Matt Ridley describes how sexual selection could lead to an increase in neoteny which leads to disproportionately bigger brains. Disproportionately larger brains led to longer childhoods which led to an increase a monogamy. An increase in male monogamy led to greater choosiness in females which led to a doubling down on the male selection of younger woman and an even greater increase in neoteny.
As neoteny and monogamy positively fueled the development the brain, it better enabled the Machiavellian Hypothesis and Geoff Miller’s Theory on the Development of Intelligence.
“I believe that Miller’s tale deserves a special twist from the neoteny theory (although he’s not convinced). The neoteny theory is well established among anthropologists. And the notion of human monogamous child rearing is established among sociobiologists. Nobody has yet put the two together. If men began selecting mates that appeared youthful, then any gene that slowed the rate of development of adult characteristics in a woman would make her more attractive at a given age than a rival. Consequently, she would leave more descendants,, who would inherit the same gene. Any neoteny gene would give the appearance of youthfulness. Neoteny, in other words, could be a consequence of sexual selection (by enlarging the brain size in adulthood), it is to sexual selection that we should attribute our great intelligence.” (342, The Red Queen)
The Red Queen by Matt Ridley