introversion

n.

1. The act or process of introverting or the condition of being introverted.
2. Psychology. The direction of or tendency to direct one's thoughts and feelings toward oneself.

introversive in'tro·ver'sive (-vûr'sĭv) adj.

A personality trait characterized by a tendency to be preoccupied with oneself; to be shy, cautious, and to be inwardly reflective more than overtly expressive. Compare extroversion.

The terms 'extroversion' and 'introversion' entered popular use in England, with pretty much their current meaning, during the 19th century; they can be found in several popular novels, referring respectively to sociable, impulsive, carefree behaviour, and unsociable, responsible, thoughtful behaviour. C. G. Jung popularized the terms on the continent, and linked them with a very complex and difficult psychoanalytic set of theories; these theories are not now widely entertained, and the Jungian meaning of the terms is only accepted by a few followers of his. Jung suggested that there were links, in neurosis, between extroversion and hysterical symptomatology, and introversion and depressive/anxiety symptomatology; this connection has been verified by later workers. The behavioural patterns which underlie the notion of extroversion/introversion were of course observed long before the 19th century; they go back at least to Hippocrates and Galen, the Greek physician who elaborated the earlier system of the four temperaments (choleric, melancholic, phlegmatic, sanguine) which has lasted in some form or other for 2,000 years, and is still popularly used by many people.

The relation of this system to modern conceptions of personality was first suggested by Wilhelm Wundt, the founder of modern psychology, just before the turn of the century. The four temperaments, until then, had been conceived of as qualitatively different types; you belonged to one or the other of these groups, and you could not show traits and qualities belonging to more than one group. This is patently untrue; many people do seem to belong to one or other of these types (otherwise the system would not have survived), but equally obviously others do not. Wundt pointed out that the melancholic and the choleric types had something in common that set them off against the phlegmatic and the sanguine types, namely a strong emotional reaction; this suggests a dimension of personality ranging from high to low emotional reactivity, or 'emotionality' (also often called 'neuroticism' or 'anxiety' by modern psychologists). Equally, the choleric and sanguine types have something in common which sets them off from the melancholic and phlegmatic types. Wundt called this quality 'changeableness', but we would nowadays call it extroversion, as opposed to introversion. Wundt was very perceptive: changeableness is indeed one of the most characteristic traits of the extrovert — but it is not the only one, and consequently a less specific name for this personality dimension was required.

With this important change from four entirely discrete personality types to two separate dimensions of personality, with individuals located at any point on each scale, Wundt rescued the ancient scheme from oblivion and made it capable of reflecting the complexity of real life. The four Greek temperaments lie in the four quadrants generated by these two intersecting dimensions of personality. Melancholics are introverted and emotional (Jung's anxious and depressed neurotics), while cholerics are extroverted and emotional (Jung's hysterics, and, as modern research has shown, psychopaths and criminals as well). Phlegmatics are introverted and stable, while sanguinics are extroverted and stable. We can now measure the degree of emotionality/stability a person shows, or the degree of extroversion or introversion, and we can show that on both these scales the distribution of people is roughly in line with the Gaussian probability curve, i.e. most in the middle, with fewer and fewer towards the extremes. We do not, then, divide the population into extroverts and introverts: we measure the degree of extroversion or introversion shown, and allocate the person in question to some place along the continuum — very much as we would do if we measured his height, or weight, or intelligence. When in what follows we speak of extroverts or introverts, we mean people falling towards one or the other end of this distribution; those in the middle are usually referred to as ambiverts.

Two problems are raised by the postulation of a concept such as extroversion/introversion. The first is a descriptive one: is it in fact true that the traits postulated to characterize the extrovert, or the introvert, actually occur together? This is fundamentally a statistical problem, concerned with correlations. We can rate the degree of sociability, impulsiveness, changeableness, talkativeness, outgoingness, activity, liveliness, excitability, optimism, etc. of a few hundred persons, chosen at random, and determine whether all these traits are positively correlated — i.e. whether a person showing one of these traits is also likely to show the others. We would not expect perfect agreement, of course, but we should expect reasonable correlation between these traits if the concept of extroversion were to have much meaning. Hundreds of empirical studies have shown that these various traits do indeed hang together to a degree that exceeds chance by a large amount; furthermore, these studies have demonstrated this general cohesion between traits for adults and children, men and women, different social classes, and different nations (including Japanese, Indian, European, and American). There is little doubt that descriptively the personality dimension of extroversion/introversion is amply supported by relevant research.

The second problem is the causal one — given that some people are extroverted, others introverted, others ambivert, can we say why these personality differences occur? The evidence suggests that genetic causes are very strongly implicated: something like two-thirds of the total variation in extroversion/introversion is probably due to heredity, with only one-third left for environmental causes. There is no evidence of dominance in the hereditary determination; this suggests that from the evolutionary point of view neither extroversion nor introversion is more successful in adapting to environmental stress. But this answer is unsatisfactory: heredity cannot determine behaviour, only structure. Underlying extroverted or introverted behaviour there must be some physiologico-anatomical structure, presumably in the central nervous system, which mediates these personality differences. Recent experimental work has suggested that this is indeed so, and that extroversion is linked with resting states of low cortical arousal, introversion with resting states of high cortical arousal. At first sight this would seem to be the wrong way round; one would have thought that the active, uninhibited extrovert would be the person with high cortical arousal. However, the main function of the cortex is one of inhibiting lower centres; effective functioning of the cortex, due to high arousal, produces inhibited (introverted) behaviour. In the same way alcohol, a depressant drug which lowers the arousal of the cortical centres, produces extroverted, uninhibited behaviour; it frees the lower centres from cortical control. Cortical arousal in turn is determined by the so-called ascending reticular activating system, a group of cells lying in the brain stem and responsible for reacting to incoming sensory messages by alerting the cortex so that it may be better able to deal with these messages. Here, theory suggests, is the causal locus of extroverted and introverted behaviour.

The evidence for this theory is by now quite strong. Part of it is direct, based on electroencephalographic measurement of resting level brainwaves and other psychophysiological measurements of arousal (see electroencephalography). Part of it is indirect, based on laboratory investigations of certain psychological functions which are known to be determined in part by arousal level (conditioning, sensory thresholds, habituation, etc.). These studies are as yet not conclusive, but they do on the whole suggest that this theory is along the right lines. Unfortunately it is still impossible to record directly from the ascending reticular activating system in humans, so that the part played by this system must rest on work done on cats and other animal preparations in the laboratory; the evidence for the cortical part of the theory is much stronger. Certainly no other aspect of personality is as widely studied, or has given rise to such clear-cut theories and experimental investigation, as has extroversion/introversion; we probably know more about this trait (or system of traits) than about any other.

(Published 1987)

— Hans J. Eysenck

A general survey of the field is offered in Eysenck, H. J. (ed.), Readings in Extraversion–Introversion, 3 vols. (1970). The genetic and physiological theories of extroversion are discussed in detail, together with the evidence, in Eysenck, H. J., Biological Basis of Personality (1967). Alternative theories are considered in Nebylitsyn, V. D., and Gray, J. A. (eds.), Biological Bases of Individual Behavior (1972). A more general approach will be found in Brody, N., Personality: Research and Theory (1972). Jung's theory is described in detail by Hall, C. S., and Lindzey, G., Theories of Personality (1968). For the most recent summary of the evidence, see Eysenck, H. J., and Eysenck, M. W., Personality and Individual Differences (1985).

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The turning outside in, more or lees completely, of an organ.