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Biology 103
2002 Third Paper
On Serendip

Extroversion, Introversion, and the Brain

Natalya Krimgold

The terms "extrovert" and "introvert" are often used to describe individuals' interpersonal relations, but what do these terms mean precisely, and is there a neurobiological basis for these personality traits?

The terms originated from psychologist Carl Jung's theory of personality. Jung saw the extrovert as directed toward the outside world and the introvert as directed toward the self (1). He characterized extroverts as being energized by being around other people and drained by being alone and introverts as the opposite (1). He recognized that most people shared characteristics of both introversion and extroversion and fell somewhere along a continuum from extreme extroversion to extreme introversion (1).

Richard Depue and Paul Collins, professors of psychology at Cornell University and University of Oregon, define extroversion as having two central characteristics: interpersonal engagement and impulsivity (2). Interpersonal engagement includes the characteristics of affiliation and agency. Affiliation means enjoying and being receptive to the company of others and agency means seeking social dominance and leadership roles, and being motivated to achieve goals (2). They also closely link extroversion to "positive affect" which includes general positive feelings and motivation (2). Extroverts, they claim are more sensitive to reward than punishment whereas introverts are more sensitive to punishment than reward (2). According to Depue, "When our dopamine system is activated, we are more positive, excited, and eager to go after goals or rewards, such as food, sex, money, education, or professional achievements", that is, when our dopamine system is activated, we are more extroverted, or exhibit more "positive emotionality" (7).

What Depue and Collins refer to as "positive emotionality" is not precisely what Carl Jung referred to as extroversion. Positive emotionality is the willingness to pursue rewards, to be more stimulated by reward than punishment. Extroversion, according to Carl Jung is enjoying the company or others and being oriented toward the external world and energized by interactions with other people. In the vernacular, an extrovert is someone who has many friends, seems to be around people all the time, and is socially dominant. While positive emotionality and extroversion describe two separate attributes, they are fundamentally related. The desire to pursue goals and being more sensitive to rewards than punishments is an integral part to enjoying relationships with other people, building large networks of friends and being socially dominant. Other people, or groups of people can be punishing of those who attempt to befriend them. One of the possible punishments an individuals risks by attempting to form a friendship is social rejection. For someone with low positive emotionality, this punishment would be enough to deter them from even attempting to form new relationships. Similarly, to achieve social dominance, one must risk losing face in front of one's peers for the possibility of appearing confident and original. Again, there is a risk of social rejection, perhaps a greater risk than in the first scenario, but the reward is also greater: the admiration of one's peer group. Someone with high positive emotionality would be willing to take this risk whereas someone with low positive emotionality would not.

In their article, "Neurobiology of the Structure of Personality: Dopamine, Facilitation of Incentive Motivation, and Extraversion", Depue and Collins argue that there is a strong case for a neurobiological basis of extraverted behavior, because it closely resembles a mammalian approach system based on positive incentive motivation which has been studied in animals (2). Animal research has provided evidence to support the theory that a series of neurological interactions are responsible for variable levels of reaction to an incentive stimulus. First, the incentive is recognized in a series of signals between the medial orbital cortex (the eye), the amygdala (the emotional control center) and the hippocampus (memory center) (3) (2). Next the brain evaluates the intensity of the incentive stimuli in a series of interactions between the nucleus accumbens, ventral palladium, and the ventral tegmental area dopamine projection system (2). This creates an incentive motivational state which can be motivate a response by the motor system (2). Differences in individuals incentive processing are thought to be due to differences in the ventral tegmental dopamine projections which are directly responsible for the perceived intensity of the incentive stimulus (2). Genes and past experience are the sources researchers believe most affect a person's dopamine projections and so, the perceived intensity of incentive stimuli and the persons motivation to pursue the incentive: their degree of extroversion.

Drugs like cocaine, alcohol, or prozac, all affect these processes and also an individual's degree of extroversion. They can artificially correct an ineffective dopamine system and make someone feel more sociable or motivated to pursue a goal. Low levels of serotonin, correlated with depression, may make people more responsive to dopamine and more susceptible to dopamine-stimulating drug use such as the use of cocaine, alcohol, amphetamine, opiates, and nicotine (7).

Impulsivity, which Depue and Collins link to extraversion, can in its extreme case cause attention deficit/hyperactivity disorder, pathological gambling, intermittent explosive disorder, kleptomania, pyromania, trichotillomania, self-mutilation, and sexual impulsivity, as well as borderline personality disorder, and antisocial personality disorder (4). Jennifer Greenberg and Eric Hollander, M.D., in their article "Brain Function and Impulsive Disorders" characterize impulsivity as "the failure to resist an impulse, drive or temptation that is harmful to oneself or others" (4). One can see why Depue and Collins see impulsivity as being linked with positive emotionality: this definition of impulsivity is almost the same as their definition of positive emotionality (more sensitivity to reward than punishment). The only addition is the inability to determine when the punishment outweighs the reward. According to Depue, "the extreme extrovert, then, is someone who has high dopamine reactivity and, as a result, easily binds rewarding cues to incentive motivation. That person will appear full of positive emotion and highly active in approaching rewarding stimuli and goals. The low extrovert will find it difficult to be so motivated and will require very strong stimuli to engage in rewarding activities" (6). It is interesting to consider that the same quality that in moderation is looked at as ambition, in excess is considered a failure to resist an impulse, drive or temptation that is harmful to oneself or others. Clearly, this extroversion/impulsivity/incentive motivation is a very influential trait which must be kept in balance to maintain emotional well-being.

The brain structures research has indicated are active in controlling impulsivity are the orbitofrontal cortex, nucleus accumbens, and amygdala regions, many of the same ones that mediate extroversion (4). Damage to these structures often results in impaired decision-making and increased impulsivity (4). In their article in the Journal of Psychiatric Research, David S. Janowsky, Shirley Morter and Liyi Hong relate novelty seeking and impulsivity to increased risk of suicidality and they correlate depression with an elevated degree of introversion (5). Impulsivity is linked to an increased risk of overt suicidality because it allows patients to avoid considering the long-term consequences of their actions (5). Research has indicated that introversion decreases as depression improves and continued introversion is associated with increased risk of relapse into depression (5). Even recovered depressed patients scored lower (more introverted) than never ill relatives or normals on the Maudsley Personality Inventory Extroversion Scale (5). Janowsky et al. infer that the social isolation associated with introversion may compound the depressed patient's need for a social support network (5). Still, the connection between introversion and depression remains ambiguous because other research has shown no correlation between them (5).

An interesting question that arises is to what extent these traits of introversion and extroversion are genetic and to what extent they are learned by through interaction with one's environment. Depue claims that it is likely that genetics make up for 50-70% of the difference between an individual's personality traits, he says, "The stability of emotional traits suggests that the extent of the interaction between environment and neurobiology is in part determined by the latter. Experience is interpreted through the variable of biology" (6). While experience may modify our response to incentives, our hardwiring, in terms of dopamine production and absorption remains intact.

It seems likely that past experience would play a larger role than Depue indicates in incentive motivation. Experiencing social rejection would seem to discourage more risk-taking in the future, but perhaps, for those with high positive emotionality, and the effective dopamine production and absorption system that it implies, the reward for succeeding is so great that they will continue the behavior even if they fail repeatedly. Perhaps there is a neurobiological basis for having "tough skin" or "thin skin"; for being resilient or oversensitive.
What happens if someone has the neurobiological make-up of someone with high positive emotionality and then suffers a traumatic, punishing, experience which damages their self-confidence. Do they become a frustrated extrovert? According to Depue, such a person would be in the 30-50% of the population whose personality is not directly related to genetics and the functioning of their dopamine reuptake receptors. What happens to this person? Do they suffer from cognitive dissonance, wanting to take more risks but unnaturally wary from what experience has taught them? Does there brain chemistry alter to adapt to their behavior or does their behavior eventually adapt to fit their brain chemistry in spite of past experience, or do they suffer internal turmoil and rely on drugs to free themselves of inhibitions and allow themselves to pursue rewards as they would naturally have done?

Neurobiologically, drugs and alcohol add something new to the mix. Do introverts or suffering extroverts self-medicate with them, or get a prescription for Prozac? First of all, all of these scholars have treated introversion as something of a disease to be medicated, which seems strange considering that Jung appeared to have a fairly egalitarian approach to introversion and extroversion. He treats them as different lifestyles rather than a disease and the lack of one. He defines introversion as enjoying solitude and the inner life of ideas and imagination; hardly a negative description. Depue and Collins probably came up with the term "positive emotionality" because they wanted to describe the quality which those typically thought of as extroverts tend to possess in a social context, but which those termed introverts may also have but manifest in different ways: the desire and ability to achieve goals. Janowsky, on the other hand, refers to introversion as a trait marker for depression, which decreases as depression improves, not as a personality trait of healthy, well-adjusted individuals.

What is introversion, and is it a bad thing? In the Jungian way of thinking most of us are at least somewhat introverted, and a good thing too, or else we would rarely get any studying done. Low positive emotionality is not really the same thing as introversion, although an introvert could have low emotionality in social settings and be more demoralized by fear of rejection than motivated by the prize of friendship or social dominance. This type of introversion is more than likely linked to depression because it does deprive a person of a necessary source of emotional release: the social support network. On the other hand, being introverted can mean that you keep a small, close circle of friends, which would definitely constitute a social support network, and there is no reason to believe that this is unhealthy or even abnormal. The term introvert implies that one is emotionally satisfied by a mostly internal life. The only really conflictual state seems to be the repressed extrovert, one who would like to forge more social relationships but is too intimidated to do so, but this is most probably not the case with all those classified as introverts.

The terms "extrovert" and "introvert" may be inherently problematic. They are so well established in the vernacular now that they have connotations that were probably never intended, for example that the extrovert is always the "life of the party" or that introverts are social outcasts. This may be the reason Depue and Collins chose to use the term "positive emotionality." Using a new term gave them a fresh opportunity to be absolutely clear about what they meant. Their term, however, still does have relevance in relation to the notion of extroversion because extroversion depends on some degree of positive emotionality.

The evidence for a neurobiological basis for all of these traits is strong. Animal research has supported the idea of a network of brain structures communicating signals in order to process and respond to incentives in the environment. In particular, there is convincing evidence that the production and absorption of the neurotransmitter, dopamine, affects the perceived intensity of the incentive stimulus, and so, how motivated the subject is to pursue the stimulus. The changes that occur within the dopamine system and their affect on personality is easily observable in people under the influence of drugs that positively affect the dopamine system like cocaine or alcohol: their fears and anxieties vanish and they are able to pursue goals (although perhaps not higher level ones) in an uninhibited fashion. In a less uninhibited way, the same affect is observable in people taking antidepressants: they are no longer dissuaded from pursuing goals by fear of negative consequences; they regain an ability to "look on the bright side" and focus on the positive aspects of achieving goals rather than the negative repercussions of failing to achieve them. It would be difficult to dispute that there is a relationship between positivity and goal oriented behavior and the dopamine system, but the reason why the dopamine system has this affect on personality remains unknown, and the precise interactions between the dopamine system and the rest of the brain, body, and the exact effect on behavioral patterns are yet to be discovered.


1) 1 Up Info: Extroversion and Introversion, Psychology and Psychiatry

2) Neurobiology of the Structure of Personality: Dopamine, Facilitation of Incentive Motivation, and Extraversion. Depue, Richard and Collins, Paul.

3) The American Heritage Dictionary of the English Language, Third Edition. Houghton Mifflin Company, New York: 1996.

4) Greenberg, Jennifer and Hollander, Eric. Brain Function and Impulsive Disorders. Psychiatric Times. March 1, 2003.

5) Janowsky, David S.; Morter, Shirley and Liyi Hong "Relationship of Myers Briggs type indicator personality characteristics to suidicality in affective disorder patients". Journal of Psychiatric Research, Volume 36, Issue 1, January-February 2002, Pages 33-39.

6) Encyclopedia Britannica Online: Development and Life Course: It's All in Your Head.

7) Cornell University: Science News: "Cornell Psychologist finds chemical evidence for a personality trait and happiness".

8) Web Paper: "Personality: a Neurobiological Model of Extraversion" by David Mintzer

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