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Hans Eysenck was an influential psychologist whose work could not be directly classified as he studied various areas of psychology. However one of the key areas Eysenck was involved in was personality. (Eysenck, 1947, 1985, 1997)
Eysenck's impact on personality psychology was one that helped shape modern day thinking (Gray, 2004)(Lucas & Diener, 2001)Even though Eysenck made contributions in developing our understanding of personality, his theory was subjected to criticism. (Rotter 1954) (Heim, 1970)
History of Eysenck's investigation
Eysenck investigation into personality spanned several decades, but the premise for his theories was over two thousand years old, being traced back to the ancient Greeks. Hippocrates and Galen made one of the first steps to defining personality by suggesting that the balance of four fluids in a humans body, predicted a person's behaviour. (Works)
Eysenck was also influenced by earlier psychologists; Carl Jung, (1933) who was one of the first psychologists to reference the terminologies, introvert and extrovert,(Jung, 1976) Gordon Allport, who identified over four thousand words in the English language that described personality(Allport, 1937) and Raymond Cattell who was able to analyse Allport's words, reducing them to 171 characteristics by the use of a statistical technique called 'Factor Analysis' and placing them into 16 key personality traits.(Cattell, 1946)
In 1942 Eysenck began work at Maudsley Hospital, which was headed by leading psychiatrists Audrey Lewis. Eysenck investigated 700 servicemen who were admitted to the hospital. He used factor analysis to identify basic aspects of personality, correlating the results with the questionnaires and experimental data. Eysenck's first and most influential book Dimensions of Personality, (1947) outlined that there were two major dimensions of personality, introversion-extroversion and neuroticism, which is illustrated in figure 1. (Eysenck, 1947) (Rushton, 1998)
The main reason Eysenck focused on just two dimensions neuroticism and introversion-extroversion; was by deliberately contrasting the continuums he expected to resolve any misconceptions there was in personality traits. (Eysenck, 1947)
Figure 1(Ltd, 2012)
Eysenck drew inspiration from Allport, Cattell and Jung's theories in order to enhance his own investigation, he expanded on his original two dimensions to include a third, 'Psychoticism.' Psychoticism was only added into the taxonomy in 1975 with contribution from Eysenck's wife Sybil.
Eysenck provided various instruments to measure his dimensions, which included the original Maudsley medical questionnaire; Eysenck's personality inventory; the most recent revised version of Eysenck's personality questionnaire and Eysenck's personality profile. (Jackson, Furnham, Forde, & Cotter, 2000)
Eysenck's inventories are comprised of self-report questionnaires, which consist of questions that concerned typical daily behaviour. Individuals were able to answer these questions on a 'Two-point Likert scale (yes/no),' thus how they answered the variety of statements, would be an indicator of how representative they were.
Biological basis of personality
Hans developed his theory of personality, by basing it initially upon the Pavlovian notion of excitation / inhibition. Eysenck defines individual differences in biological terms, as he fundamentally believed personality characteristics were genetically inherited. (Chamorro-Premuzic, 2007).
Hans used this theory in order to try and explain why personality is constant and unchanged thought out life (Chamorro-Premuzic, 2007).
Figure 2,(Pearson Education, 2010)
Eysenck argued that extraversion was a psychological consequence due to differences in the reticulo-cortical system; that determines excitation or inhibitions levels of motivation and conditioning, in the cerebral cortex. The arousal patterns of the reticulo-cortical could also be used to determine to what extent a person is extraverted or introverted (Eysenck, 1985).
Eysenck also described neuroticismin terms of arousability in the reticulo-limbic system; he believed that with arousal from an external stimulus, a neurotic individual could experience anxiety or intense emotion (Chamorro-Premuzic, 2007).
Although Eysenck did not provide a detailed account into psychoticism, he did suggest that individual differences were caused by dopamine, a neurotransmitter in the brain, which could be associated with the regulation of emotionality (Chamorro-Premuzic, 2007).
Even though Eysenck explained the biological basis for neuroticism and introversion-extroversion in great detail, the lack of information for psychoticism remained for some time, the focus of unresolved psychometric disputes.
With advances in neuroscience many of Eysenck's suggestions have been discredited however he set down foundations for other psychologists such as Jeffery Gray to develop new theories (most notably; his 'reinforcement sensitivity theory.') and for new taxonomies to become established.
Eysenck received criticism towards his theory as many psychologists believed that where Cattell focused on too many traits, Eysenck focused on too little. This resulted in an expanded version of personality traits being produced, namely 'The Big Five' (Costa & McCrae, 1985, 1992).
The Big Five is a statistical approach that provides a descriptive classification of individual differences. There has been a collective consensus and empirical evidence to support The Big Five as the major model for the dimensions of personality (Funder, 2001)(John & Srivastava, 1999)
Furthermore Eysenck also received criticism by using the two-point Likert scale in his questionnaires, Heim (1970) criticised the limited ability to respond (yes/no), as she felt that Eysenck's questionnaires did not do justice to the complex nature of human personality. She also felt that the questions themselves could result in subjective answering by recipients, thus leading to unrepresentative data being collected. It was suggested that by using a peer questionnaire the investigator could gain a better representative sample, however even this suggestion was subjected to criticism, as ultimately it depended on how much the person liked the individual under investigation.
Further criticisms came to light by Walter Mischel (1968) and Julian Rotter (1954), both suggested that personality depends on situations; personality and behaviour are changeable, if the environment is changed then the individual's personality also changes and this criticism went against Eysenck's genetic based theory.
Even though Eysenck's theory was subjected to criticism, it is still used in modern day psychology as the basis for the agreed upon 'Big Five Model.' The vision Eysenck had in psychology, especially in personality, brought about a greater understanding of factor analysis and how personality has dimensions that ultimately can determine an individual's behaviour.