W. W. Norton & Company | The Personality Puzzle, 4th ed.

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Chapter 8: The Anatomy and Physiology of Personality

Studies of the biology of personality raise the philosophical issue called the mind-body problem, which concerns the degree to which all aspects of human nature can be understood as processes of our physical brains and bodies, making humans no different from any other animal.

Both brain anatomy and neurophysiology are relevant to personality. Knowledge about the brain comes from studies of the effects of brain injury and brain surgery, from studies of direct brain stimulation, and from newly developed imaging tools such as PET scans and fMRI.

Complex computerized data analysis can combine data gathered from instruments such as PET and fMRI scanners to provide pictures of the brain areas that are most active during various mental tasks and emotional reactions. Researchers have also used these techniques to compare brain activity in people with different personality attributes.

The ascending reticular activating system (ARAS), part of the brain stem, was hypothesized by Hans Eysenck to be the basis of extraversion and introversion. According to his theory, people whose ARAS cuts them off from stimulation may seek out exciting people, environments, and activities, perhaps to the point of danger.

The amygdala plays a special role in generating emotional response. Based on its computation of whether the environment seems to offer impending threat or reward, it can respond by making the heart beat faster and raising the blood pressure, among other effects. Traits associated with functioning of the amygdala include chronic anxiety, fearfulness, sociability, and sexuality.

The frontal lobes are the basis of uniquely human abilities such as language and foresight, and they also are important for understanding the self and other people, and for regulating emotion. fMRI studies show strong activity in this area in people who are prone to negative emotions, but also in people who are consistently cooperative. Cases such as Phineas Gage, Elliott, and victims of Capgras syndrome show how basic emotional responses and cognitive functioning must work together for meaningful experience and adaptive decision making.

Psychosurgeries on the frontal lobes, such as lobotomies, may have helped some desperately ill people in the past, but overall seemed to damage patients' ability to reason and to function, especially in their emotional lives and relations with others.

Recent fMRI research suggests that personality may be affected more by systems or circuits of different areas of the brain acting at the same time, rather than by single areas' relevance to particular traits.

The chemical bases of behavior include neurotransmitters and hormones, both of which play a role in communication between and stimulation of the cells of the nervous system.

The neurotransmitters epinephrine and norepinephrine are an important part of the fight-or-flight response to threatening situations. Some psychologists have recently proposed that in women the instinctive response to a threat may instead be a tend-and-befriend response.

Dopamine is important for responding to rewards and, according to Jeffrey Gray, may be the basis of the trait of impulsivity. Other evidence suggests that dopamine is also relevant to the broader trait of extraversion.

Serotonin aids in regulating emotions, and some widely prescribed anti-depressant drugs are designed to increase its prevalence in the brain. When its level is raised via Selective Serotonin Reuptake Inhibitors (SSRIs) such as Prozac, the result is often a general lessening of neurotic over-reaction to negative events. However, attempts to improve personality-relevant brain functioning through "cosmetic psychopharmacology" may have side effects.

The male sex hormone testosterone plays a role in sexuality, aggression, and dominance, especially in people who have not been socialized against physical aggression. Testosterone level is an effect as well as a cause of certain social behaviors; e.g., it rises after the experience of victory over an opponent.

Cortisol is an important part of the fight-or-flight (or tend-and-befriend) response. Excess production may lead to chronic anxiety and even brain damage, whereas a shortage can lead to dangerously impulsive behavior.

It is important to remember that biological processes affect behavior, but both behavior and the social environment also affect biological processes. An understanding of each will be helpful for understanding the others.