Trait theory was developed by Eysenck and Rachman (1965,cited in Butt,2007) and belongs to the mainstream, experimental approach to individual differences (Butt,2007). The aim of trait theory is to produce general principles of why people behave differently in different situations. Questionnaires, for example Eysenck’s Personality Inventory (EPI), are used to produce psychometric inventories, which are a measure of personality traits. This is a scientific approach, facilitating prediction of how a particular person will react in a specific situation. Other trait theorists (e.g. Kant) considered traits to be categorical. Their understanding was that each individual could be assigned to one particular category; no one could be a mixture of two or more categories. However, Eysenck’s use of criterion analysis discredited this belief in categorization, and suggested a continuum of traits. His understanding was of individuals being measured along a two continuums; extraversion-introversion and neuroticism-stability. He proposed that a person’s score on these continuums allows prediction of how they will react in a particular situation.
Trait theorists believe traits are biologically determined