The eye of the peahen has very little in common with the tail of the peacock. They are at opposite ends of the body. They are constructed of different materials. They grow under the influence of different genes. During runaway, the genes underlying the sexually selected trait (the tail) may become correlated with the genes underlying the mechanism of sexual choice (the eye), but that is about the limit of their acquaintance.
The same is not true of the mental capacities used in human courtship, such as creative intelligence. There is much more overlap between those aspects of the brain used for producing sexually attractive behavior, and those aspects of the brain used for assessing and judging that behavior. Speaking and listening use many of the same language circuits. The production and appreciation of art probably rely on similar aesthetic capacities. It takes a sense of humor to recognize a sense of humor. Without intelligence, it is hard to appreciate another person's intelligence. The more psychologically refined a courtship display is, the more overlap there may be between the psychology required to produce the display and the psychology required to appreciate it.
This overlap suggests that runaway sexual selection for psychologically refined courtship may produce much smaller sex differences than runaway sexual selection for long bird tails. Consider the case of language. Suppose that human language evolved through a pure runaway process. Let's say males talked, and females listened, and females happened to favor articulate conversationalists over tedious mumblers. Male language abilities would then improve by sexual selection: their vocabularies might grow larger, their syntax more complex, their story plots more intricate, their ideas more imaginative. But for runaway to work, female choosiness would have to increase as well. How could that happen? Female language abilities would have to keep one step ahead of male abilities, to remain discerning. Females would have to be able to judge whether males used words correctly, so their vocabularies would keep pace. They would have to be able to notice grammatical errors, so their syntax abilities would keep pace. Most importantly, the females would have to understand what the males were saying to judge their meaning. Even if males exerted no sexual selection whatsoever on female language abilities, those abilities would have to evolve as part of the female mate choice mechanism.
To a psychologist like me, this is a much more promising sort of overlap than a mere genetic correlation between the sexes. There is a profound functional reason why males and females evolve in psychologically similar ways when courtship turns psychological. They use the same mental machinery to produce displays that they use to judge the displays produced by others.
There are two further reasons for the overlap between display-producers and display-judgers. To produce a really effective display, it helps to anticipate how the display will be judged. One might mentally rehearse a joke before telling it, to see if it will work, and find another joke if it won't. A painter could look at a picture while painting to see if it's beautiful. A musician could listen to the melody being played to see if it's tuneful. When trying to impress someone during courtship, we routinely do this sort of anticipatory filtering and correcting. Even if only males produced courtship displays, they would benefit by evolving psychological access to the same judgment mechanisms that females use.
Conversely, to be a really good judge of something, it helps to be able to do it oneself. For females to judge which male tells the best jokes, they may benefit by evolving joke-telling ability. We shall see later that mental anticipation is closely related to creativity. To be capable of judging someone's creativity, one must develop expectations about their behavior. Without expectations that can be violated, there can be no sexual selection for novelty and creativity. The mental machinery for generating expectations about someone else's stories, jokes, or music may overlap considerably with the mental machinery that is used in producing stories, jokes, and music.
So, even given a pure runaway process based on male courtship and female choice, male minds will tend to internalize the sexual preferences of females in their own courtship equipment, in order to produce better displays. And female minds will tend to internalize the display-production abilities of males in their own sexual choice equipment, in order to be better judges of male displays. This should lead to many mental capacities being shared by both sexes, even if males are more motivated to use their mental capacities to produce loud, public courtship displays. At present this argument is speculative, but it could be supported if neuroscience research found overlap between the brain areas used in producing and judging particular forms of courtship behavior, and if behavior genetic research were to show that the same genes underlie culture-production and culture-judgment abilities in both sexes.
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