Emilie Capes from Randwick has three children under 5. Thomas, 5, is in kindergarten while Josephine, 3, and Charlotte, 2, attend a Goodstart Early Learning long day care centre a couple of days a week.

"They're definitely learning, it's not just a free-for-all playtime," she says. "There's a structure to their day which is really good preparation for school.

"I think the most important for me is the social skills they're learning, how to share and how to be independent. They have to get their own glass of water when they want it, they have to look after their own property, manage where their hats are and their clothing. That gives them really good skills."

Professors Melhuish and Barnett argue Australia is being left behind while other leading OECD countries, including the UK, have mandated two years of quality preschool. Broadly, Australia provides for one year (16 hours per week of preschool for 4 year olds). NSW last year removed subsidised preschool places for 3 year olds to better target 4 year olds.

"What children experience before school has long-term implications for their education right through school, and when they leave school," says Professor Melhuish. "Those effects are maintained right through into adulthood, into higher salaries, better self-regulation, better social adjustment."

Or as Professor Barnett puts it: "In the US disadvantaged kids are 18 months behind when they enter kindergarten. Almost all the achievement gap, which is what produces the income gap, is there before they start school."

It's particularly important for disadvantaged kids from lower socio-economic backgrounds as the home environment, particularly the mother's education level, has the biggest impact on learning outcomes. Preschool - quality, play-based early learning - can level the playing field for these kids before they start school.

Professor Melhuish has been involved in early childhood research in 12 countries, including a major longitudinal study that tracked a cohort of teenagers since they were born. His research showed that even low or average quality preschool boosts 5-year-olds' reading skills by months compared with their peers who had no preschool; and that preschool's positive effects continue all the way through to late highschool and beyond.

"Across the developed world, all of Northern Europe has a policy of compulsory early education for children. Other countries are doing this, like China," he says. "In Australia, it makes a lot of sense for you to extend that down to 3 year olds, because the evidence is indisputable that early play-based education benefits children very clearly all the way up."

High-quality early education is all about the teachers, says Professor Barnett, who assisted the Obama Administration's initiative to improve access to preschool in the US.

"At its core it's about what the teacher does with the child. Can they deliver rich content and interactions that are personalised to each child, one-on-one and in small groups, in an intentional planned way that accumulates over the year?

"If you have children in a program for two long daycare days and they have different teachers each day, clearly that doesn't happen. If the teachers themselves are not well educated or if the groups sizes are too large this doesn't happen."