In psychology, denialism is a person's choice to deny reality as a way to avoid believing in a uncomfortable truth. Denialism is an essentially irrational human behavior that withholds the validation of a historical experience or event when a person refuses to accept an empirically verifiable reality.
In the sciences, denialism is the rejection of basic facts and concepts that are undisputed, well-supported parts of the scientific consensus on a subject, in favor of ideas that are radical, controversial, or fabricated. Examples include Holocaust denial, AIDS denialism, and climate change denial. The forms of denialism present the common feature of the person rejecting overwhelming evidence and trying to generate political controversy in attempts to deny the existence of consensus.
The motivations and causes of denialism include religion, self-interest (economic, political, or financial), and defence mechanisms meant to protect the psyche of the denialist against mentally disturbing facts and ideas; such disturbance is called cognitive dissonance.