Denial is a defense mechanism frequently employed by the mind to cope with painful, uncomfortable, or embarrassing aspects of our psyche.
We can now use technology to directly see processes that we could previously only abstractly imagine. We can watch a virus enter a cell, see actual DNA strands, watch neurons make connections, unravel the intricate geometry of a protein. We have a vividly clear three-dimensional look at the entire period of gestation for animals and humans. We can trace the interactions of subatomic particles and map the entire universe, with a trillion galaxies, by dating the background radiation.

Being able to directly observe natural processes should make it harder to deny certain types of evidence. Yet, many people still doubt the most basic facts such as human-caused climate change and evolution, even with melting ice sheaths, and natural selection of bacteria occurring in the lab, in front of their eyes.

That is the nature of selective perception, denial and dissonance, confirmation bias, motivated reasoning, and belief perseverance—people adhere even more strongly to invested beliefs when they are threatened by contradictory data. 

Also, the prefrontal cortex creates justifications for our opinions after we have formed them.

A vivid example of denial is portrayed in the film Blue Sky with Jessica Lange, which is set in the days of above-ground nuclear testing in the 1950s. In one scene, two cowboys unwittingly ride through a testing ground during a test, and are warned by a concerned civilian that they have been exposed to lethal doses of radiation. They react angrily, insisting that the government would have warned them of any danger, and blithely ignore the woman’s pleas that they insist on accountability from those at fault.
Denialism
On a broader level, members and leaders of an entire group or society can be in denial in concert, ignoring the long-term consequences of decisions or actions made by earlier generations or officials. The use of selective propaganda and persuasion techniques, plus discrediting of contradictory evidence and isolation from original sources exacerbate public denial. On the whole, however, opinion persuaders appeal to a society’s pride, honor, prejudice, fear, and inflated sense of superiority to instill group denial.

Holocaust deniers are particularly destructive. But, there are also many other groups, long forgotten by history, who have been slaughtered without recompense, reparation, or acknowledgement.

The Turkish government continues to insist that the Armenian genocide never took place. This denial has been one of the reasons that Armenians hand down the memories of injustice to their children, generation after generation. There has been no official acknowledgment and, thus, no resolution. Recently, a famous Turkish writer who announced that Turkey had killed one million Armenians was charged with “insulting the national character.” An Istanbul conference examining the historical record of the genocide brought out angry protests against “imperialist propaganda.” This reaction was due to years of indoctrination by the Turkish government through media and omissions and revisions in history books and school texts.