Does my brain make me shy?


If you look at your group of friends, you can probably divide them into introverts and extraverts – those who are quite happy with their own company, and those who need to be kept busy by others; those who are quiet and shy within a group, and those who love to be the centre of attention. What makes these people different? What is it that makes some people more extraverted than others?

I think we can all generally agree that to be introverted/extraverted is to do with ones personality. It’s a difference between people, to describe someone as introverted/extraverted is to describe their behaviour and how they are, which is generally created by their personality. If someone has a sociable, bubbly, outgoing personality they are generally described as being extraverted. Someone’s personality is generally made up of descriptive words – outgoing, caring, anti-sociable, snobby, etc. However, studying personality is a funny thing. There is no unversally accepted definition of what a personality is. Most psychologists generally agree however that whatever the definition of a personality is, it contributes to an individuals behaviour, thoughts and feelings. For example, Gordon Allport (1897-1967) believed that a personality is a “dynamic organisation” which consists of “psychological systems” and these systems create the individual – their behaviour, thoughts and feelings.

Carl Jung (1875-1961) believed that there were two personality types. There is a difference between personality TYPES and personality TRAITS. Types tend to be discontinuous – as in you are one or you are the other, x or y, black or white. Whereas traits are more continuous, you can be inbetween two starting and finishing points, they are on a continuum from x to y, from black to white. Now, Jung believed there were two personality types – introversion and extraversion. Therefore he believed that all individuals were either extraverted or introverted, and all of their personality traits and characteristics stemmed from this. However, this has many flaws. Some people, I can think of a few off the top of my head, don’t fit into being either introverted OR extraverted. They are somewhere in the middle. They may be introverted in certain situations, but become more extraverted in others – I find that I actually work this way. This doesn’t fit Jung’s theory of introversion and extraversion being the two personality types.

Hans Eysenk (1916-1997) believed there to be two personality traits. These were extraversion and neuroticism. Now, because these are traits rather than types they ran on a continuum. So extraversion ran from being extraverted to being introverted, and neuroticism ran from being neurotic to being emotionally stable, with individuals being one extreme, or the other, or somewhere in between. Later, Eysenk added psychoticism as another personality trait and it’s continuum ran from psychoticism to socialisation. However this was really only added because extraversion and neuroticism did not adequately explain all of Eysenk’s data. In this blog post,  I will be focusing on the personality trait of extraversion (and its scale to introversion) and looking at 3 different theories – a biological theory, a behavioural theory and an evolutionary theory.

Eysenk gave a biological explanation for the difference between those who were more extraverted than introverted.

He assumed that the brain has two distinct sets of neural mechanisms. There is the excitatory mechanism which keeps us alert and ready for action, and the inhibitory mechanisms which is related to relaxation, inactivity and sleep. He also assumed that individuals wish to maintain a balance between the excitatory and inhibitory mechanisms and this balance is regulated by the system called the ascending reticular activating system – “ARAS” for short.
Within this system there are two key circuits; one called the reticulo-limbic circuit which is responsible for neuroticism/emotional stability. The other is called the reticulo-cortical circuit which is responsible for controlling cortical arousal produced by stimuli incoming from the environment – this creates extroversion/introversion.
Eysenck thought of the ARAS as a gate which regulated the level of arousal in the brain, and that to be extraverted or introverted was down to the result of differences within the brain and in this system. He suggested that the ARAS operated differently in extroverts and introverts in order to maintain a balance of arousal.

  • Introverts: ARAS always lets too much arousal in from external stimulation -> Introverts are more easily cortically aroused -> experience high levels of cortical arousal in the brain -> behaviourally will avoid situations where further stimulation is likely.
  • Extraverts: ARAS does not let enough external arousal in -> Extraverts are less easily aroused -> experience low levels of cortical arousal -> behaviourally will seek out situations where further stimulation is likely.

There is plenty of behavioural evidence to back this biological theory up. For example, introverts are less tolerant of painful electric shocks than extroverts because their cortical arousal is already high. Also, when introverts are given a high dose of caffeine they perform a given task worse than when given a placebo because they go over the optimal (perfect, or well suited) level of cortical arousal. Whereas extraverts perform better when given caffeine as this boosts their cortical arousal up to the optimal level.

In neuropsychological research, Johnson et al (1999) found in a PET scan (a type of brain scan that involves injecting a glucose/sugar into the body and recording where this glucose goes) that the frontal lobes of introverts, where the reticulo-cortical circuit is located, is more active than the frontal lobes of extraverts. Furthermore Hagemann et al (2009) measured the electrical activity in the brain using EEG (records brain activity) and found that introverts experienced a higher baseline (original, without any stimulation) level of cortical arousal than extraverts.

Jeffery Gray (1935-2004) developed an alternative, behavioural model of personality called “the reinforcement sensitivity theory” which involved extraversion and introversion as personality traits. Like Eysenck, Gray also believed that personality stems from the interaction between two basic systems in the brain, but rooted his theory in behavioural psychology rather than biological:

  • Behavioural approach system (BAS)  – Responsible for our sensitivity to potential rewards and motivates us to seek those rewards, the BAS activity is linked to an individuals level of impulsivity
  • Behavioural inhibition system (BIS) – Responsible for our sensitivity to punishment or potential danger and motivates us to avoid these, the BIS activity is linked to an individuals level of anxiety.

According to this theory:

  • Extraversion – is a combination of high BAS activity and low BIS activity -> Sensitive to potential rewards and easily motivated, and insensitive to punishment/potential danger – > highly impulsive with low levels of anxiety.
  • Introversion – is a combination of low BAS activity and high BIS activity -> insensitive to potential rewards and not easily motivated, and sensitive to punishment/potential danger -> highly anxious with low impulsivity.

Marvin Zuckerman (professor in personality psychology) also suggested a slightly alternative behavioural model to personality, but also put forward the idea that extroversion was high sensitivity to reinforcement, which supports Gray’s model which believed extraverts are sensitive to rewards and introverts are not.

Finally, evolutionary psychologists believe that human behaviour is a product of a process of natural selection – survival of the fittest. Therefore, only behaviour that increases the chances of survival and/or reproductive success is to be passed on to the next generation of offspring. The general evolutionary benefits of being extroverted included higher mating success, and meeting more social allies. However an evolutionary cost of being extroverted may have involved physical risks to both the individual and their family, and a lack of family stability. Genetic variations in personality may have adapted throughout evolution due differing levels of extraversion being adaptive in differing environmental conditions. Therefore, individual differences in personality may have evolved to meet the needs of different situations.


Realistically, looking at the theories all together, it can be said that a combination of all three of them (biological, behavioural and evolutionary) can explain why different individuals are more extraverted than others; we have evolved to have differing systems in our brains which then leads to our behaviour being different. However, in my experience of studying psychology for 5 years, psychologists do not agree on anything and tend not to agree with an area of psychology they don’t study therefore, using the evidence and theories I have presented, I’m afraid for now you’ll have to make your own mind up as to why your friendship group is a collection of the really shy ones, the really outgoing ones, and the mixtures of both!



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