In the psychology of human behavior, denialism is a person’s choice to deny reality, as a way to avoid a psychologically uncomfortable truth. Denialism is an essentially irrational action 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 radical and controversial ideas. The terms Holocaust denialism and AIDS denialism describe the denial of the facts and the reality of the subject matters, and the term climate change denial describes denial of the scientific consensus that the climate change of planet Earth is a real and occurring event primarily caused by human activity. The forms of denialism present the common feature of the person rejecting overwhelming evidence and the generation of political controversy with attempts to deny the existence of consensus. The motivations and causes of denialism include religion and self-interest (economic, political, financial) and defence mechanisms meant to protect the psyche of the denialist against mentally disturbing facts and ideas.