It turned out that staying close to grandma paid off in family size. Women who lived 200 miles from mom had, on average, 1.75 fewer children than their sisters who lived in the same parish as their mother. “Women in those days had a lot of kids, on average almost eight,” says Engelhardt. But times were tough, and about half of a woman’s offspring died before age 15. Such harsh conditions led to a range of reproductive success; the number of grandkids per grandmother in this database ranged from one to 195.

Being geographically close to grandma curbed child mortality too and allowed mothers to start having kids at a younger age.

Altogether, these results are what you’d expect if the grandmother hypothesis is true. “These results are really interesting,” says Hawkes. “They took a much more fine-grained approach, and it gives us a clearer picture of the effect of grandmothers.”