Individual Differences


Biological Processes in Personality

Last updated:
01 Sep 2003

Biological systems & processes in personality

Eysenck: Extraversion & Neuroticism


Hormones & Personality

Neurotransmitters & Personality

Biological systems and processes in personality

Evolutionary/genetic perspectives do not generally account for the biological mechanisms between genes and personality.  Theorists use biological processes in an attempt to fill in the gap between personality and genetics by inferring, theorizing and researching biological links with behaviour
Anatomical approach examine functions of various structures of brain.

The biochemical approach examines hormones & neurotransmitters.  This is a complex, difficult area of personality with no clear and simple answers. At a second year level, the expectation here is to get your head around the basic principles that have been proposed. Also, look for places where you can link the theories to each other and to other perspectives of personality. In this way, the content of the biological perspective is most likely to make sense and become part of your understanding of human psychology.

Eysenck: Extraversion and Neuroticism

One of the pioneers in attempting to relate personality to biology was the British psychologist Hans Eysenck (e.g. 1967, 1987). His theory is complex and has evolved over the years, but one of its basic assumptions is that the human brain has excitatory and inhibitory neural mechanisms.
A basic assumption is that the human brain has excitatory and inhibitory neural mechanisms.

Excitatory --> individual alert, awake, aroused

Inhibitory mechanisms --> sleepy, drowsy, sluggish

Balance between the two produces level of psychological arousal at any given moment.  Eysenck hypothesised balance regulated by ARAS Ascending Reticular (aka Reticulocortical) Activating System which is a structure in the brain stem.

Research suggests that function of ARAS is to regulate the amount of information or stimulation that goes into the brain. It activates and deactivates higher parts of the brain (cerebral cortex) and is involved in maintaining alertness and concentration and in controlling the sleep-waking cycle.

Eysenck stated each individual’s ARAS functions differently:

  • ARAS that lets in LOTS of stimulation --> introvert
  • ARAS that cuts brain off from stimulation --> extrovert

This may seem counterintuitive at first but think about it. A person whose ARAS causes him/her to be to be chronically overraroused is an INTROVERT. If you are getting lots of sensory input you AVOID stimulation as you already have more than you need. So, avoid loud noises, social stimulation, noisy parties and exciting situations.

However, a person whose ARAS causes him/her to be to be chronically underaroused is an EXTRAVERT. He or she will CRAVE stimulation looking for sociable situations, loud parties and may be a sensation seeker.

The resting level of ARAS activity is higher for introverts than extraverts.

Introverts are:

  • naturally more aroused
  • prone to overarousal
  • stimulus shy
  • withdrawing

Extraverts are:

  • naturally underaroused
  • seek arousal
  • stimulus hungry
  • approaching

Evidence for ARAS theory

  • Animal research
  • Laboratory research
  • extraverts chose louder noise
  • extraverts take more pauses during in a repetitive task (due to 'boredom')
  • Drugs: introverts require more depressant to get 'unalert'
  • Saliva production (Lemon Juice Test)


  • It has been reported that introverts produce more saliva than do extraverts (Corcoran, 1964, in Phares, 1991):
  • Tie a thread to the centre of a double-tipped cotton swab so that it hangs perfectly horizontal
    Person swallows three times then put one end of the swab on the tongue, holding it there for 30 seconds
  • Then 4 drops of lemon juice are placed on the tongue. After swallowing the person places the other end of the swab on the same portion of tongue for 30 seconds.
  • Swab will remain horizontal for extraverts, hang down on the lemon juice end for introverts (indicating a relatively large amount of saliva produced in response to the lemon juice)
  • Nonetheless, other more sophisticated tests give conflicting results.

Eysenck’s theory is an impressive effort to conceptualize and test brain functioning and personality. It is the oldest modern comprehensive attempt to create model of personality based on biological processes. It was however developed prior to modern methods of testing brain functioning, thus may be wrong in some respects. And it is certainly likely to be an oversimplification. The ARAS is a not as GENERAL a system as was first thought. It does not just turn neural stimulation to the brain on and off like a tap (Zuckerman, 1991). Differing levels of arousal across different parts of the brain are typical.

Gray's BAS/BIS Theory

Many theorists in this area believe that there is a set of brain structures that cause animals to move towards things they desire. We are going to focus on the work of neuropsychologist Jeffrey Gray (e.g. 1981). Gray has tried to improve on Eysenck’s theory. Gray extrapolated the findings of his early research on animals to human personality. Gray, like Eysenck, proposes that personality is based on the interaction of two basic systems in the brain:

1. The first system is the Behavoural Approach system (BAS): this is the approach motivation system or the GO system. The approach system causes one to be both sensitive to potential rewards and motivated to seek those rewards. Attraction to a person or a chocolate cake and a desire to approach the person or cake comes from this system (BEHAVIOURAL APPROACH SYSTEM)

2. The second system is the Behavioural Inhibition System (BIS): this is the avoidance motivation or STOP system. This is the system that causes one to be sensitive to potential punishment and motivated to avoid those punishments. Fear of rejection by someone you fancy or fear of a snake and the motivation to avoid these things comes from this system.
• BAS / BIS independent

• Behavioural Approach/Activation System
• reward-seeking
• positive emotions (hope, joy) (anticipation of good events)
• high BAS (reactivity)  'impulsive'
• low BAS (reactivity)  'not impulsive'
• leads to sociopathy if no inhibition (whereas Eysenck says it is a combination of extraversion and emotional instability)

Work by Richard Davidson (1992, 1995) has shown that positive feelings involve activity in the left frontal lobe of the cortex. Higher levels of frontal activity have been found to occur in adults presented with an incentive. Davidson and colleagues (p. 163) assert that 1. The tendency to experience happiness relates to the Behavioural Approach System and 2. That the system is partly based in the left frontal cortex.

BIS: behavioural inhibition system
• inhibits movement towards goals
• responsive to cues of punishment or danger
• high BIS high trait anxiety
• low BIS low trait anxiety
• leads to anxiety-based disorders (which Eysenck says are a combination of high introversion and emotional instability)
• Research on cortical activity shows an increase in activity in the RIGHT frontal areas of the cortex when people are anxious, fearful or disgusted.

Biological bases of disorders

oversensitive BIS - highly anxious
oversensitive BIS or undersensitive BAS - depression

Antisocial behaviour:
overactive BAS or BIS deficits - antisocial

Hormones & Personality

Uses endocrine system, rather than brain processes, to explain personality.

Our body chemistry also influences our personality. Several studies have revealed a link between hormone levels and aggression for example (e.g. Berman, Gladue, & Taylor, 1993; Van Goozen, Frijda, & Van de Poll, 1995). See Farrington 1994.

Females have 40 nanograms of testerone in each deciliter of their blood; males have 300 to 1000 nanograms of testosterone per deciliter of blood (Ridley, 1999).

Mothers exposed to male hormone during pregnancy had children who were more likely to self-report aggression as a way of dealing with interpersonal conflict, as opposed to their same sex siblings (Reinisch, 1981). This was found for both boys and girls, and suggests that hormonal influences alone may impact upon aggressiveness. One caveat however is that this study used self-report measures of aggression.

In a study by Dabbs and Morris (1990), men higher in testosterone were found to be more likely to abuse alcohol and drugs, assault people, be dominant, have large numbers of sexual partners, be single, or divorced if married, and have trouble with teachers, parents, classmates whilst growing up. Mostly these effects were stronger for low SES, perhaps because high SES socializes out undesirable behaviours.

As mentioned, females also produce testosterone. In a study by Dabbs, Ruback, Frady, Hopper and Sgoritas (1988) female prisoners who had committed violent crimes which were unprovoked were founder to have higher levels of testosterone than females who had committed violent crimes due to provocation or had committed non-violent crimes.

It should be noted however that the findings on the links between testosterone and aggression are by no means conclusive. It has been suggested, for example, that increased testosterone level may be the result of aggression rather than the cause of it (Zuckerman, 1991). Levels of testosterone are altered by situational factors, e.g. increases following success at a competitive event, sexual intercourse, and decreases following failure.

Another hormone implicated in personality is cortisol. People who suffer from severe stress, anxiety and depression tend to have very high levels of cortisol in their system. This would appear to be a RESULT of stress and depression rather than a CAUSE of it.

Neurotransmitters and personality

Communication between neurons in the brain is based upon substances called neurotransmitters. Some of the neurotransmitters you may be familiar with are epinephrine, norepinephrine, dopamine and serotonin. Norepinephrine for example is likely to be involved in both anxiety and anger.

High levels of norepinephrine have for example been associated with anxiety-proneness, dependency and sociability. On the other hand, low levels of norepinephrine are associated with disinhibition & impulsivity (Zuckerman).

Criminals have been found to have low levels of epinephrine and norepinephrine.

The neurotransmitter, dopamine, is implicated in bodily movements, amongst other things. A lack of dopamine is associated with Parkinson’s disease (l-dopa and Sacks).

The enzyme MAO which I mentioned early and which regulates the breakdown of the neurotransmitters serotonin, norepinephrine and dopamine has been found to be implicated in aggression, extraversion, sensation seeking, such that low levels of this enzyme lead to increased levels of aggression, extraversion, and sensation seeking.

Another neurotransmitter, serotonin has a role in the inhibition of behavioural & emotional impulses. Low serotonin levels are implicated with obsessive worry, depression, irrational anger, and chronic pessimism. Marazzitti, et al. (1993) found that highly aggressive individuals had reduced levels of serotonin when compared with controls.

High serotonin levels are associated with compulsive behaviours and obsessive tidiness. However the workings of serotonin are extremely complex and not fully understood. Dean Hamer, who has researched serotonin extensively, suggests that serotonin is a chemical that ABETS rather than alleviates anxiety and depression. Serotonin levels have been linked to aggression such that when serotonin levels rise so to does aggression (Wright, 1995).


Burger, J. M. (1993). Personality (3rd ed.) Pacific Grove, CA: Brooks/Cole.

Carver, C.S., & Scheier, M.F. (2000). Perspectives on Personality (4th ed.) Needham Heights, MA: Simon & Schuster.

Funder, D. C. (1997). The personality puzzle. New York: W. W. Norton.

Ornstein, R. (1993). The Roots of the Self: Unraveling the mystery of who we are. New York: HarperSanFrancisco.

Phares, J.E. (1991). Introduction to Personality (3rd ed.). New York: Harper Collins.

Ridley, M. (1999). Genome: The autobiography of a species in 23 chapters. London: Fourth Estate.