Personality refers to the stable patterns of thought, feeling, and behavior that make each person unique, while still allowing people to be compared with one another. Psychologists have tried to measure and explain these differences since the 18th century.
Key Takeaways
- Early History: Personality tests date back to the 18th century, when phrenology (measuring skull bumps) and physiognomy (analyzing facial appearance) were used to assess personality (Goldstein & Hersen, 2000).
- Lexical Roots: Psychologists have tried counting personality-descriptive adjectives in English since the 19th century. Thurstone (1934) reduced 60 such adjectives, rated by 1,300 people, to five common factors.
- Cattell’s 16PF: British-American psychologist Raymond Cattell developed the Sixteen Personality Factor Questionnaire, a self-report measure of all sixteen factors used in both clinical and non-clinical settings.
- The Big Five: In the 1980s, after a roughly two-decade hiatus, Goldberg (1980) revived Tupes and Christal’s (1961) finding of five major personality traits: openness, conscientiousness, extraversion, agreeableness, and neuroticism (OCEAN).
- Lasting Influence: This model is central to the wide acceptance and popularity the five-factor approach still enjoys today.
“Personality is the dynamic organization within the individual of those psychophysical systems that determine his characteristics behavior and thought” (Allport, 1961, p. 28).
“The characteristics or blend of characteristics that make a person unique” (Weinberg & Gould, 1999).
Both definitions treat personality as something that belongs to the individual, making them unique but still open to comparison with others.
Defining and Classifying Personality
Nomothetic vs Idiographic Approaches
The idiographic approach assumes each person has a unique psychological structure. Some traits belong to only one person, so it is sometimes impossible to compare one person with another. This approach relies on case studies, biography, and personal documents.
The nomothetic approach takes the opposite view. It assumes traits carry the same psychological meaning in everyone, and that people differ only in where they fall along a shared set of dimensions.
This approach relies on self-report questionnaires and factor analysis, a statistical technique that identifies a smaller number of underlying dimensions from patterns of correlation among many test items.
Eysenck and Cattell, covered below, both worked in this nomothetic, psychometric tradition. Allport’s theory sits closer to the idiographic side.
This nomothetic, trait-based tradition rests on the lexical hypothesis: the idea that if a personality characteristic matters in everyday life, people will have invented a word for it. Allport’s trait catalogue, Cattell’s sixteen factors, and the Big Five all descend from this assumption.
Consistency, Change, and the Role of Nature and Nurture
Two further questions cut across the idiographic-nomothetic split.
The first is whether personality consists of permanent traits, so behavior stays consistent, or whether behavior is driven by situations and so can change. The second is how much of personality is inherited versus learned.
Trait theories treat personality as substantially biologically based. State theories, such as Bandura’s (1977) Social Learning Theory, instead emphasize the role of nurture and environmental influence.
Sigmund Freud’s psychodynamic theory takes a middle position, assuming an interaction between nature (innate instincts) and nurture (parental influences).
The evidence favors neither extreme. Behavior-genetic studies attribute roughly half of the reliable variance in trait scores to genetic differences. The remainder goes to non-shared environment and measurement error, a split the person-situation debate below explores further.
Trait Approach to Personality
This approach assumes behavior is determined by relatively stable traits, the fundamental units of one’s personality.
Traits predispose one to act in a certain way, regardless of the situation. This means that traits should remain consistent across situations and over time, but may vary between individuals.
It is presumed that individuals differ in their traits due to genetic differences.
These theories are sometimes referred to as psychometric theories, because of their emphasis on measuring personality by using psychometric tests. Trait scores are continuous (quantitative) variables.
A person is given a numeric score to indicate how much of a trait they possess.
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Gordon Allport analyzed language to identify thousands of trait words, ultimately categorizing them into cardinal traits (rare, all-consuming traits that dominate a life), central traits (the core characteristics of an individual’s personality), and secondary traits (situational preferences and attitudes).
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Raymond Cattell utilized factor analysis to narrow down personality to 16 fundamental source traits, developing the 16PF questionnaire.
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Hans and Sybil Eysenck believed personality was biologically governed and described it along two primary dimensions: extroversion/introversion and neuroticism/stability. They later added a third dimension: psychoticism/superego control.
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Big Five (Five-Factor Model): Personality is best captured by five broad dimensions, easily remembered by the acronym OCEAN: Openness to experience, Conscientiousness, Extraversion, Agreeableness, and Neuroticism. These traits are largely stable over the lifespan and exist on a continuum in every individual.
Eysenck’s Personality Theory
Eysenck (1952, 1967, 1982) proposed a biological theory of personality. He argued that people inherit a type of nervous system that shapes how readily they learn and adapt to their environment.
In the 1940s, Eysenck worked at a psychiatric hospital in London. His job was to assess patients before diagnosis.
Aim: How many personality dimensions best describe a large psychiatric sample?
Method: Eysenck (1947) compiled a battery of questions about behavior and personal history, and applied it to 700 soldiers being treated for neurotic disorders at the hospital.
He then used factor analysis. This technique reduces many measured variables to a smaller number of underlying dimensions.
Results: The soldiers’ answers clustered into two broad dimensions, not separate diagnostic categories. Two dimensions emerged: Introversion-Extraversion (E) and Neuroticism-Stability (N). Patients differed in degree, not in kind.
Conclusion: Personality is best described by a few continuous dimensions. Diagnoses are best understood as extreme regions of that dimensional space, not as separate kinds.
The sample was neurotic wartime servicemen, so generalizing to the wider population is an inference, not a finding. The items were also assembled by one clinician. Factor analysis can only recover dimensions the item set actually samples.
Each aspect of personality (extraversion, neuroticism, and psychoticism) can be traced back to a different biological cause.
Personality is dependent on the balance between the excitation and inhibition process of the autonomic nervous system (ANS).
Extraversion/Introversion
- Extraverts are sociable and crave excitement and change, so they can become bored easily. They tend to be carefree, optimistic, and impulsive.
- They take more risks and seek thrills. Eysenck traced this to the ascending reticular activating system (ARAS): extraverts are chronically under-aroused, so they seek stimulation to feel normal.
- Introverts lie at the other end of this scale, being quiet and reserved. They are chronically over-aroused, so they avoid extra stimulation.
- Introverts are reserved, plan their actions, and control their emotions. They tend to be serious, reliable, and pessimistic.
Neuroticism/Stability
A person’s level of neuroticism is determined by the reactivity of their sympathetic nervous system and the limbic (“visceral”) brain.
A stable person’s system responds sluggishly to stressors and returns quickly to baseline. They stay calm and level-headed.
Someone high in neuroticism reacts quickly and strongly to stress, and settles slowly. They are prone to worry, anger, or fear.
This produces the subjective experience of being easily upset and hard to calm down. Neuroticism is a disposition to emotional reactivity, not a disorder.
Psychoticism/normality
Eysenck added a third dimension, psychoticism, later than the first two. Eysenck and Eysenck (1969) first reported it in children, and Eysenck and Eysenck (1971) presented factorial evidence that it was separable from neuroticism.
The high-P scorer is described as solitary and uncaring, lacking empathy, aggressive, and drawn to the unusual.
Eysenck did not claim high scorers are psychotic. He defined P as a disposition to develop psychotic symptoms given other contributing factors.
Eysenck speculated about a hormonal link. He proposed androgenic hormones such as testosterone. This link is often reported as an established finding, but it is not: the evidence is sparse, inconsistent, and drawn largely from small cross-sectional samples.
According to Eysenck, the two dimensions of neuroticism (stable vs. unstable) and introversion-extroversion combine to form a variety of personality characteristics.
Critical Evaluation
Twin studies can be used to see if personality is genetic. However, the findings are conflicting and non-conclusive.
Shields (1976) found that monozygotic (identical) twins were significantly more alike on the Introvert – Extrovert (E) and Psychoticism (P) dimensions than dizygotic (non-identical) twins.
Loehlin, Willerman, and Horn (1988) found that only 50% of the variations of scores on personality dimensions are due to inherited traits. This suggests that social factors are also important.
One strength of Eysenck’s theory is that it accounts for both nature and nurture. Biology and environment work together. Predispositions combine with conditioning and socialization in childhood to shape personality.
This interactionist approach may be more valid than a purely biological or environmental account alone.
It fits well with the diathesis-stress model, which holds that a biological predisposition combines with an environmental trigger to produce a particular outcome.
The theory is also genuinely testable. It makes risky predictions, and several have failed.
Contemporary Research
Soto (2019) directly tested how well trait-outcome findings replicate. His Life Outcomes of Personality Replication Project pre-registered a high-powered replication of 78 published trait-outcome associations, using roughly 1,500 adult participants.
Of the 78 associations, 87% replicated in the same direction at statistical significance.
Replication effect sizes, however, averaged only about 77% of the original size, so trait-outcome links are real but were originally overstated.
The genetic evidence tells a similar story. Vukasović and Bratko (2015) meta-analyzed 134 behavior-genetic studies and estimated the heritability of personality at approximately .40, confirming a substantial but partial biological basis.
Trait stability has also been tested directly.
Roberts, Walton, and Viechtbauer (2006) meta-analyzed 92 longitudinal samples and found that traits change in a patterned way across adulthood, with the largest shifts between ages 20 and 40.
The Eysenck Research Record
Some of Eysenck’s later work has faced formal scrutiny. A body of his papers linking personality to cancer and cardiovascular mortality, some funded by the tobacco industry, reported unusually large effects.
A 2019 institutional enquiry concluded a set of these publications were unsafe and recommended retraction (Pelosi, 2019; Craig, Pelosi, & Tourish, 2021).
This concerns Eysenck’s later health-outcome work specifically, not the factor-analytic personality research described above.
Eysenck Personality Inventory (EPI)
Cattell’s 16PF Trait Theory
Raymond Cattell (1965) disagreed with Eysenck’s view that personality can be understood by looking at only two or three dimensions of behavior.
Instead, he argued that it was necessary to look at a much larger number of traits in order to get a complete picture of someone’s personality.
Whereas Eysenck based his theory based on the responses of hospitalized servicemen, Cattell collected data from a range of people through three different sources of data.
- L-data – this is life record data such as school grades, absence from work, etc.
- Q-data – this was a questionnaire designed to rate an individual’s personality (known as the 16PF).
- T-data – this is data from objective tests designed to “tap” into a personality construct.
Cattell analyzed the T-data and Q-data using a mathematical technique called factor analysis to look at which types of behavior tended to be grouped together in the same people. He identified 16 personality traits (factors) common to all people.
Cattell made a distinction between source and surface traits. Surface traits are obvious and easily identified by other people.
Source traits are less visible. They underlie several different aspects of behavior and are recovered only by factor analysis.
Cattell viewed source traits as more important.
Because his sixteen source traits are intercorrelated, factor-analyzing them yields a smaller number of second-order factors. The two largest, exvia-invia and anxiety, correspond closely to Eysenck’s extraversion and neuroticism.
This convergence is the key to the Eysenck-Cattell dispute. Their systems are not contradictory; they are pitched at different levels of the same hierarchy.
Cattell produced a personality test similar to the EPI that measured each of the
sixteen traits. The 16PF (16 Personality Factors Test) has 160 questions in total, ten questions relating to each personality factor.
Allport’s Trait Theory
Gordon Allport’s theory of personality emphasizes the uniqueness of the individual and the internal cognitive and motivational processes that influence behavior.
For example, intelligence, temperament, habits, skills, attitudes, and traits.
Allport (1937) believes that personality is biologically determined at birth, and shaped by a person’s environmental experience.
He categorized traits into three levels. Cardinal traits are dominant traits that shape a person’s entire life. Central traits influence behavior across various situations, and secondary traits are narrower, situation-specific preferences with minimal impact.
Allport emphasized the importance of studying individuals holistically and understanding the complexity of human personality beyond mere trait labels.
The Person-Situation Debate
Trait theory assumes behavior stays fairly consistent across situations, no matter what a person is doing or who they are with. Walter Mischel challenged that assumption directly in the 1960s, sparking a debate that reshaped personality psychology for decades.
Mischel’s Situationist Critique
Mischel (1968) reviewed the personality-assessment literature. He compared test scores against behavior across different settings.
The results were weak. Correlations between behavioral measures of the same trait in different situations typically fell around .10 to .20, and trait scores rarely predicted behavior above .30, the so-called “personality coefficient.”
Mischel concluded that situations predict behavior better than personality does. This position became known as situationism.
The critique was influential, but later work found the comparison unfair. Trait measures were aggregated across many items, while behavioral criteria were single, unreliable acts, and that gap artificially lowered the correlations.
Funder and Ozer (1983) later found that classic situational manipulations in social psychology, once converted to correlation coefficients, were no larger than the trait effects Mischel had criticized.
The Aggregation Reply
Epstein (1979) made the decisive methodological point. A single test item is noisy: it mixes measurement error in with the trait it is meant to capture.
A single behavioral act is noisy too. It mixes situational noise in with the person’s underlying disposition.
Epstein tracked participants for several weeks.
He used daily self-report and observational data, and found that correlations rose substantially once behavior was averaged across many occasions, often into the .50 to .80 range.
This finding rescued trait theory without disputing Mischel’s original data. It simply showed that a single observation was the wrong unit of measurement.
Traits, on this view, predict behavioral tendencies over time, not individual acts. Asking a trait measure to predict one specific occasion is asking it the wrong question.
Interactionism and a Resolution
The field settled on interactionism: behavior is a joint product of person and situation, and the interaction between the two is often larger than either alone.
Fleeson (2001) tested this using experience-sampling. Participants reported their behavior several times a day for one to three weeks.
Within-person variability was large. People routinely spanned most of the extraversion range within a single week.
Yet each person’s average level stayed highly stable across weeks. Bem and Allen (1974) found a related pattern: some people are simply more behaviorally consistent than others, so trait measures predict their behavior better.
Mischel himself later moved toward this view, replacing broad traits with situation-specific cognitive variables such as expectancies and self-regulatory plans. Contemporary personality psychology treats traits and situations as complementary levels of description, not rivals.
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