Grandparents hold their baby grandchild.
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Parents with close emotional ties to their children may be increasing their chance of having grandchildren down the road, a new study finds.
The results support the theory that humans evolved to live past their reproductive years because grandparenting might help ensure that grandchildren continue the genetic line. But it's not yet clear how supportive parents lead adult children to decide to have kids of their own.
"It may be that supportive parents make you feel that having a family or having a larger family is the thing to do," study researcher David Waynforth, a biological psychologist at the University of East Anglia in the United Kingdom, told Livescience.
The reason for grandma
Unlike most animals, humans have life spans that stretch decades past the point when reproduction becomes a thing of the past.
Scientists have argued that perhaps these long twilight years evolved because longer-lived individuals were able to pitch in to help their own children raise children, thus increasing the likelihood that longevity genes would be passed on. Studies of traditional tribes such as the Hadza in Tanzania have suggested that grandmothers can help their daughters gather food, saving their offspring energy and making it more likely that more grandkids will be born. A 2008 review of 45 studies on the topic published in the journal Evolution and Human Behavior found that having a maternal grandmother alive tends to improve a child's likelihood of survival.
Waynforth and his colleagues looked at the question in a population of British adults who took part in the 1970 British Cohort Study, a collection of information on all the babies born in Britain during one week in 1970. About 8,900 people in the sample had answered questions in the year 2000 about how much assistance they received from their parents, how emotionally and physically close to them they were, and whether their parents helped them with child-care duties. [11 Facts Every Parent Should Know About Their Baby's Brain]
Waynforth was interested in whether any of these factors changed the likelihood of a person deciding to give birth in the next five years after the data was collected. The researchers controlled for other factors that would influence reproductive decisions, including marriage, income and the presence of existing children in the household.
The results, published today (Sept. 13) in the journal Proceedings of the Royal Society B, revealed that parents do influence their kids' reproductive decisions, albeit mildly. People who reported a close emotional relationship with parents or reported seeing their own parents frequently were more likely to have children during the following five years than those with more distant attachments. The effect was modest, Waynforth said, but surprisingly stronger than the effect of having parents who provided real help in the form of child care or money. That sort of help did not increase people's likelihood of having kids, he said.
"It turns out that it's simply being a supportive parent and grandparent that seems to be associated with the higher likelihood of having a kid," Waynforth said.
Why a close relationship with parents makes child rearing more likely but day-to-day help doesn't is still a mystery. It could be that if you're hard up enough to need parental handouts, you wouldn't want to add another mouth to feed, Waynforth said.
People in close families, on the other hand, might view launching their own parent-child relationship with more optimism, or they might feel that their parents will be there for them if times get tough. Alternatively, they might be under pressure from baby-crazy grandparent wannabes who urge them to continue the family line, Waynforth said.
More study of different demographics is necessary to ensure that the results hold, Waynforth said. Changing economic circumstances over time will likely change the influence grandparents hold, he added.
Today, "it's pretty hard for a family starting out," Waynforth said. "Things may have changed since this 1970 birth cohort, and it's probably a constantly changing picture of what kind of effect grandparental assistance has."