Personality and individual differences in psychology

A key concept in psychology is understanding personality and individual differences. This includes the complex relationship between personality and behaviour, the effects of personality disorders, and broader societal issues relating to individual differences in personality. 

So integral to psychology is the idea of personality and individual difference that there is a book of the same name by Hans Eysenck as well as a peer-reviewed academic journal (founded by Eysenck) with the title. Personality and Individual Differences is the official journal of the International Society for the Study of Individual Differences and is published sixteen times a year by Elsevier. There are also numerous textbooks, which expand upon the subject including Personality, Individual Differences and Intelligence by Day, Macaskill, and Maltby, published by Pearson.

Differential psychology looks at the ways in which individuals’ behaviour creates personality differences and the processes that underlie it. The variability in this differentiation can include metrics like personality, motivation, intelligence, emotional intelligence, interests, values, self-concept, self-efficacy, and self-esteem. From this grew Eysenck’s model of individual differences.

Hans Eysenck’s model of individual differences

Often referred to as “the father of psychology”, Hans Eysenck is credited by many for bringing psychology into the scientific sphere, but also into the public sphere. Interestingly, his theories of personality were originally based on the Greek physician Galen’s four temperaments: sanguine, choleric, melancholic, and phlegmatic. This approach doesn’t feel that scientific, but Eysenck built his research upon an empirical foundation and ran numerous experiments to test his theories. These tests and his interest in how to measure behavioural traits contributed considerably to the field of psychometrics.

Eysenck proposed that each individual has a range of characteristics that remain stable over time, but that each person’s nervous system defines what these characteristics are. Although Eysenck took sociocultural influences and social psychology into consideration, he believed that physiological and genetic make-up dictated the function of the nervous system and consequently was responsible for personality traits. His work focused on three key dimensions of the personality:

• Extraversion-Introversion: Traits considered within this scale include sociability, impulsiveness, vitality, exploration, dynamism, dominance, and dogmatism.
• Neuroticism: This dimension considers traits such as instability, irrationality, anxiety, emotiveness, guilt, low self-esteem, and shyness.
• Psychoticism: This includes traits such as egocentrism, aggression, cruelty, coldness, and difficulty feeling empathy.

These dimensions and the study of individual differences provided the basis for much of our understanding of personality types in psychology, but Eysenck is not without his detractors. 

In 2019, many of Eysenck’s studies came under scrutiny for suspected data manipulation. The studies were with regard to personality-providing predictors for health, and specifically, links with cancer and cardiovascular disease. The idea that an optimistic personality has more chance of surviving cancer is one that has some influence in society, and Anthony Pelosi, whose critique instigated further inquiry, is pursuing a fuller investigation. 

Eysenck’s work still has a high impact factor; for example, a highly criticised 2008 meta-analysis reporting a link between stress and cancer, included reference to two questionable Eysenck papers. According to Google Scholar, this meta-analysis has been cited 966 times to date.

As our knowledge of the nervous system grows, so does our understanding of the effect of biochemistry upon personality. Equally, our knowledge of genetics is still developing and changing, sometimes overlapping with research in neuroscience. It’s currently estimated that 20 to 60 percent of temperament is determined by genetics and the rest is down to environment. So, the determinants of personality are a mixture of nature and nurture.  

The Five-Factor Model

The Five-Factor Model (FFM) is sometimes referred to as the Big Five and it elaborated on Eysenck’s three-factor theory (amongst others), which many began to feel was potentially limiting in scope. Today, it is generally believed that there are five primary factors of personality and FFM is the most widely accepted personality theory by psychologists. 

Rather than presenting binary polarities in personality, each of the Big Five represents a continuum of personality traits:

• Openness (imagination, feelings, actions, ideas)
• Conscientiousness (competence, self-discipline, thoughtfulness, goal-driven)
• Extroversion (sociability, assertiveness, emotional expression)
• Agreeableness (cooperative, trustworthy, good-natured)
• Neuroticism (tendency towards unstable emotions)

The acronym OCEAN is an easy way to remember the Big Five. Each represents a spectrum which could contain various levels of the trait. For example, agreeableness may express itself as someone being trusting, altruistic, compliant, modest, sympathetic, empathetic, unsympathetic, stubborn, belittling of others, demanding, uncaring of others’ feelings, or sceptical. This represents a wide range of personality rather than simply classing an individual as being at a positive or negative end of a spectrum.

A study in 2012 by Soto and John tried to establish developmental changes for the Big Five traits. They found that agreeableness and conscientiousness increased with age. Decreases are seen in openness to experience and neuroticism from teenage years to middle adulthood. Soto and John came to the conclusion that there were more significant trends relating to specific facets of the traits (such as adventurousness and depression) rather than the traits as a whole.

Studies have attempted to find a relationship between forms of neurodiversity like ADHD and specific personality types, but there is also the possibility that personality traits are consequently shaped by someone having ADHD from birth, as it is believed to be heritable. The large body of neurobiological research around ADHD points to this. In personality questionnaires, those with ADHD often rate themselves as lower in conscientiousness and higher on the neuroticism scale. Using the diagnostic criteria in the Diagnostic and Statistical Manual of Mental Disorders (DSM), someone with ADHD may identify symptoms such as poor organisation and lack of attention to detail as being unconscientious, when in fact, it’s a consequence of their brain function.

In psychopathology, empathy is recognised as often lacking in personalities that may be affected by psychopathy, Machiavellianism, and narcissism. This would be represented as not caring about how other people feel on the agreeableness scale. Neuroticism and a lack of conscientiousness may also become apparent in a personality assessment.

What is evolutionary psychology?

Evolutionary psychology is the study of human cognition and behaviour with respect to their evolutionary origins. Core to the methodology of this biologically informed line of thinking is that the behaviour of human beings today can be better understood by thinking about the context in which early humans evolved. It follows the uncontroversial assumption that the brain causes behaviour, but the controversial assumption that Darwinian evolution applies to the human mind as well as the brain.

A thought leader in the field, Todd K. Shackelford is an American psychologist and professor at Oakland university. He is also a co-founder (with Vivianna A. Weekes-Shackelford) of Evolutionary Psychology Lab. According to the homepage: 

“Evolutionary psychology is a hybrid discipline that draws insights from modern evolutionary theory, biology, cognitive psychology, anthropology, economics, computer science, and paleoarchaeology.”

Anthropologist John Tooby and psychologist Leda Cosmides elaborate in Chapter 1 of David Buss’s The Handbook of Evolutionary Psychology:

“Evolutionary psychology is the long-forestalled scientific attempt to assemble out of the disjointed, fragmentary, and mutually contradictory human disciplines a single, logically integrated research framework for the psychological, social, and behavioral sciences – a framework that not only incorporates the evolutionary sciences on a full and equal basis, but that systematically works out all of the revisions in existing belief and research practice that such a synthesis requires.”

Evolutionary psychology has many critics who believe it is reductionist. A frequent criticism is that its theories and assumptions are not falsifiable, but this doesn’t mean it should be rejected completely.

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