Helpless babies make for smart parents
Useless infants select for smart parents who make useless infants
Her model suggests that helpless infants demand intelligent parents, in order to ensure their survival. But in order for a species to produce large-brained, intelligent parents, infants must be born early and relatively undeveloped - or else their heads won't pass through the birth canal.
This evolutionary feedback links intelligence and undeveloped infants, and applies to many species of primates she's compared.