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.
There also exists something called the "grandfather hypothesis".
( z1 exist :ARG1 ( z2 something :ARG1-of ( z3 call :ARG2 ( z4 thing :name ( z5 name :op "grandfather" :op "hypothesis" ) ) ) ) :mod ( z6 also ) )