It's time for some plain talk.
Humans have no natural affinity for truth (reality) and therefore can not have a deteriorating relationship with truth (reality) now. Therefore, we are not in some new "post-truth" era (and here).
Humans are not truth-seekers, and never were. For our purposes here, humans are story-telling social animals. There is no relationship between telling stories and having an affinity for "objective" truth in so far was we can determine it. Story-telling is subjective by definition.
By the way, I have no evolutionary "just-so" story to tell which purports to explain why humans are story-telling social animals. Close and repeated observation tells us so. (See 2016 in America.) The fact that humans are deluded about what they are does not change those observations.
In particular, and in the cases I'm interested in, humans have no natural affinity for truth (reality) when when there is something important at stake. First, what does "important" mean in this context?
Something important is at stake when instinctual drives are in play, especially when there are conflicts with instinctual drives. For example, there may threats to group or individual psychological coherence (existential threats). Or the social distribution of status and power is on the line (These two examples are not mutually exclusive).
It is clear that there is always something important at stake in politics (in-group cooperation, elites versus out-groups, inter-group conflict). Politics is far more pervasive in human social interaction than you might think.
When something important is at stake, you get bullshit. You get self-serving stories. I will quote from the third Flatland essay, which I wrote in 2014. Bullshit is ubiquitous in human life. I wrote that essay in part to answer the question why is there so much bullshit?
In the Flatland model, bullshitting is not some random phenomenon; it is characteristic human behavior. There is more bullshit now (at least in absolute terms) than there used to be because there are so many more people than there used to be. Moreover, thanks to huge advances in communications technology, all these people have a much greater ability to express themselves in public and find an audience for their nonsense. Spin is ubiquitous in modern life because there are far more bullshitting opportunities clamoring for our attention...
In individuals or social groups, bullshit, or spin, is a self-serving post-hoc rationalization of some unconscious motivation. Bullshit arises when there is an unconscious need to—
(1) maintain a positive self-image or manage positive impressions of the self by others;
(2) deflect existential threats to the self or group; or
(3) advance the agenda of the bullshitter (representing himself or the group).
This list is not meant to be exhaustive, nor are these needs mutually exclusive.
Bullshit stands opposed to a reasoned, objective search for truth. Spin often departs from reality in a serious way, or, more commonly, contains "half-truths" which advance the agenda of the bullshitter. Otherwise, if speech or writing does not depart from reality by the 1) assertion of falsehoods or 2) omission of crucial facts, even when unconscious motivations exist—unconscious motivations always exist—the discourse in question is not bullshit.
Of course, not everything humans say or write is bullshit. It is clear that humans do sometimes reason without an ax to grind or some easily identifiable agenda, although it is also clear that unfettered rationality is far rarer than commonly assumed. In short, bullshit arises when there is something important at stake, just as "bad news" filtering does...
Repeating myself, if this model is correct, we can not be in the "post-truth" era because there was never a time when humans had a positive relationship with truth which can deteriorate at critical times in human history (like now in the United States).