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For irrationality as it relates to numbers, see irrational number.

Irrationality is talking or acting without regard of rationality. Usually pejorative, the term is used to describe thinking and actions which are, or appear to be, less useful or logical than the rational alternatives. There is a clear tendency to view our own thoughts, words, and actions as rational and to see those who disagree as irrational.

Types of behavior which are often described as irrational include:

• fads and fashions
• crowd behavior
• offense or anger at a situation that has not yet occurred
• unrealistic expectations
• falling victim to confidence tricks
• belief in the supernatural without evidence
• stock-market bubbles
• irrationality in the form of a mental illness, such as obsessive-compulsive disorder, major depressive disorder, and paranoia.

These more contemporary 'normative' conceptions of what constitutes a manifestation of irrationality prove difficult to empirically demonstrate because it is not clear by whose standards we are to judge rational or irrational behaviour.

Celestial divination: Roman augurs were priests who used omens to predict the future. Many Roman writers explain history in terms of these auguries, but modern historians consider belief in auguries to be irrational.

The study of irrational behavior is of interest in fields such as psychology, cognitive science, economics, game theory, and evolutionary psychology, as well as of practical interest to the practitioners of advertising and propaganda.

Theories of irrational behavior include:

• people's actual interests differ from what they believe to be their interests.
• mechanisms that have evolved to give optimal behavior in normal conditions lead to irrational behavior in abnormal conditions.
• In situations outside of one's ordinary circumstances, one may experience intense levels of fear, or may regress to a Fight or flight mentality.
• people fail to realize the irrationality of their actions and believe they are acting perfectly rational, possibly due to flaws in their reasoning.
• apparently irrational decisions are actually optimal, but made unconsciously on the basis of "hidden" interests that are not known to the conscious mind
• an inability to comprehend the social consequences of one's own actions, possibly due in part to a lack of empathy.
• Some people find themselves in this condition by living "double" lives. They try to put on one "mask" for one group of people and another for a different group of people. Many will become confused as to which they really are or which they wish to become.

Factors which affect rational behavior include:

• stress, which in turn may be emotional or physical
• the introduction of a new or unique situation
• intoxication

This short section requires expansion.

Irrationalist is a wide term. It may be applied to mean one without rationality, for their beliefs or ideas. Or, more precisely, it may mean someone who rejects some aspect of rationalism, variously defined. For example religious faith may be seen as, in part, a rejection of complete rationalism about the world; this would be contested by some religious thinkers, in that the rational is a debatable term. On the other hand, it might be considered irrationalist to buy a lottery ticket, on the basis that the expected value is negative.

Irrational thought was seen in Europe as part of the reaction against Continental rationalism. For example Hamann is sometimes classified as an irrationalist.

Irrational behaviour has always been a notable target to satirical writers and philosophers.

Look up irrationality in Wiktionary, the free dictionary.

• God
• Irrationalism and Aestheticism
• Bounded rationality
• Cognitive Bias
• Logical Fallacy
• behavioral economics
• fan death
• Dysrationalia
• Stupidity
• Nicolai

• Stuart Sutherland, N. S. Sutherland. Irrationality: Why We Don't Think Straight, reissued 2007 by Pinter & Martin ISBN 978-1-905177-07-3
• Transwiki:Irrationalist

• Craig R. M. McKenzie. Rational models as theories – not standards – of behavior. TRENDS in Cognitive Sciences Vol.7 No.9 September 2003
• REBT-CBT NET- Internet Guide to Rational Emotive Behavior Therapy