"[S]ince around 1980, English speakers have been more given to writing about feelings than writing from a more scientific perspective."

"From around 1850 on, [researchers] found, the frequency of words such as 'technology,' 'result,' 'assuming,' 'pressure,' 'math,' 'medicine,' 'percent,' 'unit' and 'fact' has gone down while the frequency of words such as 'spirit,' 'imagine,' 'hunch,' 'smell,' 'soul,' 'believe,' 'feel,' 'fear' and 'sense' has gone up. The authors associate their observations with what Daniel Kahneman has labeled the intuition-reliant 'thinking fast' as opposed to the more deliberative 'thinking slow.' In a parallel development, the authors show that the use of plural pronouns such as 'we' and 'they' has dropped somewhat since 1980 while the use of singular pronouns has gone up. They see this as evidence that more of us are about ourselves and how we feel as individuals — the subjective — than having the more collective orientation that earlier English seemed to reflect."

Writes John McWhorter, in "Don’t, Like, Overanalyze Language" (NYT), discussing a study published in the Proceedings of the National Academy of Sciences that purported to detect a "surge of post-truth political argumentation" and a "historical rearrangement of the balance between collectivism and individualism and — inextricably linked — between the rational and the emotional.” 

McWhorter thinks the authors of the study are overdoing it, because, he says, often "the process by which language changes is that something starts out being about objective observation and drifts into being, as it were, all about me." He focuses on the question: "Why would this have happened to such an unusual extent since 1980?"  

And he guesses that it's a matter of increasing informality, which naturally entails being "more open about the self, less withholding about the personal, more inclined to the intimate." In that light, it's hard to believe the researchers think they've shown a move from individualism to collectivism. 

Something McWhorter doesn't talk about is that some people adopt a rationalistic tone for rhetorical purposes. They're not actually more rational, just trying persuade other people by posing as reasonable and unemotional. Ironically, that's an emotional move, and it can work if we respect the speaker's sincerity and good will, but it can stimulate wariness and irritation if we mistrust the speaker. And it's entirely rational to mistrust all speakers in the American political discourse that's developed in the last 40 years. In that light, it's not surprising that speakers have been abandoning the less effective rhetorical strategy.