Rich children start school 'five months ahead'
Children from high-income families start school with skills that are already five months ahead of their middle and low-income classmates.
The Resolution Foundation think tank analysed data on 15,000 children who turned five in 2006.
The better-off groups of children had wider vocabularies and fewer behavioural problems.
The authors found wealthier parents were able to create a "richer learning environment" for their children.
For example they found that 75% of higher-income children were read to daily aged three, compared with 62% of children in low to middle-income households.
While 42% of more affluent children visited a library at least once a month, this figure was 35% for worse off families.
Mothers in low to middle-income groups were at greater risk of post natal depression and had lower self-esteem and less sense of control over their lives, they said.
The researchers also said there was a statistical link between children's behavioural problems and poorer mental health and social isolation among their mothers.
Squeezed for time
The research, by Prof Jane Waldfogel of Columbia University and Jane Washbrook of University of Bristol, was based on data from the Millennium Cohort Study of 15,000 children born in 2000 and 2001.
The researchers said the lag in attainment by the less wealthy groups could also be explained by their parents' age and qualifications.
Low and middle-income group parents were three times more likely to have no formal qualifications beyond GCSE.
Vidhya Alakeson, director of research at the Resolution Foundation, said the problems faced by children in this group were not as bad as those in the lowest income groups, but they should be taken seriously, as there was a danger that without support some in this group could fall further behind.
She said "policy makers rightly focus on trying to improve outcomes for children in the very poorest families... but this new study shows the perils of ignoring the low to middle group who are a third of our future workforce.
"With parents increasingly squeezed for time and money, this only creates more stress and even less positive environments for their children."
Families on low to middle incomes are defined as having a household income of between £24,000 and £42,000 with two or more children. The data shows the average income of these families is around half that of the high-income group.
Ms Alakeson wants the government to ensure there are no further cuts to tax credits which have had a big impact on lower-income families.
She wants employers to be encouraged to allow working parents as much flexibility as possible. GPs and schools should be allowed to target support at low and middle income families. as well as the poorest, she argues, and there should be more awareness of post-natal depression.
"For these parents a relatively light-touch parenting support programme can be very effective," she said.