Bohannon imagines a big cat in southern Africa killing and dragging a dead hominin into her den two million years ago. She starts eating the dead hominin’s brain in the safety of the den, savoring the large brain’s taste. She does this and nurses her kittens, unaware that her descendants will become pets for the hominin’s descendants. Bohannon then details the brain growth that allowed hominins to become smarter, more social, and able to better control their fertility. She explains that she was reluctant to add to the topic of human brain evolution but must do so to address whether there are significant differences in male and female brains. She states that of all mammals, humans have the least significant brain differences based on sex. She then states that she must address the sexist ideas about female brains to judge their accuracy.

Bohannon first challenges the idea that men are smarter than women. She explains that people tend to associate being smart with problem-solving, frequently using the IQ test to determine intelligence. While both male and female individuals tend to have equal IQs before age 15, male individuals tend to have higher IQs after puberty. While IQ tests are given great emphasis and IQs are shown to be hereditary, they are also controversial because white Americans tend to score higher than African Americans unless there is a control for income.