I have read (no cite handy) that couples attempting to get pregnant do better if one or both sets of parents are nearby. IF this is so, and I don’t know for sure that it is, it would make some evolutionary sense in that having grandparents nearby would increase the chances that the offspring would be well cared-for. But, what subtle mechanisms within men or women might be affected by proximity to parents that would increase fertility?
How nearby is nearby? I know why chances of getting pregnant would be seriously decreased if the potential grandparents were too nearby. For more moderate distances, I suppose nagging about wanting a grandchild might be a factor. Apart from that, I don’t really believe it happens. If some study showed it, it was probably a statistical fluke. Most correlational studies showing small effects are.
Free time.
There are lots of unproven and often discredited ideas about human fertility - synchronized menstruation, pheromones, stress affecting conception, etc. Nearby parents could affect any of those ideas, but a lot of people think the jury is out on all of the research on those issues. (Cecil discusses and mostly dismisses the synchronized menstruation issue, for example.)
I’d look for some kind of social mechanism or even confirmation bias first, before I would assume that fertility rates have actually changed.
When Patrick Moore was having an argument with an astrologist, the astrolger said that the stars had a gravitational influence on a child at the point of birth. Moore pointed out that a fat midwife would have more influence than the stars.
Perhaps Grandparents have some cosmic influence.
In truth, and my own opinion, I suspect that for every beneficial grandparental influence there is an equal and opposite one.
Perhaps what you read was the fertility and child spacing in hunter gatherers or nomads? That makes more sense, and could be extrapolated by idiot pop-science journalists (Daily Mail, we’re looking at you) to apply to people now.
Hunter-gatherers usually spaced children further apart: at least four years, because you can only really carry one child when you’re on the move. A child needs to be able to walk before the next child can come along. There could be studies that indicate that with grandparents, such spacing was not necessary?
You may be confusing the definitions of fertility and fecundity as used for the purposes of demographic studies on the subject. In demography, fertility doesn’t refer to the physical ability or potential to reproduce, but whether or not reproduction occurs. (not as a result of fecundity but factors like finances, education, culture, etc.)
Studies have shown that women who live within the same town or region as their parents or in-laws are more likely to reproduce - not that they are more fertile in the biological use of the word.
Here isone such study that concluded women in Germany are about 4% more likely to have children when living nearby the future grandparents. This is basically due to the obvious benefit of having free childcare nearby, and the resulting ability that provides for the mother to work or continue school.
If you mean that there is a theory that proximity to parents increases fecundity you would probably need to provide a cite to the data for some context if you want any educated guesses about why that may be.
Specifically in Germany, according to a study I read about perhaps 3 years ago, the average flow of money from parents to their adult children is equal to $7000 per year. I didn’t read the linked study, but I imagine there are all sorts of possible mechanisms.
Pressure to have children along with financial and moral support could produce this result. There’s nothing that would be surprising here, or a need for an evolutionary explanation.