London - Grandparents who act as unpaid babysitters for their children could be rewarded by living longer.
Looking after the grandchildren, even only occasionally, may cut the risk of dying early by a third, scientists found.
Research suggests it could add as much as five years to someone’s life. Half of those who spent time with their grandchildren were still alive a decade after being surveyed. For those who did not help out, half had died within five years.
The study suggests grandparenting could reactivate the same "care-giving" brain pathways that older people had decades previously as parents. These pathways are believed to buffer the negative effects of stress on the body, preventing their health from going downhill.
Researchers from Germany, Switzerland and Australia tracked more than 500 people aged 70 and older to examine their life chances if they helped with grandchildren, did not help or did not have any grandchildren.
The risk of dying for those who helped with care was a third lower than those who did not, and 37 percent below those without grandchildren.
Co-author Dr Ralph Hertwig, from the Max Planck Institute for Human Development in Berlin, said it is not clear exactly why they might live longer but added: "Possibly [grandparenting] activities are helpful in the context of cardiovascular disease, simply because you are more in motion.
"They may help to slow the cognitive degradation because you have to plan, organise, have to deal with each other."
He said oxytocin, the "cuddle hormone" released by bonding, may play a part in keeping people healthy, but cautioned that older people who look after their grandchildren full time or have more intense involvement may reverse the positive effects.
"Helping shouldn’t be misunderstood as a panacea for a longer life," he added. "A moderate level of care-giving involvement does seem to have positive effects on health.
"But previous studies have shown that more intense involvement causes stress, which has negative effects on physical and mental health."
The study backs up the "grandmother hypothesis", which suggests women have evolved to live well beyond menopause because they help to raise their grandchildren. It suggests hunter-gatherer grandmothers were of evolutionary use to humanity by boosting the success of the next generation.
The grandparents considered, from the Berlin Aging Study, saw their grandchildren less than every day but their "helping behaviour was associated with reduced mortality".
The effect was not only seen in family members, as older non-grandparents who helped others, with housework or fixing things in the house for example, also lived longer. Half had died within around seven years of the 1990 survey, compared with five years for other non-grandparents or grandparents who did not help with grandchildren.
The study, published in the journal Evolution and Human Behaviour, was done by researchers from the University of Basel, Edith Cowan University in Australia, the University of Western Australia, the Humboldt University of Berlin and the Max Planck Institute.