Couples who live near their mother-in-law have more babies, according to new research.
Previous studies also found that children born to households where their grandmothers live close by are also less likely to die in infancy.
Dr. Virpi Lummaa of Sheffield University studied 300 years' worth of church records, with the most recent being from 2000, to track births, marriages and deaths in farming communities in Finland.
The study, presented at the Euroscience 2012 conference in Dublin, found that not only did sons and daughters marry younger if their mothers were alive, they also had more children and had smaller gaps in time between each birth.
Researchers found that couples only had more children when a mother-in-law lived nearby, suggesting that the affected families probably benefited from an extra person willing to help with raising and providing attention to the child.
Lummaa did not find a difference between maternal and paternal mother-in-laws. However, previous studies on African families found that a mother's mother provides the greatest benefits.
The positive effect of maternal grandmothers may purely be due to age, as men generally marry later than women, paternal grandmothers are also more likely to be older, and less able to help raise their grandchildren.
Researchers say that grandfathers may seem like they make less of an impact not because they don't do anything, but because their help may not translate into better survival of their grandchildren.
Lummaa is also studying why women lose the ability to have children when they are still relatively young, while mammals are able to reproduce into old age.
Researchers add that the latest findings support the theory that menopause is nature's way of keeping women from having children while they are still young so that they can protect their genes by directing their efforts on their grandchildren, without having to go through the pain of childbirth again.