Personality psychology, and thus personality neuroscience, rests on the presupposition that all meaningful ways in which individuals differ have been nested in the world’s languages. These differences are found in the lexicon of the world’s languages. With the lexical hypothesis as a point of departure, we can factor-analyze trait adjectives and type nouns. The result is the emergence of five broad factors or dimensions of personality.

What is a Personality Factor?

A personality factor can be conceived of as a probabilistic description of the intensity and frequency with which individuals experience divergent cognitive, behavioral, affective, and motivational states.

The Big Five Theory of Personality

An interesting fact about the Big Five theory is that it was derived without a guiding theory. This means that the factor structure predated the underlying theory. However, after decades of consistent results, we can fairly confidently assume that as far as self-reports go, there only are five factors of personality.

Numerous studies have assessed the heritability of personality. The results estimate genetic influence to account for roughly 50% of trait variability, which in turn directly implicates neuroscience and physiology to account for the biological systems informing personality synthetically.

Cybernetic Big Five Theory

At the forefront of research in personality neuroscience is Collin DeYoung. He asserts that for a theory about personality to be comprehensive, it necessarily must be nested in cybernetic theory. Cybernetic systems are goal-oriented systems that are able to self-regulate in order to achieve a set goal. A classic example of a cybernetic system is a self-regulated thermostat; a thermostat has the goal to reach a certain temperature and in order to fulfill that goal it either sends out a signal to warm or cool a room. This is essentially what our temperament does, although it is an order of magnitude more complex.

A simple example can be to think of a person who desires social stimulation; one way to achieve that goal is to arrange a social gathering with friends. By the same token, if the person feels overly stimulated, they can leave and regenerate. As such, cybernetic systems are adaptable and adjustable systems that inform behavior to meet a desired goal, whether it be social stimulation, cognitive exploration, or something more primal like actively defending from a perceived threat.

Theoretical Framework for Extraversion

Extraversion is the positive emotion dimension of personality. Highly Extroverted people enjoy being around people, are more exploratory, more easily aroused, and more sensitive to rewards. Extraversion breaks down into two aspects: Assertiveness and Enthusiasm, where Assertiveness reflects a drive for goals and Enthusiasm reflects an enjoyment of goals.

• • Assertiveness: Incentive reward sensitivity: drive toward goals.
  • Enthusiasm: Consummatory reward sensitivity: enjoyment of actual or imagined goal attainment.

The reason for breaking Extraversion into two aspects is simple. The aspects predict different things.

For instance, Enthusiasm is a better predictor for feeling positive emotions and Assertiveness is a better predictor for seeking out positive experiences; this can be compared with a feeling of wanting something versus liking it. Notably, sensitivity to the effects of rewarding stimuli appears to be the cardinal feature of Extraversion. But, not all rewards are made the same and for our purposes, we need to distinguish between incentive rewards and consummatory rewards.

Incentive rewards are rewards in the form of a cue that a person is moving towards a target. Meanwhile, consummatory rewards are rewards that arise for actually attaining a goal. Notice how this aligns with the definition of cybernetic systems outlined above. That is, we have goals that are attained or sought after, and your level of Extraversion moderates the rewarding effects that you feel as a result of signals for their attainment and the rewarding effects of actually attaining them.

But how does this all link back to psychophysiology? When we are rewarded, a surge of dopamine is released. The bigger the reward, the larger the burst of dopamine. But, by the same token, the higher the level of Extraversion, the smaller the reward needs to be for an equivalent release of dopamine in a person with low levels of Extraversion. We can therefore think of Extraversion as an indicator of reward sensitivity; Extraverts feel more rewarded per unit of reward, which in turn secretes more dopamine. To substantiate this claim, research has shown that when people are given dopamine agonists, their reward-seeking behavior increases. By the same token, when people are given dopamine antagonists, their reward-seeking behavior decreases.

What is interesting about the biological theories of personality is how they originated from different places but ended up converging at the same definitions. Case in point, a remarkable scientist by the name of Jeffrey Gray conceived of a biological personality theory called reinforcement sensitivity theory. One of the components of that theory is called the behavioral approach system. Gray defined it as a reward-orienting approach system. It mediates reactions to appetitive stimuli. Gray also claimed dopamine to be the main culprit of the behavioral approach system. Now, somewhat obviously, this definition is the same as the definition for Extraversion. What is also interesting is that self-report scales designed to measure the behavioral approach system show high convergent validity with scales measuring Extraversion. In short, researchers have reached the consensus of the two being conceptually equivalent.

I want to end this post by telling you about a few interesting results concerning Extraversion from the neuroscientific literature. First, Extraversion appears to be associated with an increased volume of the orbitofrontal cortex (OFC). The largest study showing this included 116 participants. The OFC is implicated in assigning value to environmental stimuli, and it makes sense for it to be greater in volume among people high in Extraversion because they are continuously approaching novel situations and consequently assigning value to them. However, in a study with older adults, this finding was not replicated which does put the claim of increased OFC volume into question. Nonetheless, it should also be noted that levels of Extraversion do decline with age, especially after age 50.