On its own, the idea of ornamental evolution through sensory biases has about the same number of strengths and weaknesses as the runaway brain theory and the healthy brain theory do. We probably need to combine all three perspectives to understand human evolution. I would not have spent a whole chapter on the runaway process if I did not think it was important in explaining the capricious divergence of courtship behavior between different ape and hominid species. I would not have spent a chapter on fitness indicators if I did not think that the pressure to advertise good genes was important in mental evolution. And I would not have discussed sensory biases, pleasure, and entertainment if I did not think that the psychological quirks of our ancestors had influenced our psychological capacities through the sexual choices they made.
Later, when I come to discuss particular human abilities like language and creativity, I shall draw on all three viewpoints. Biologists sometimes compare runaway theory, indicator theory, and sensory bias theory as if they were competing models of sexual selection. Such debates helped revive sexual selection theory, but I think that each of the theories now has enough support for them to be considered as overlapping sexual selection processes, not competing models. They all really happen in nature.
Runaway happens because sexual preferences really do become genetically correlated with the sexual ornaments they favor. It helps to explain human mental traits that are extreme, unusual, attractive, and useless for survival, and why such traits evolved in our lineage and not in other ape species. Runaway is endemic to sexual selection, always happening, or just finished, or just about to happen. It explains much of sexual selection's power, speed, and unpredictability.
Sexual ornaments really do evolve higher costs and higher condition-dependence in order to work better as fitness indicators. Indicator theory explains why some sexual ornaments stick around for many generations rather than disappearing as transient runaway effects. It gives sexual selection much of its direction, explaining why individuals usually prefer large tails to small, loud calls to whispers, good territories to bad, winners to losers, health to sickness, and intelligence to stupidity.
Sensory biases really do influence in which direction runaway is most likely to go, and which indicators are most likely to evolve. Sexual selection for pleasure and entertainment explains why so many sexual ornaments like the human mind are pleasing and entertaining. It draws attention to the role of sensation, perception, cognition, and emotion in sexual choice.
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