The so-called "grandmother hypothesis", based on studies of African hunter-gatherer groups, suggests that infertile women are vital for successful child-rearing despite being unable to produce children themselves.
The "grandmother hypothesis" is based on nomadic groups
The  "grandmother hypothesis was based on hunter-gatherer societies
( z1 base :ARG1 ( z2 thing :ARG1-of ( z3 hypothesize ) :mod ( z4 grandmother ) ) :ARG2 ( z5 group :mod ( z6 nomad ) ) )