Self-deception

Self-deception is a process of denying or rationalizing away the relevance, significance, or importance of opposing evidence and logical argument.
It has been argued that humans are, without exception, highly susceptible to self-deception, as everyone has emotional attachments to beliefs, which in some cases may be irrational. Some evolutionary biologists, such as Robert Trivers, have even suggested that, because deception is such an important part of human behaviour (and animal behaviour generally), an instinct for self-deception can give a person a selective advantage: if someone can believe their own "lie" (i.e., their presentation that is biased toward their own self-interest), the theory goes, they will consequently be better able to persuade others of its "truth'.
This notion is based on the following logic. In humans, awareness of the fact that one is acting deceptively often leads to tell-tale signs of deception. Therefore, if self-deception enables someone to believe their distortions, they will not present such signs of deception and will therefore appear to be telling the truth.