One of the most important theorists of the study of personality is Hans Eysenck. A psychologist born in Germany, but who at the age of 18 settled in the United Kingdom where he grew up professionally. He carried out a lot of research, although he became famous for his theory of personality .

Its approach is framed within the theory of traits , which assumes that behavior is determined by relatively stable attributes that are the fundamental units of one’s personality, because they predispose a person to act in a certain way. This means that traits must be consistent across situations and over time, but can vary between individuals.

Eysenck and individual differences

For Eysenck, individuals differ in their traits due to genetic differences, although he did not rule out environmental and situational influences on personality, such as family interactions in childhood. Therefore is based on a biopsychosocial approach in which these genetic and environmental factors determine behaviour .

What the author proposes is that each person is born with a specific structure at the brain level, which causes discrepancies in psychophysiological activity and, therefore, causes the individual to develop differences in the psychological mechanism, determining a specific type of personality.

Personality according to Hans Eysenck

Hans Eysenck developed a theory based on the results of the factor analysis of the answers of some personality questionnaires. Factor analysis is a technique that reduces behavior to a series of factors that can be grouped together under a heading called dimension, since they share common attributes.

In conclusion, he identified three independent dimensions of personality that I will explain later: Neuroticism (N), Extraversion (E) and Psychoticism (P), which is called the PEN model .

This model is intended to be explanatory and causal, as it determines the biological basis of these dimensions and confirms them experimentally.

Eysenck’s studies

During the 1940s, Eysenck worked at Maudsley Psychiatric Hospital (London, UK). His job was to perform the initial evaluation of each patient before their disorder was diagnosed by a psychiatrist. In this position he collected a battery of behavioural questions, which he later applied to 700 soldiers who were being treated at the same hospital for their neurotic disorders.

After passing the questionnaires, he noticed that there seemed to be a link between the soldiers’ answers , suggesting that there were personality traits being revealed.

The structure of personality according to Eysenck

Following the results of his research, Eysenck proposes a hierarchical model of personality in which behavior can be ordered on four different levels. This is the order from the lowest to the highest level:

  • First level : At this level are the responses that can be observed once, and which may or may not be characteristic of the person (for example, experiences of daily life).
  • Second level : These are the usual answers, which often occur in similar contexts (for example, if you answer a test a second time, similar answers will be given).
  • Third level : These are the habitual acts that are ordered by traits (sociability, impulsiveness, vivacity, etc.).
  • Fourth level : This level is the most extensive in the sense of generality, and there are the superfactors I mentioned before: Neuroticism, Extraversion and Psychoticism.

People can score high or low on these superfactors. A low score in Neuroticism refers to high emotional stability. Low scores on Extraversion refer to Introversion.

The three types or superfactors are sufficient to describe the personality in an adequate way, since from these it is possible to make predictions both on a physiological level (e.g. cortical activation level), psychological level (e.g. performance level) and social level (e.g. criminal behaviour).

The dimensions of the Eysenck Model

Neuroticism (emotional stability/instability)

People with emotional instability show anxiety, hysteria and obsession. They often tend to overreact emotionally and have difficulty returning to a normal state after emotional activation. At the other extreme, the person is equable, calm, and has a high degree of emotional control.

Extraversion (extraversion-introversion)

The extraverted are characterized by sociability, impulsiveness, disinhibition, vitality, optimism and sharpness of wit; while the introverted are calm, passive, unsociable, attentive, reserved, reflective, pessimistic and quiet. Eysenck thinks that the main difference between the extraverted and the introverted is found in the level of cortical excitement .

Psychoticism

People with high scores in psychoticism are characterized by being insensitive, inhumane, antisocial, violent, aggressive and extravagant. These high scores are related to various mental disorders, such as susceptibility to psychosis. In contrast to the other two dimensions, psychoticism does not have the opposite extreme, but is a component present at different levels in individuals.

Biological basis of the PEN model: causal aspects

Taking into account this descriptive model of personality, the PEN model also provides a causal explanation. It focuses on the biological, hormonal and psychophysiological mechanisms responsible for the three dimensions, in order to test this theory experimentally.

Theory of Cortical Activation and its relation to extraversion

The theory of cortical activation appears later than another proposal by Eysenck himself, the Excitation-Inhibition model , since the latter did not allow empirically contrastable predictions to be made.

Excitation-inhibition model

The excitation-inhibition model proposes that extraverted people have weak excitation potentials and strong reactive inhibition . In contrast, introverted people have strong excitatory potentials and weak reactive inhibition.

Theory of Cortical Activation

Eysenck cortical activation proposes a biological explanation of extraversion taking into account the ascending reticular activation system (SARA). SARA activity stimulates the cerebral cortex, which, in turn, increases the level of cortical activation.

The level of cortical arousal can be measured through skin conductance, brain waves or sweat. Taking into account the different activity levels of SARA, introverts have higher activity levels than extraverts . Some research has shown that extraverts seek out sources of external stimulation that cause them to have a higher level of stimulation.

Neuroticism and activation of the limbic system

Eysenck also explains neuroticism in terms of the activation thresholds of the sympathetic nervous system or the visceral brain . The visceral brain is also known as the limbic system, which consists of the hippocampus, amygdala, septum, and hypothalamus, and regulates emotional states such as sex, fear, and aggression. It is responsible for the fight or flight response in the face of danger.

Heart rate, blood pressure, skin conductance, sweating, breathing rate, and muscle tension (especially in the forehead) can be used to measure levels of visceral brain activation. neurotic people have low thresholds of visceral brain activation and are unable to inhibit or control their emotional reactions. Therefore, they experience negative effects in stressful situations, are annoyed even in situations with minor stress and get upset very easily.

Psychotic and gonadal hormones

Eysenck also provides a biological explanation for psychoticism, specifically gonadal hormones such as testosterone and enzymes such as monoamine oxidase (MAO). Although there is not much research on psychoticism compared to extraversion and neuroticism, some current studies show that people with psychotic episodes have high levels of testosterone and low levels of MAO.

Furthermore, in these studies, impulsiveness and aggressiveness, two characteristic features of individuals who score high in psychoticism, correlated negatively with MAO, since this enzyme plays a fundamental role in the degradation of the monoamines noradrenaline, dopamine and serotonin. In these studies, it was also shown that low levels of MAO are a characteristic of psychotic patients .

Eysenck Personality Questionnaires

As a result of Eysenck’s theory of personality, several questionnaires have emerged that are the result of more than forty years of development and many psychometric and experimental studies carried out in many countries.

  • Maudsley Medical Questionnaire (MMQ): Contains 40 items and evaluates Neuroticism.
  • Maudsley Personality Inventory (MPI): Contains 48 items and evaluates Extraversion and Neuroticism.
  • Eysenck Personality Inventory (EPI): Contains 57 items and evaluates Neuroticism and Extraversion
  • Eysenck Personality Questionnaire (EPQ): Contains 90 items and evaluates the three superfactors: Extraversion, Neuroticism and Psychoticism.
  • Revised Eysenck Personality Questionnaire (EPQ-R): Contains 100 items and evaluates the three superfactors.

Bibliographic references:

  • Eysenck, H.J. and Eysenck, S.B.G. (1994). Manual of the Eysenck Personality Questionnaire. California: EdITS/Educational and Industrial Testing Service.
  • Gray, J. A. (1994). Three fundamental emotion systems. In P. Ekman & R. Davidson (Eds.). The nature of emotion (pp. 243-247). New York: Oxford University Press. Gutiérrez Maldonado, J. (1997). Personality psychology and experimental synthesis of behaviour. Revista Latinoamericana de Psicología, 29, 435-457.
  • Pueyo, A. A. (1997). Manual de Psicología Diferencial. Madrid: Mc Graw Hill.
  • Schmidt, V., Firpo, L., Vion, D., De Costa Oliván, M. E., Casella, L., Cuenya, L, Blum, G.D., and Pedrón, V. (2010). Eysenck’s Psychobiological Model of Personality: a history projected into the future. International Journal of Psychology, 11, 1-21.