First and foremost, the hallmark of this great regression would be a preoccupation with implicit meanings and ignorance of those explicit; all in all, a heightening of suggestibility happening concurrently to a lessening of rational thought: a concretizing of the mind. So firstly, their speech would become convoluted, as they'd come to place ever more priority on reacting to supposedly non-verbal signals and disregarding those which are not — this is so because as abstract ability begins to fade, too goes the ability to understand complex notations — that is, language. Secondly, due their rising need for in-group agreement — which is due their decreasing capacity for creative thought — they'd give rise to a culture that both prizes and produces work that more closely resembles failed attempts at abstraction than anything that is coherent: their music, being inseparable from noise; their paintings, identical to accidents; their prose, thinly veiled riddles and recycled cliches. And thirdly, lacking insight as they would into their own condition, promiscuity would advance under the guise of sexual liberation. Men, being of the sex which is unable to bear children, and as a result, of the sex which is most sensitive to changes in mating preferences, would respond to this shift most strongly: meaning that if by virtue of a desire to sustain high amounts of sexual activity, or status, large swaths of women began using contraception and found feminine features more attractive as a result, large swaths of men would come to exhibit feminine traits; meaning that given sufficiently long, or intense capture by this dynamic, high numbers of men would begin to transition genders; meaning that given how susceptible to adopting tribal dogmas this group would become, alongside how culturally molded the psyches of the women that would result from this change would be, the most charismatic of the women that'd be caused from this shift would be hailed as Christ figures, with their ideas as touted as revelations and their foreseers as prophets — moreover, I suspect that by this time the average neocortical ratio of the population would've decreased by about 12%.