Apparently Oxford Dictionary's "word of the year" in 2016 was post-truth:
... relating to or denoting circumstances in which objective facts are less influential in shaping public opinion than appeals to emotion and personal belief.
Google is filled with articles alarming about a recent change (a new "era") in which truth is less influential than it was, usually citing Brexit and Trump as the main evidence.
Years ago I watched Adam Curtis's documentary Century of the Self, about Edward Bernays, the father of public relations, and a pioneer in the field of propaganda, who (in the 1920s):
... described the masses as irrational and subject to herd instinct—and outlined how skilled practitioners could use crowd psychology and psychoanalysis to control them in desirable ways.
That is, I had been under the impression that "post-truth" was always the state of humanity, and nothing has changed.
I realize that there are lots of arguments to be made about this, but I've been unsuccessful in finding actual research on this topic, such as longitudinal studies showing that people's ability to discern truth from non-truth in media has changed, that people's beliefs have become less aligned with objective facts over time, that public opinion has gradually diverged from the truth, that people have lost interest in truth over time, or that people's political leanings have less to do with factual evidence than they did in the past.
Are we really in a new "post-truth era"?