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.
Only five studies have ever been conducted on African hunter-gatherer groups.
( z1 conduct :ARG1 ( z2 study :quant 5 :ARG1 ( z3 group :mod ( z4 continent :name ( z5 name :op "Africa" ) ) :mod ( z6 hunter-gatherer ) ) :mod ( z7 only ) ) :time ( z8 ever ) )