You don't need to know a complete biologically-based theory of personality to compare with Freud's psychodynamic model, but you might find it easier to compare approaches if you know a bit more about Eysenck's personality theory.

THE P.E.N. THEORY OF PERSONALITY
PERSONALITY TRAITS & THE BRAIN

One of the most influential theories of personality was proposed by Hans Eysenck​, a German who came to live in Britain to escape from the Nazis.

Hans Eysenck carried out research in the 1960s that suggested people can be measured in terms of extrovert-introvert (E) and stable-neurotic (N).
​
Extroversion is based on the level of stimulation we need. This is caused by a brain process known as the Ascending Reticular Activating System (ARAS). The job of the ARAS is to maintain an ideal level of alertness. If the ARAS is under-active, a person does not receive enough stimulation and needs to go looking for it in the environment. These people are extroverts: outgoing and easily bored. At the other end of the scale are the introverts, whose brains receive too much stimulation; they stay away from crowds and excitement and prefer calm and quiet.

Neuroticism is based on the Autonomic Nervous System (ANS), which enables us to respond to stress. If the ANS does its job effectively, a person will be stable, with a consistent mood and an ability to learn easily. If the ANS is inefficient, the person will be neurotic, with changeable moods and difficulty learning from experience.
Eysenck suggests we all fall somewhere on the E scale and the N scale. Stable extroverts make good athletes but neurotic extroverts are more likely to be criminals.
Eysenck created a questionnaire to measure E and N. This was the Eysenck Personality Inventory (EPI). Eysenck carried out his original research on soldiers suffering from battle-fatigue; he found that the most traumatised soldiers scored high on N and low on E, which is what he predicted. E ratings have been found to go up when people take alcohol or drugs and N ratings go down when people are treated for stress.
Eysenck (1975) later collaborated with his wife Sybil and they proposed a third dimension called psychotic (P) which is a trait to do with compassion and morality. Psychoticism affects the type of relationships and attitudes you have. High P scores show antisocial traits like selfishness, coldness, hostility and impulsivity. However, they may also show creativity.

With P, E and N, Eysenck claimed to have a complete model of human personality, the PEN Model.
This shows the PEN model as a 3-D graph with E on the left-right x-axis, N on the up-down y-axis and P on the near-far z-axis.
​Eysenck incorporated questions to measure P into his questionnaire and called it the Eysenck Personality Questionnaire (EPQ). He tested prisoners and found they scored higher for P than non-convicts, as he predicted.
This interactive test scores you on P, E and N and draws a diagram to show where you are placed
Eysenck is not a complete nativist. He recognises that environment can change your personality (after all, his first research was on soldiers who had become neurotic due to battle shock). Eysenck suggests your personality traits are 75% nature (intrinsic, fixed and unchanging) and 25% nurture (changeable depending on your experiences).

THE "BIG FIVE"

Following on from Eysenck, other researchers have suggested 5 crucial personality traits (known as the "Big Five" or the "Five Factor Model" or FFM).

​These can be remembered through the code OCEAN:
  • ​Openness
  • Conscientiousness
  • Extroversion
  • Agreeableness
  • Neuroticism

Eysenck's E and N traits appear in the Big Five but his P trait has been broken down into three separate traits: Openness (creativity), Conscientiousness (impulse control) and Agreeableness (empathy, concern for others).

THE PEN MODEL & THE BRAIN
PERSONALITY TRAITS & NEUROCOGNITION

Eysenck explains how the PEN Model is supposed to relate to brain functioning, so let's look at that in more detail:

THE TWO HEMISPHERES

The brain's left and right hemispheres communicate through the corpus callosum. This seems to be important for long-term planning, which is an important component in Stability (low-N).

​Colin DeYoung (2010) used MRI scans and reports a link between the right hemisphere and N scores. This makes sense since the right hemisphere is associated with emotional intelligence and high N scores means more emotionality.

Raine et al. (1997) found that murderers showed less activity in the corpus callosum, which links to Eysenck's idea that criminals may have high N traits.

THE CEREBRAL CORTEX

As the Phineas Gage case suggests, the brain's frontal lobe plays an important part in decision-making and self-restraint. In particular, a region called the pre-frontal cortex seems to be particularly important.

Impulse control is an important component in having a low P score. In the Classic Study by Raine et al. (1997), brain scans of a group of murderers revealed they had much less activity in the frontal lobe compared to a control group of non-murderers. This ties in with Eysenck's idea that criminals may have high P traits.
The Phineas Gage case also seems to illustrate brain plasticity; the brain tends to heal or re-shape itself so that if one part is damaged, another part steps in to "pick up the slack". For example, Raine found that the murderers had more activity in the occipital lobe, perhaps compensating for the lowered activity in the frontal lobe.

THE LIMBIC SYSTEM

The limbic system is a sub-cortical area - part of the "old brain" that we share with other animals. It is also the brain's emotion centre where our most basic urges and desires (appetite, sleep, sex drive, fear) are regulated.

However, the Ascending Reticular Activating System (ARAS) is mostly located beneath the limbic system in the oldest part of the brain - the brainstem.

The ARAS controls alertness and restfulness and sends out messages to the other parts of the brain, via the thalamus, which acts like the brain's "switchboard" or "router".  This makes the brainstem and the thalamus important for determining your E trait.
This shows how pathways from the ARAS gather in the thalamus then "branch out" to the rest of the brain.
Your N trait is determined by activity in your limbic system. The Autonomic Nervous System (ANS) is controlled by the hypothalamus. Other nearby structures contribute to neuroticism. For example, the amygdala regulates the "fight or flight" response and high-N people tend to interpret things as threats; the hippocampus deals with memory and high-N people are slow to learn from experience.

PERSONALITY & HORMONES

Your P trait is linked to hormones like testosterone. This would explain why women tend to score lower than men for P - they have less testosterone (but they do have testosterone; it's produced in women's ovaries).
A study of prisoners (James Dabbs et al., 1987, 1995) found testosterone levels were higher in those who had been convicted of a violent crime. Those with high testosterone levels were rated higher by other prisoners for being "tough". This links with Eysenck, who found higher P ratings in his study of prisoners compared to non-offenders.
The link between hormones and mood is pretty well known - we use the phrase "feeling hormonal" to describe mood swings. However, personality is more enduring than mood. Hormones which may affect personality include:
  • ​Oestrogen: the female sex hormone, which regulates mood and emotional wellbeing (low N scores)
  • Vasopressin: a hormone that affects men more strongly than women and promotes social behaviour and dominance (high E and P scores)
  • Oxytocin: a hormone that affects females more strongly than males and promotes bonding and empathy (low P scores)

PERSONALITY & NEUROTRANSMITTERS

The ARAS is responsible for making the brain alert and the neurotransmitter that does this is dopamine. Colin DeYoung (2010) used MRI scans to study the brain and reports a correlation between E scores and dopamine pathways in the brain, which is what Eysenck's theory would predict.

​Serotonin inhibits (restrains) mood and emotion and DeYoung reports a negative correlation between serotonin and N-scores - in other words, high N scores link with low serotonin levels.

There's no specific neurotransmitter for psychoticism, but high levels of dopamine and low levels of serotonin would encourage the sort of impulsive personality that Eysenck describes. Eysenck (1990) links this to monoamine oxidase (MAO), which is a protein that helps in the re-uptake of both serotonin and dopamine. Low levels of MAO might lead to fluctuating levels of dopamine and serotonin in different parts of the brain and Eysenck claims there are low levels of MAO in psychotic patients and their relatives.

APPLYING PERSONALITY THEORY TO REAL LIFE
AO2

Addiction​
As part of the Biological Approach, you study the effects of drugs on the brain. Personality theory explains some of the features of addiction.
Remember there are non-biological explanations of aggression, such as Social explanations in terms of obedience to authority or discrimination against out-groups or Learning explanations in terms of role models. There is also the Freudian explanation you must consider.
The Biological Approach is good at explaining why drugs are addictive - they affect the reward pathways in the brain. It's not so good at explaining two other things:
  1. why people start taking drugs in the first place, and
  2. why some people quit drug-taking but other people don't
This is where individual differences and personality theory comes in.

Experimenting with drugs is clearly linked to having a psychotic (high-P) personality, since drug-taking is risky, antisocial and unrestrained and these characteristics all link to P. Some creative types also believe (rightly or wrongly) that drug-taking unlocks their imagination and, according to Eysenck, creativity is linked to high P-scores too.

​Drugs are also stimulating and people with strongly extrovert (high-E) personalities crave stimulation. However, most extroverts get the stimulation they need from sport, socialising, etc. The other function of drugs is to regulate mood and this is something strongly neurotic (high-N) personalities find difficult. The combination of high-E and high-N along with high-P might be a "perfect storm" for drug-taking and this ties in with Eysenck's idea that criminals are neurotic extroverts with psychotic tendencies.

​Quitting drugs is much easier for introverts (low-E) who don't require as much stimulation anyway. It would also be easier for stable personalities (low-N) who can regulate their mood better and don't experience the same "lows" from withdrawal. Empathic people (low-P) are much more likely to be influenced by friends and loved ones who are hurt by their drug habit and put pressure on them to quit.
People often say they have an "addictive personality" but that's usually just an excuse for them to do what they want to do while making out it's beyond their control. There's not much evidence for "addictive personalities" but some people may find it harder than others to resist temptation.
Psychometric Tests

Psychometric tests are instruments that measure mental characteristics (like personality, but also intelligence or mental health) and give it a number score (quantitative data). In the Biological Approach, most psychometric tests are questionnaires or brain imaging techniques.
In 1985, Eysenck produced the Revised Eysenck Personality Questionnaire (EPQ-R) which has 100 yes/no questions. He also produced a short-scale (B) version with only 48 questions that could be used more quickly.

The EPQ-R measures P, E and N but the large-scale (A) version also has a L-scale, which consists of 21 "lie questions" to check for demand characteristics.
"Do you never gossip?" - example of a "Lie question"
Eysenck, Eysenck & Barrett (1985) tested men and women on the EPQ-R, with these results:
You'll notice that women are slightly more extroverted and more neurotic than men, but men are significantly more psychotic than women. They performed similarly on the lie questions.
There seem to be cultural variations too. Hu & Gong (1990) compared writers and mathematicians. The writers scored higher in P, E and N, but lower on L.
What's going on here? Does writing attract "creative types" with high P-scores, as Eysenck suggested? Or does a life spent dealing with numbers rather than people gradually reduce individual differences, which might be what a situationalist would say? Or were the mathematicians just bigger liars?

EVALUATING INDIVIDUAL DIFFERENCES
AO3

Credibility

Eysenck's ideas have stood the test of time and are still being researched. The "Big Five" have replaced Eysenck's simple PEN Model, but Extroversion (E) and Neuroticism (N) are backed up by studies of  brain functioning and strongly predict people's behaviour in different situations. For example, Colin DeYoung (2010) used MRI scans on 116 men and women who had completed personality tests to measure the Big Five traits; there was a significant correlation between activity in certain parts of the brain and high or low scores on certain traits.
Eysenck's PEN Model is supported by his own research into battle-fatigued soldiers and prisoners.

​The EPI and later EPQ have been proven to be reliable since they have been used by many other researchers. The questionnaire has two formats: a longer A format and a shorter B format. Results from the two correlate strongly. This suggests they are measuring the same thing (validity).

​Research by DeYoung suggests there is a link between PEN and brain processes, such a dopamine being linked to extroversion and testosterone to psychoticism.
Eysenck takes into account brain plasticity. Someone who is highly extrovert doesn't have​ to go to night clubs and get involved in cage fighting. They can discipline themselves to get stimulation from books and classical music, but it's just harder for them. They get impatient and distracted more easily.

​Someone who is introverted could accustom themselves to stimulating activities (like going to rock concerts).

​This is why Eysenck argues personality is 75% biology and 25% environment.
Wolverine: craves stimulation
​Beast: not so much
Objections

​A common objection to Eysenck's theory is that it is based on research using questionnaires. The EPI and the EPQ use simple yes/no questions which offer a "forced choice" between an answer which is high in the trait or an answer which is low.

​However, more recent versions of the questionnaire offer Likert-style questions where participants tick a box saying whether they "never" or "frequently" feel a certain way or something in between.

The aspect of the PEN Model that attracts the most criticism is psychoticism (P), which is accused of being too vague. Eysenck seems to be lumping very different types of people together as high-P personalities: creative artists, live-for-the-moment romantics and cold-hearted thugs.

If Eysenck's idea of P is vague, his theory becomes non-falsifiable (just how could you prove there isn't​ a link between P and criminality if P is so vaguely defined it can mean anything). Theories like this lack scientific status, according to Karl Popper.

​Teresa Amabile (1993) argues that creativity is really a different personality trait from just having poor impulse-control. Many artists are very self-disciplined.
True. I don't usually think of poets as spoiling for a fight!

OK, maybe Byron...
Most psychologists follow the Big Five traits instead of using Eysenck's P trait. Low scores in Openness, Conscientiousness and Agreeableness cover the behaviours Eysenck lumped together as "psychotic" while making them much clearer and easier to measure.
A deeper criticism of personality theory is that there are in fact no consistent traits that define us. Walter Mischel (1968) argues that the apparent consistency in people’s behaviour is an illusion caused by the fact that we usually observe people in similar situations. This is a situationalist view - that situations (environments) produce out behaviour, not personality traits.
Differences
The Exam Board expects you to compare the biological explanation of personality with the Freudian explanation of personality. Freud's ideas are explained on another page. For now, I'll compare and contrast the biological explanation with the situationalist explanation that "personality" is learned.
The biological explanation of personality supports the nativist (nature) view of human behaviour, but the Learning Approach takes the situationalist (nurture) view that all our behaviour comes from our environment.

There are many studies supporting the idea that personality is a learned behaviour. Bandura's "Bobo Doll" studies show that children imitate the behaviours they see in role models. Classical and Operant Conditioning both offer explanations for behaviour on which personality is based.
  • Classical Conditioning shows that aggression is an unconditioned response to some stimuli but may become a conditioned response to a neutral stimuli. This ties in with the Biological Approach because of brain plasticity; because of conditioning, your brain alters in the way it responds to things.
  • Operant Conditioning shows that personality may come about through reinforcement, either positive reinforcement (where behaving in an extroverted way brings you admiration and respect) or negative reinforcement (introverted behaviour gets you away from unpleasant situations and psychotic behaviour makes people keep their distance).

Strict Behaviourists like B.F. Skinner would argue that ALL behaviour is learned and that human beings are born as tabula rasa (a bank slate), with individual differences like personality being added later.

Biological determinists would argue that ALL behaviour is hereditary and that human beings are born with a genetic destiny they cannot help but obey.

Most psychologists take a middle way between these extremes. Genetics gives us predispositions to behave a certain way, but we can resist these impulses if we try. This seems to be Eysenck's position, but he thinks biology is more influential than learning (75% as opposed to 25%).
Applications

If the nativist viewpoint is correct, then personality is innate and difficult to change. However, the PEN Model recognises that people have some​ capacity to change, so the biggest application of this theory has been in psychotherapy (counseling), in particular Cognitive Behavioural Therapy.
You normally think of the nativist perspective and the Biological Approach as opposed to counselling, recommending drugs or surgery instead. This just goes to show the crossover going on in neurocognitivism these days.
The PEN model explains why people might have predispositions to feel a certain way or react in a certain way. It also offers reliable questionnaires like the EPQ to measure these traits. If people in therapy know what their traits are, they can work to counteract them. This is because psychoticism doesn't force​ you to be an inconsiderate jerk and neuroticism doesn't force you to be flaky and passive-aggressive. If you know you've go the tendency, you can work at changing it.
CBT for criminals often focuses on encouraging them to feel empathy and think about the impact of their behaviour on other people. If that turns into a mental habit, their P score will drop and they'll become less unpleasant (and perhaps less likely to reoffend).
Because the EPQ can measure the P, E and N traits, it can be used to test to see if therapy is working.

Of course, it's not too difficult to figure out the EPQ and give the answers you think your therapist is looking for. This is called ​demand characteristics​ and it's a particular problem with applying personality questionnaires.

Eysenck was aware of the danger of demand characteristics. He added "lie questions" to the EPQ-R which didn't add to the score on any of the three traits but would "catch out" anyone trying to give ideal answers. Out of the 100 questions in the large-scale (A) version of the EPQ-R, 21 were "lie questions" and the higher the "lie score", the less reliable a respondents results would be.

EXEMPLAR ESSAY
How to write a 8-mark answer

Evaluate one biological explanation of individual differences. (8 marks)
  • An 8-mark “evaluate” question awards 4 marks for describing individual differences from the biological perspective (AO1) and 4 marks for evaluation (AO2). You need a conclusion to get a mark in the top band (7-8 marks).
  • ​Note: the question doesn't ask about "personality" because the Specification only states "individual differences". It's up to YOU to remember that this means personality.

Description
One example of individual differences is personality which is explained by Eysenck's PEN Model.
​Extroversion is a trait that measures how much stimulation your brain needs. Extrovert people need stimulation but introverts are easily over-stimulated and prefer quiet.
​Neuroticism measures how emotionally unstable you are. Neurotic people have mood swings and don't learn easily but stable personalities have more self-control.
​Psychoticism was the last trait Eysenck added in 1975. It measures impulsivity and aggression but also creativeness. People low in psychoticism may be empathic and inclined to follow rules.

Evaluation
Eysenck found a link between high N-scores and battle-fatigued soldiers and high P-scores and prisoners.
​DeYoung has found links between E and dopamine, N and serotonin and P and the MAO enzyme. There's also a link between N and the right hemisphere.
​However, these scores depend on questionnaires which may be unreliable. This is because respondents may suffer from demand characteristics and give the answers they think the researchers are looking for.
P has been criticised by Amabile for being too vague. Most psychologists think artistic creativity is different from psychoticism.

Conclusion
An interactionist approach includes both cognitions and biology. This is why the PEN Model is a bit out of date and the "Big Five" are preferred instead, taking out P and adding Agreeableness, Conscientiousness and Openness.
  • Notice that for a 8-mark answer you don’t have to include everything about personality and the brain. I haven’t mentioned the other parts of the limbic system, testosterone or the case study of Phineas Gage. But it is a balanced answer - one third description, one third application and one third evaluation.