Australasian Science: Australia's authority on science since 1938

Personality Influenced by Brain Structure

By Tim Hannan

Individual differences in personality have been associated with structural variation in the cortex.

Individual differences in patterns of thinking, feeling and behaving are what makes each of us unique, and the variety of personality types has long been a field of study for psychologists. While traits have traditionally been measured using questionnaires and observations of behaviour, it may soon be possible to assess an individual’s personality through examination of patterns of brain structure and function. In a recent study, researchers have found associations between personality traits and structural aspects of the brain, such as the thickness, size of surface area and degree of folding of specific regions of the cortex.

The history of psychology is littered with theories of personality, ranging from the quasi-astrological to the rigorously scientific. From his therapeutic analysis of neurotic Viennese, Sigmund Freud proposed that human behaviour reflects the interaction of three unconscious structures: the id, ego and superego. Later approaches developed theories from statistical analyses of people’s responses to lists of words or statements.

In the 1950s, Hans Eysenck concluded that personality comprised three main factors that he termed extraversion–introversion, neuroticism–stable and psychoticism. In contrast, Raymond Cattell argued that three dimensions were insufficient to conceptualise the diversity of personality, and he developed a 16-factor model. Gordon Allport went even further, combing a dictionary for personality-related adjectives to argue that more than 4000 traits could be discriminated.

Since the 1990s, modern personality theory has been dominated by the “five-factor model”. Derived from factor analyses of questionnaires completed by very large samples, these “big five” dimensions are labelled Neuroticism (moodiness and emotional instability), Extraversion (sociability, enthusiasm and expressiveness), Openness (imagination, creativity and open-mindedness), Agreeableness (prosocial behaviour, trust and empathy) and Conscientiousness (thoughtfulness and goal-directed behaviour). Cross-cultural studies have shown that these five factors are present in people from diverse backgrounds, with longitudinal studies demonstrating that these dimensions are relatively stable through the adult years.

Like earlier personality theories, the five-factor model assumes that traits have developed as a result of an interaction between innate, biologically determined dispositions and environmental variables, especially parenting and other early experiences. Family and twin studies have suggested that these personality traits are largely heritable but are also affected by experience. However, evidence of associations between personality dimensions and biological markers has remained elusive, with correlations between specific traits and brain structure or function found only occasionally and inconsistently.

In the recent study published in Social Cognitive and Affective Neuroscience, a team of Italian, American and British researchers speculated that prior research has been constrained by samples either of small size or significant heterogeneity, and by limitations in the methods of quantifying the size of specific brain regions. To address these methodological problems, they engaged 507 healthy young adults to complete the NEO Five-Factor Inventory, a 60-item questionnaire designed to assess the “big five” personality traits, and to undergo a brain scan that enabled the measurement of variability in cortical thickness, surface area, cortical volume and folding.

Analyses revealed strong associations between each of the five personality domains and the brain measurements, especially in the frontal regions of the brain. As had been previously shown in other studies, the thickness of a region of the cortex was inversely related to the size of its surface area and degree of folding: put simply, as a part of the cortex develops it either grows thicker or folds over upon itself.

However, the present study added the new finding that this structural variability is correlated with personality dimensions. To take the two strongest associations, high Neuroticism was associated with a thicker cortex in prefrontal–temporal regions and thus a smaller surface area and less folding, while Openness was related to a thinner cortex in prefrontal–parietal regions, with greater surface area and increased cortical folding.

While the study replication, its demonstration of associations between these personality dimensions and specific anatomical variables supports the validity of the five-factor personality theory. More importantly, it provides hope that further research will identify biological markers for personality dis­orders or other behavioural, emotional or cognitive problems, opening the possibility of early diagnosis and targeted therapeutic intervention.

A/Prof Tim Hannan is Head of the School of Psychology at Charles Sturt University, and the Past President of the Australian Psychological Society.