Eysenck's three traits
H.J Eysenck (1967, 1982) developed a very influential model of personality. Based on the results of factor analyses of responses on personality questionnaires he identified three dimensions of personality: extraversion, neuroticism and psychoticism. Two of these also form part of the "Big Five" model of personality (McCrae & Costa, 1985). Eysenck considered that psychological disorders were related to the extremes of personality.
Extraverts are sociable and active, they enjoy meeting people and going to parties. The original conception of extraversion linked it to arousal (Eysenck, 1967). Eysenck described extraverts as showing low levels of cortical arousal, while introverts were seen as over-aroused. Later explanations focussed on proposed differences in conditioning. Because of their higher arousal introverts were claimed to condition more readily and were therefore more socialised, more sensitive to social constraints.
Gray (1981) reconceptualised the biological bases of extraversion and neuroticism as reflecting differences in sensitivity to reward and punishment. He argued that extraverts were more sensitive to reward, whereas introverts (especially neurotic introverts) are more sensitive to punishment
Eysenck described neuroticism as reflecting differences in the intensity of emotional experience. As described above Gray (1981) argued that neuroticism was indicative of a higher sensitivity to punishment. Neuroticism is close to a number of other traits such as trait anxiety or negative emotionality. It might be expected that individuals high in neuroticism might be more likely to use drugs in order to reduce or avoid negative emotional states.
The third of Eysenck's dimensions is a more recent addition and is less well defined than extraversion and neuroticism. Individuals high on psychoticism are tough-minded, non-conformist, willing to take risks and may engage in antisocial behaviour. The name of the scale reflects Eysenck's original suggestion that the trait tapped personality traits related to psychosis, just as neuroticism seems to measure traits related to anxiety and depression. Later revisions to the scale have moved away from this view and recent explanations emphasise impulsive nonconformity or tough-mindedness. The scale has obvious similarities to sensation seeking and if the trait relates to a disorder it is psychopathy/antisocial personality disorder, rather than psychosis. On the "Big Five" model high psychoticism overlaps with low scores on the traits of agreeableness and conscientiousness.
Methodological issues 2: Different, yet overlapping, scales
As discussed above, personality scales with different names may tap very similar aspects of behaviour. Conversely the same scale may change over time, as different revisions emphasise slightly different aspects of the trait. This can make it difficult to draw conclusions from studies undertaken at different times or using different scales. As an example the sub-trait of impulsivity was originally part of Eysenck's Extraversion scale. When he introduced the Psychoticism scale in 1978 many of the items related to impulsivity moved to the Psychoticism scale. On McCrae & Costa's (1985) big five, on the other hand, impulsivity is linked to neuroticism.