In psychology, trait theory is an approach to the study of human personality. Trait theorists are primarily interested in the measurement of traits, which can be defined as habitual patterns of behavior, thought, and emotion. According to this perspective, traits are relatively stable over time, differ across individuals (e.g. some people are outgoing whereas others are shy), and would influence behavior.
Gordon Allport was an early pioneer in the study of traits, which he sometimes referred to as dispositions. In his approach, central traits are basic to an individual's personality, whereas secondary traits are more peripheral. Common traits are those recognized within a culture and may vary between cultures. Cardinal traits are those by which an individual may be strongly recognized. Since Allport's time, trait theorists have focused more on group statistics than on single individuals. Allport called these two emphases "nomothetic" and "idiographic," respectively.
There is a nearly unlimited number of potential traits that could be used to describe personality. The statistical technique of factor analysis, however, has demonstrated that particular clusters of traits reliably correlate together. Hans Eysenck has suggested that personality is reducible to three major traits. Other researchers argue that more factors are needed to adequately describe human personality including humor, wealth and beauty. Many psychologists currently believe that five factors are sufficient.
Virtually all trait models, and even ancient Greek philosophy, include extraversion vs. introversion as a central dimension of human personality. Another prominent trait that is found in nearly all models is Neuroticism, or emotional instability.
The Two taxonomies 
Both approaches extensively use self-report questionnaires. The factors are intended to be orthogonal (uncorrelated), though there are often small positive correlations between factors. The five factor model in particular has been criticized for losing the orthogonal structure between factors. Hans Eysenck has argued that fewer factors are superior to a larger number of partly related ones. Although these two approaches are comparable because of the use of factor analysis to construct hierarchical taxonomies, they differ in the organization and number of factors.
Whatever the causes, however, psychoticism marks the two approaches apart, as the five factor model contains no such trait. Moreover, psychoticism, unlike any of the other factors in either approach, does not fit a normal distribution curve. Indeed, scores are rarely high, thus skewing a normal distribution. However, when they are high, there is considerable overlap with psychiatric conditions such as antisocial and schizoid personality disorders. Similarly, high scorers on neuroticism are more susceptible to sleep and psychosomatic disorders. Five factor approaches can also predict future mental disorders.
Lower-order factors 
There are two higher-order factors that both taxonomies clearly share: extraversion and neuroticism. Both approaches broadly accept that extraversion is associated with sociability and positive affect, whereas neuroticism is associated with emotional instability and negative affect.
Many lower-order factors, or facets, are similar between the two taxonomies. For instance, both approaches contain factors for sociability/gregariousness, for activity levels, and for assertiveness within the higher order factor extraversion. However, there are differences too. First, the three-factor approach contains nine lower-order factors and the five-factor approach has six.
Eysenck's psychoticism factor incorporates some of the polar opposites of the lower order factors of openness, agreeableness and conscientiousness. A high scorer on tough-mindedness in psychoticism would score low on tender-mindedness in agreeableness. Most of the differences between the taxonomies stem from the three factor model's emphasis on fewer high-order factors.
Although both major trait models are descriptive, only the three-factor model offers a detailed causal explanation. Eysenck suggests that different personality traits are caused by the properties of the brain, which themselves are the result of genetic factors. In particular, the three-factor model identifies the reticular system and the limbic system in the brain as key components that mediate cortical arousal and emotional responses respectively. Eysenck advocates that extraverts have low levels of cortical arousal and introverts have high levels, leading extraverts to seek out more stimulation from socializing and being venturesome. Moreover, Eysenck surmised that there would be an optimal level of arousal, after which inhibition would occur and that this would be different for each person.
In a similar vein, the three-factor approach theorizes that neuroticism is mediated by levels of arousal in the limbic system and that individual differences arise because of variable activation thresholds between people. Therefore, highly neurotic people when presented with minor stressors, will exceed this threshold, whereas people low in neuroticism will not exceed normal activation levels, even when presented with large stressors. By contrast, proponents of the five-factor approach assume a role of genetics and environment but offer no explicit causal explanation.
Given this emphasis on biology in the three-factor approach, it would be expected that the third trait, psychoticism, would have a similar explanation. However, the causal properties of this state are not well defined. Eysenck has suggested that psychoticism is related to testosterone levels and is an inverse function of the serotonergic system, but he later revised this, linking it instead to the dopaminergic system.
List of personality traits 
|Openness to experience||Composed of two related but separable traits, Openness to Experience and Intellect. Behavioral aspects include having wide interests, and being imaginative and insightful, correlated with activity in the dorsolateral prefrontal cortex. Considered primarily a cognitive trait.|
|Conscientiousness||Scrupulous, meticulous, principled behavior guided or conforming to one's own conscience. Associated with the dorsolateral prefrontal cortex. Anorexics are noted to have higher levels of conscientiousness.|
|Extraversion||Gregarious, outgoing, sociable, projecting one's personality outward. The opposite of extraversion is introversion. Extraversion has shown to share certain genetic markers with substance abuse. Extraversion is associated with various regions of the prefrontal cortex and the amygdala.|
|Agreeableness||Refers to a compliant, trusting, empathic, sympathetic, friendly and cooperative nature.|
|Neuroticism||"Refers to an individual's tendency to become upset or emotional" (Hans Eysenck) "Neuroticism is the major factor of personality pathology" (Eysenck & Eysenck, 1969). Neuroticism has been linked to serotonin transporter (5-HTT) binding sites in the thalamus: as well as activity in the insular cortex.|
|Self-esteem (low)||A "favorable or unfavorable attitude toward the self" (Rosenberg, 1965). An individual's sense of his or her value or worth, or the extent to which a person values, approves of, appreciates, prizes, or likes him or herself" (Blascovich & Tomaka, 1991).|
|Harm avoidance||A tendency towards shyness, being fearful and uncertain, tendency to worry. Neonatal complications such as preterm birth have been shown to affect harm avoidance. Those with BED, AN, and BN exhibit high levels of harm avoidance. The volume of the left amygdala in girls was correlated to levels of HA, in separate studies HA was correlated with reduced grey matter volume in the orbito-frontal, occipital and parietal regions.|
|Novelty seeking||Impulsive, exploratory, fickle, excitable, quick-tempered, and extravagant. Associated with addictive behavior.|
|Perfectionism||"I don't think needing to be perfect is in any way adaptive" (Paul Hewitt, PhD)
Socially prescribed perfectionism – "believing that others will value you only if you are perfect."Self-oriented perfectionism – "an internally motivated desire to be perfect.
|Alexithymia||The inability to express emotions. "To have no words for one's inner experience" (Rený J. Muller PhD).In studies done with stroke patients, alexithymia was found to be more prevalent in those who developed lesions in the right hemisphere following a cerebral infarction. There is a positive association with post-traumatic stress disorder (PTSD), childhood abuse and neglect and alexithymia. Utilizing psychometric testing and fMRI, studies showed positive response in the insula, posterior cingulate cortex (PCC), and thalamus.|
|Rigidity||Inflexibility, difficulty making transitions, adherence to set patterns. Mental rigidity arises out of a deficit of the executive functions. Originally termed frontal lobe syndrome it is also referred to as dysexecutive syndrome and usually occurs as a result of damage to the frontal lobe. This may be due to physical damage, disease (such as Huntington's disease) or a hypoxic or anoxic insult.|
|Impulsivity||Risk taking, lack of planning, and making up one's mind quickly (Eysenck and Eysenck). A component of disinhibition. Abnormal patterns of impulsivity have been linked to lesions in the right inferior frontal gyrus and in studies done by Antonio Damasio author of Descartes Error, damage to the ventromedial prefrontal cortex has been shown to cause a defect in real-life decision making in individuals with otherwise normal intellect. Those who sustain this type of damage are oblivious to the future consequences of their actions and live in the here and now.|
|Disinhibition||Behavioral dis-inhibition is an inability or unwillingness to constrain impulses, it is a key component of executive functioning. Researchers have emphasized poor behavioral inhibition as the central impairment of ADHD. It may be symptomatic of orbitofrontal lobe syndrome, a subtype of frontal lobe syndrome which may be an acquired disorder as a result of traumatic brain injury, hypoxic ischemic encephalopathy (HIE), anoxic encephalopathy, degenerative diseases such as Parkinson's, bacterial or viral infections such as Lyme disease and neurosyphilis. Disinhibition has been consistently associated with substance abuse disorders, obesity, higher BMI, excessive eating, an increased rate of eating, and perceived hunger.|
|Psychoticism||Psychoticism is a personality pattern typified by aggressiveness and interpersonal hostility, one of four traits in Hans Eysenck's model of personality. High levels of this trait were believed by Eysenck to be linked to increased vulnerability to psychosis such as schizophrenia. He also believed that blood relatives of psychotics would show high levels of this trait, suggesting a genetic basis to the trait.|
|Obsessionality||Persistent, often unwelcome, and frequently disturbing ideas, thoughts, images or emotions, rumination, often inducing an anxious state. Obsessionality may result as a dysfunction of the basal ganglia.|
See also 
- 16 Personality Factors
- Big Five personality traits
- Hans Eysenck
- HEXACO model of personality structure
- Personality psychology
- Alternative five model of personality
- Cultural schema theory
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