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.
African groups have never been studied.
( z1 study :polarity - :ARG1 ( z2 group :mod ( z3 continent :name ( z4 name :op "Africa" ) ) ) :time ( z5 ever ) )