Sexual selection has powerfully influenced psychology and psychology has powerfully influenced evolution.
RSS is debilitating to a species. Pheasants and peafowl are ground-dwelling and ground-nesting species that can barely still fly. This is why they make good game birds for humans. They rarely take flight when startled (-prefer running instead), and they only bother to really fly in order to take to trees to roost. They can survive just in somewhat atypical environments with sufficient food lying around but few ground competitors or predators. Obviously they can't be migratory, which greatly inhibits their ability to find new habit to exploit.
The parental investment analysis is equally dire. The peahen's predicament is that she's loaded the peacock up with "honesty" to the point where he expends all his energy in being a seducer -- and it's a herculean task if you've ever seen her -- and therefore he has nothing left over to pay child support with. All she gets is his genes. And because of the extreme polygamy -- in monogamous species the sexes are difficult to tell apart and this is the other end of that spectrum -- at least a third of the female's parental investment is wasted on sons who never reproduce. Mom's best strategy is to have fewer boys to begin with (which is not a good thing for other reasons), so this should drive the sex-ratio-at-birth to produce many more females, but the data are yet mixed.
The evidence seems to suggest this class of birds suffered an episode of RSS in the past, and is now evolving away from the nearly crippling consequences. Molecular phylogeny indicates the most recently evolved species of peafowl/pheasants are the least ornamented. The processes and preferences Shaner represents as being currently in operation, according to your quotes, are not observed. Takahashi, Arita et al (2008) say "peacocks' train is an obsolete signal for which female preference has already been lost or weakened" (Wikipedia paraphrasing of their work).
Under such post-RSS conditions, one can use all the smarts you can get hold of, so this line of reasoning suggests the evolution of more intelligence should be a secondary, downstream effect of RSS. Perhaps something like a rebound resulting from overshoot.
To take the evolution of bigger brains and more intelligence (in humans) as the primary effect of RSS is really some radical anti-Darwinian thinking, when you get right down to it.
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Allen Frances, M.D., was chair of the DSM-IV Task Force and is currently professor emeritus at Duke.
It can take a radical reboot to get past old hurts and injustices.