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A study has found maternal employment has a positive effect on children's academic performance.

It's a trade-off that troubles even the most hard-headed working parent: time at work versus time with the kids.

Mothers are especially prone to anxiety over the clash between family and job. Even though the workforce participation rate of mothers has surged in the past 30 years, there's still a strong expectation in Australia that mothers will be very present with children in their early years.

But now researchers have made a discovery that will comfort guilt-ridden career women: children with a working mother do better in high school than those with a stay-at-home mum.

That's what a team of policy analysts and economists from the US and Denmark found when they examined unique survey data that tracked 135,000 Danish children from birth until the age of 15.

The survey recorded children's school grades over time as well as detailed information about their households, including the work status of their parents.

This allowed the researchers to compare the academic results of 15-year-olds whose mothers worked with those who had stay- at-home mothers.

Because the survey sample was so large the researchers were able to control some of the natural variables that affect children's educational outcomes.

This meant they could compare children who were otherwise similar except for the employment status of their mothers.

They examined the association between a child's ''grade point average'' in year 9 and his or her mother's work patterns during the first three years of the child's life, and separately during the first 15 years of the child's life.

After an exhaustive analysis the research team, led by Professor Rachel Dunifon from the US's Cornell University, concluded ''maternal employment has a positive effect on children's academic performance'' even for those whose mothers worked during the first three years of life.

The best performers had mothers who were part-time employees.

The grades for a child whose mother worked between 10 and 19 hours a week when they were aged under four years of age had grades that were on average 2.6 per cent higher when they reached year 9 than an ''otherwise similar'' child whose mother did not work at all.

The researchers found the beneficial effect of maternal employment was even larger when a mother's full 15-year work history was taken into account.

These results challenge the conventional wisdom in Australia and raise an important question: why might children with a mother who works do better over time than those with a mother who stays home?

The most obvious reason is the additional financial resources. Dual-income families have more resources to spend on the education and ''enrichment'' of their children, including coaching classes, music and sport.

But a factor often ignored in debates about work and family is the benefits that mothers derive from working.

Professor Dunifon and her co-authors note that the higher grades achieved by teenagers with working mothers could be the result of ''improved mental well-being'' among the mothers who work.

Women who have satisfying jobs ''reap psychological, social or other benefits of employment in ways that benefit children'', they conclude in their paper The Effect of Maternal Employment on Children's Academic Performance, published by the National Bureau of Economic Research.

Studies from a variety of countries and cultures have shown that work - and the financial security that comes with it - can improve mothers' sense of self-esteem and confidence. This, in turn, benefits their children.

However, there was another dynamic at work in the Danish study: good childcare.

Denmark has invested heavily in its childcare system, which has become internationally renowned for high quality, easy access and low cost.

This first-class childcare seems to have been a factor in the superior academic results achieved by year 9 students with working mothers.

There's an important lesson here for Australia.

Now that most families need two incomes to survive, improving our childcare system is more important than ever.