"Older women who are socially engaged have better cognitive function ... but too much of a good thing just might be bad" ... Researcher Margery Gass Photo: Getty Images

There is no doubt children gain a lot from time spent bonding with their grandparents. But a new Australian study shows the benefits go both ways, with post-menopausal women who look after their grandchildren at least once a week less likely to develop cognitive disorders such as Alzheimer's disease than their peers.

But mums and dads shouldn't cancel their childcare arrangements and drop the kids off with grandma every day just yet - the study also found that grandmothers who look after their grandchildren five days a week or more performed worse on a range of mental sharpness tests than others the same age.

"We know that older women who are socially engaged have better cognitive function and a lower risk of developing dementia later, but too much of a good thing just might be bad," said North American Menopause Society Executive Director Doctor Margery Gass.

Researchers studied 186 women ages 57 to 68, of which 120 were grandmothers. The women were asked to undertake three different tests of mental sharpness, and they also noted whether they thought their own children had been particularly demanding of them in the previous 12 months.

Out of the grandmothers in the group, those who spent one day a week taking care of grandchildren performed best on two of the three tests, while those taking care of grandchildren for five or more days a week did significantly worse on the test which assessed working memory and mental processing speed.

However, researchers say mood may also play a role in the test results, with grandmothers who look after their children five or more days a week also reporting their children were more demanding of them.

The study, which was part of the Australian Women's Healthy Aging Project and published in the US journal Menopause, is the first time the connection between mental sharpness and grandmothering has been investigated.

The results will be of interest to many Australian families, with a recent report showing that four in 10 young children are cared for by a grandparent on a weekly or more frequent basis.

According to a submission by the Australian Institute of Family Studies to a Senate committee, grandparents are increasingly sharing the role of caring for grandchildren, and are also providing financial support for their children's families.

"While the nuclear family continues to predominate in Australia and other Western countries, research over the years has consistently indicated that key relationships extend beyond household boundaries with much 'caring and sharing' occurring between members of the extended family network," the AIFS said in its submission.

"Grandparents are important providers of informal childcare, especially for children under the age of five years."

In addition to those caring for their grandchildren on a weekly basis, the the AIFS says there are about 46,680 families in Australia where children are cared for full-time by grandparents and not their parents.