Miller finds his answer to these questions in sexual selection. This theory was advanced by Darwin as a complement to his theory of natural selection, but for nearly a century it was neglected and only since the 1980s has it begun to attract the attention of biologists. Miller devotes a chapter to the reasons for this neglect. Sexual selection is important because it is capable of producing features that seem to go into runaway by a process of positive feedback. If females prefer a particular male characteristic, this may increase in size and showiness even to the point where it imperils the survival of its possessors.
Examples are the tail of the peacock and, perhaps, the antlers of the extinct Irish elk.
Something similar could explain the development of the human brain, and also human mental attributes such as humour, benevolence, and so on. However, it can't be the whole explanation, because men and women have very similar brains. This may be partly attributable to the fact that women require large brains in order to appreciate these attributes in their mates. Another aspect of sexual selection, which receives a lot of attention in Miller's theory, is that the possession of showy items like brains is an indication of fitness. Females would therefore be expected to favour males who gave evidence of intelligence because this was also evidence that they were fit in other ways.