Eysenck then devised a series of personality questionnaires, not as diagnostic tools, but as research tools. Kline (1981) said that the Eysenck Personality Inventory (which had a Lie Scale to test for the extent to which an individual would give socially desirable answers) was generally acceptable and valid. Eysenck then attempted to validate these questionnaires by using criterion analysis, whereby the questionnaire was given to groups of individuals who were known to differ on the dimensions e.g. diagnosed neurotics, and a match was found. This criterion analysis was at first criticised because it only seemed to validate the ends of the scale without validating the "normal" middle of the scale. However, Gibson (1970) found significant overall correlations in so-called "normal" people of the middle range between the EPI and friend's self ratings, which seems to suggest that the dimensions are valid across the population, and could indeed be normally distributed.
A further questionnaire, the Eysenck Personality Questionnaire (Eysenck and Eysenck EPI, 1975) incorporated a P scale to test individuals in that dimension, but this was found to be weak and inferior to diagnostic methods. Heim (1970) also criticises the EPI and other related questionnaires because of the forced choice yes/no questions. She argues that such forced answer questions can not do justice to the complex issue of personality analysis, and indeed shows a bad example of reductionism in Eysenck's theory formation. She also criticises the Lie Scale for its lack of subtlety, and indeed it may also be testing the intelligence of the individual, in whether they will notice the hidden test in the first place, and thus conform to it or not.
There was also a biological basis for Eysenck's theory. He suggested that the nervous system (which is inherited genetically, which suggests that personality is also to some extent inherited) can explain the people's differences in dimensions. In the (E) dimension he proposed that there was a balance between excitation and inhibition in the CNS, in which specifically the Reticular Activating System (RAS) acts to maintain the optimum level of arousal. He argues that extroverts have a "strong" nervous system which acts to inhibit the impulses of stimuli, this means that they are under-aroused and their behavioural patterns turn to seeking more arousal. Introverts, conversely, have a CNS in which impulses build up strongly and quickly, and are so over-aroused and seek ways of decreasing arousal. In the N dimension, Eysenck argues, the autonomic system is involved, and differences in the limbic system here we need to mention the similarity of these differences in the limbic system i.e. see m eysenck for account of how limbic mediates many kinds of emotion and thus one kind such as n cant specifically be located here which controls the ANS means that an individual will be faster or slower to react in a stressful situation ("fight or flight"), and this affects especially the sympathetic branch of the ANS which controls adrenaline production breathing rate etc.. Eysenck predicts that high scoring on the N dimension will mean a strong and quick reaction to stressful situation in the individual. As for the P dimension Eysenck (1980) could only say that it could be related to hormones such as androgen.
There is some evidence to support this theory: Eysenck (1967,1971) and Hawkins and Green (1975) predicted that according to the stimulus seeking/avoiding part of the theory, on a vigilance task extroverts should do better at the task at the beginning but then it will decline as they become bored of it and search for a new exciting stimuli, whereas introverts would remain reasonably good at the task all the way through, and this is what the experiments actually found. There is a better description of vigilance
Wilson (1976) pointed out that introverts should be harder to sedate because they are already more aroused to start off with, and indeed Claridge and Herrington (1963) found that this was the case: extroverts were more easily sedated than "normals", who were more easily sedated than introverts. Eysenck (1967) also predicted that regardless of the initial level of arousal in the individual, raising the arousal level artificially (for example, through stimulant drugs) would cause the individual to seek ways of compensating for this arousal increase by avoiding arousal, and the opposite would occur in the event of a decrease in the level of arousal (through depressant drugs), and he claimed to have found sufficient support to back this hypothesis. Yet Claridge (1967) found no simple relationship between the E dimension and physiological arousal. All of this is a bit inconclusive
The stimulus part of Eysenck's theory has seriously consequences when considering the influence of personality on the extent of conditioning. Conditioning relies on a strong and rapid build up of excitation in the nervous system to learn the stimulus-response connection, and according to the theory this would mean the over-aroused introverts would be much easier to condition than the under-aroused extroverts. Reviews of the evidence have found mixed results in the influence on conditioning and this seems to be a major flaw in Eysenck's theory. Don’t include
Eysenck also came to the conclusion through his theory that criminality was related to personality dimensions. He predicted that criminals would be neurotic extroverts, because they are under-socialised and have an under-developed conscience. Cochrane (1974) reviewed studies of prison inmates and found that although prisoners were generally higher on N than a compared control, they were also generally lower on E. Eysenck said in response to this that the EPI measures more the "sociability" part of introversion rather than "impulsivity", and he seems to be changing his view that sociability and conditionablity are related. This discredits the original theory and there are problems with this research method: even if the prisoners were of certain dimensions this could be for other reasons; perhaps it is only the introverted neurotics that get caught, the nature of the offence might also be a factor, as well as the offender's social status, and it does not take into account the first time or one-off offender. Heather (1976) thought it was wrong to blame such a complicated social phenomenon as crime on the variance of one or two dimensions, which could be related to genetic inheritance. Don’t include
With the foundation of the P dimension Eysenck argued for a dimensional, as opposed to categorical or classificatory, approach to psychiatric diagnosis. With P and N forming a scale on which individuals could be placed. However, P as a separate dimension had been questioned, especially as a high overlap has been found between low N scorers and High P scorers. The theory relies upon the differences in conditionability (which is questionable in itself, as we have seen) between individuals. For example, a highly conditionable individual will be more susceptible to form a phobia, being a conditioned anxiety response to a previously neutral stimulus formed through classical conditioning. Overall, however, the general lack of evidence has led to a lack of faith in the standardised tests such as the EPI as predictors of response of as a measure of change (Peck and Whitlow, 1975).
In conclusion, although there does seem to be some biological evidence that points in the direction of Eysenck's theory of personality dimensions, there does seem to be a major limitation in that these dimensions should, as Eysenck predicted, affect conditioning in the individual, and there is not enough significant evidence to prove this to be the case. That is not to say that Eysenck's theory is without its use; the scales of E and N do seem to be a reasonable way of assessing an individual, even if only in basic terms, but the P scale does not seem to be sturdy enough. It seems as though the theory in itself is no longer adequate and is too simplistic to cover the complexities of human personality and behaviour, the research that it has generated, and the standing ground it has provided may stand the test of time. No mention of genetic twin studies
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