The social gap can persist throughout schooling
Poor parenting is the key factor behind the significant gaps in readiness for school between children from low and middle income families, a study says.
Analysis of ability test scores of children in the UK and US showed the poorer their families, the less well prepared they were for school.
But the study found up to half of these differences in the US were due to poor parenting and home environment.
It suggested early years "compensatory education programmes" could help.
The study led by Professor Jane Waldfogel of Columbia University, New York, sought to establish what ability gaps there were across income groups, how large they were, and what factors explained them.
It found similarly stark gaps in ability between children from poor and rich families in the US and the UK.
For example, four-year-olds from the poorest families in the US scored 34 points out of 80 in literacy tests, compared with 69 points out of 80 for the richest families.
And in the UK, three-year-olds from the poorest families scored 32 points out of 80 in school readiness tests, compared with 63 points for those from the the richest homes.
In England, differences between the social classes persist all the way up the educational ladder.
The study said: "The environments of low-income children differ in many dimensions from those of more affluent children. For example, access to toys, books, computers and learning-related activities may be directly affected by lack of income.
"But other dimensions, such as parental sensitivity and responsiveness to children's needs, may be linked more strongly to parents' education, knowledge of child development and psychological well-being."
'Making the difference'
Prof Waldfogel said her findings were "very concerning because with children starting out at school so unequal, it's very difficult to imagine they will end school on an equal footing".
She said what surprised her the most was the extent to which the way children were parented and how they were influenced by their home learning environment had an impact on their ability to learn once they reached school.
The researchers looked at only how parenting affected children's ability levels in the US.
Here, between a third and half of the gap between low and middle income groups were to due to factors related to parenting and the home environment, rather than income itself, she said.
These included, for example, how sensitive and responsive a parent was to their child's needs, whether they read to the child and whether they took them on outings.
"It is these factors that make the difference," she said, adding that higher income mothers were interacting more positively with their children from when their offspring were as young as nine months.
They have not yet studied the impact of this factor in the UK.
Prof Waldfogel said the research in the US suggested there were some useful "compensatory education" programmes which could help bring children from poorer backgrounds forward.
For the US, providing universal access to good quality pre-school child care would help.
But in England, where all three and four-year-olds are entitled to free part-time pre-school day care, she suggested ministers might consider improving parenting help for poor families when their children were very young.
She cited a scheme called Nurse Family Partnerships, which targeted parenting help and developmental awareness at low income first-time mothers during pregnancy and the first two years after a child was born.