Biological basis of personality

The biological basis of personality is the collection of brain systems and mechanisms that underlie human personality. Human neurobiology, especially as it relates to complex traits and behaviors, is not well understood, but research into the neuroanatomical and functional underpinnings of personality are an active field of research. Animal models of behavior, molecular biology, and brain imaging techniques have provided some insight into human personality, especially trait theories.

Much of the current understanding of personality from a neurobiological perspective places an emphasis on the biochemistry of the behavioral systems of reward, motivation, and punishment. This has led to a few biologically based personality theories such as Eysenck's three factor model of personality, Grey's reinforcement sensitivity theory (RST), and Cloninger's model of personality. The Big Five model of personality is not biologically based; yet some research in the differences in brain structures provided biological support also for this model.

Defining personality in a biological context

Personality can be defined as a set of characteristics or traits that drive individual differences in human behavior. From a biological perspective, these traits can be traced back to brain structures and neural mechanisms. However, this definition and theory of biological basis is not universally accepted. There are many conflicting theories of personality in the fields of psychology, psychiatry, philosophy, and neuroscience. A few examples of this are the nature vs. nurture debate and how the idea of a 'soul' fits into biological theories of personality.^[1]

History of biology-based personality research

Hans Eysenck

Since the time of the ancient Greeks, humankind has attempted to explain personality through spiritual beliefs, philosophy, and psychology. Historically, studies of personality have traditionally come from the social sciences and humanities, but in the past two decades neuroscience has begun to be more influential in the understanding of human personality.^[2]

However, the most cited and influential figures in publishing the first biology-based personality theories are Hans Eysenck and Jeffrey Alan Gray. Eysenck used both behavioral and psychophysiological methodologies to test and develop his theories.^[3] He published a book in 1947 called Dimensions of Personality, describing the personality dimensions of Extraversion and Neuroticism. Gray, a student of Eysenck, studied personality traits as individual differences in sensitivity to rewarding and punishing stimuli.^[3] The significance of Gray's work and theories was his use of biology to define behavior, which stimulated a lot of subsequent research.^[4]

The idea of biology-based personality research is relatively new, but growing in interest and number of publications.^[5] In August 2004, there was a conference specifically on the topic, called the The Biological Basis of Personality and Individual Differences.^[6] This allowed for presenting and sharing of ideas between psychologists, psychiatrists, molecular geneticists, and neuroscientists, and eventually gave birth to the book under the same title.^[6] The book is a collection of current research (as of 2006) in the field contributed by many authors and edited by Turhan Canli. Recently, psychology professor Colin G. DeYoung has even named the idea as the field of 'Personality Neuroscience'.

Experimental techniques

There are many experimental techniques for measuring the biology of the brain, but there are five main methods used to investigate the biological basis of personality.^[7] The biological data from these methods are commonly correlated with personality traits. These personality traits are often determined by personality questionnaires. However, personality questionnaires may be biased because they are self-reported. As a result, scientists emphasize using several different measures of personality,^[7][8] rather than solely self-reported measures of personality. For example, another measure of personality traits is observation of behavior. Both humans and animals have been observed to measure personality traits, but animals are particularly useful for studying the long-term behavioral-biological relationship of personality.^[9]

Another interesting method that has become more sophisticated and affordable to researchers is the method of whole genome expression analysis. This method involves collecting data for a large number of genes simultaneously which provides many advantages in studying personality. In an article written by Alison M. Bell and Nadia Aubin-Horth, they describe the advantages very clearly by stating, "For one, it is probable that the genetic basis of personality is polygenic, so it makes sense to simultaneously study many genes. In addition, gene products rarely act alone. Instead, they perform their function by interacting together in pathways and networks. As a result, the molecular changes that characterize a phenotype are frequently not based on a single marker or gene, but rather on an entire pathway. Whole genome expression profiling therefore has the potential to reveal new candidates genes and pathways."^[10]

Genetic and molecular correlations to personality

Neurotransmitters

Dopamine and Serotonin pathways

The biology-based personality theories (discussed below) are based on correlating personality traits with behavioral systems related to motivation, reward, and punishment. On a broad level, this involves the autonomic nervous system, fear-processing circuits in the amygdala, the reward pathway from the ventral tegmental area (VTA) to the nucleus accumbens and prefrontal cortex. All of these circuits heavily rely on neurotransmitters and their precursors, but there has been the most research support for dopamine and serotonin pathways:

• Dopamine: Dopamine is a monoamine neurotransmitter that has been found to promote exploratory behavior.^[11] Dopaminergic pathways have been specifically correlated with the extraversion trait of the Five Factor Model of Personality.^[7] The monoamine oxidase (MAO) enzyme has a preferential affinity for dopamine, and its levels are inversely correlated with sensation seeking.^[8]
• Serotonin: Serotonin is a monoamine neurotransmitter, and has been found to promote avoidance behavior through inhibitory pathways.^[11] Specifically, serotonin has been associated with Neuroticism, Agreeableness, and Conscientiousness (traits defined by the Five Factor Model of Personality).^[7]

Genes

Previous studies show that genes account for at most 50 percent of a given trait.^[1] However, it is widely accepted that variance in gene sequence affect behavior, and genes are a significant risk factor for personality disorders.^[12] With the growing interest in using molecular genetics in tracing the biological basis of personality,^[6] there may be more gene-trait links found in the future.

Varying polymorphisms and sequence repeats in the gene for dopamine receptor D4 and serotonin transporter gene 5-HTTLPR, have both been found to influence the extraversion trait in adults. Specifically, study participants with at least one copy of the 7-repeat variant of the dopamine receptor D4 gene had higher scores of self-reported extraversion.^[6] This suggests that dopamine and serotonin interact to regulate the conflicting behavioral traits of careless exploration vs. cautious inhibition.^[11]

Synaptic plasticity

Synaptic plasticity refers to the ability of neurons to strengthen or weaken the connections between them. According to Hebbian theory, these connections are strengthened and maintained through repeated stimulation between neurons. Specifically, there is an emphasis on long-term potentiation (LTP), which is the prolonged strengthening of synaptic connections that facilitate learning from experience.

On a larger scale, there are many pathways and brain regions that are interdependent and contribute to a cohesive, stable personality. For example, the amygdala and hippocampus of the limbic system mediate emotional intensity and consolidate memory of these experiences. But the basic mechanism by which these pathways and brain regions perform these functions, is synaptic plasticity. Ultimately, it boils down to this feature of neurons that allows the brain to learn from repeated experiences, retain memories, and ultimately maintain personality.^[13] Joseph LeDoux, an award-winning neuroscientist, asserts that although humans share the same brain systems, it is the unique wiring of neurons that is different in each person and makes their personality.^[13]

Personality theories with biological basis

There are many theories of personality that on the identification of a set of traits that encompass human personality. Few however, are biologically based. This section will describe some theories of personality that have a biological basis. Additionally, it will present biological support for a popular non-biologically based personality theory, the Five Factor Model.

Eysenck's three-factor model of personality

Eysenck's three-factor model of personality was a causal theory of personality based on activation of reticular formation and limbic system. The reticular formation is a region in the brainstem that is involved in mediating arousal and consciousness. The limbic system is involved in mediating emotion, behavior, motivation, and long-term memory.

1. Extraversion (E) – degree to which people are outgoing and are interactive with people, which is mediated by the activation of the reticular formation.
2. Neuroticism (N) – degree of emotional instability, which is associated with the limbic system.
3. Psychoticism (P) – degree of aggression and interpersonal hostility.

Gray's reinforcement sensitivity theory

Gray's reinforcement sensitivity theory (RST) is based on the idea that there are three brain systems that all differently respond to rewarding and punishing stimuli.^[3]

1. Fight-flight-freeze system (FFFS) – mediates the emotion of fear (not anxiety) and active avoidance of dangerous situations. The personality traits associated with this system is fear-proneness and avoidance.
2. Behavioral inhibition system (BIS) – mediates the emotion of anxiety and cautious risk-assessment behavior when entering dangerous situations due to conflicting goals. The personality traits associated with this system is worry-proneness and anxiety.
3. Behavioral approach system (BAS) – mediates the emotion of 'anticipatory pleasure,' resulting from reactions to desirable stimuli. The personality traits associated with this system are optimism, reward-orientation, and impulsivity.

Cloninger's biological dimensions of personality

Cloninger model of personality

This model of personality is based on the idea that different responses to punishing, rewarding, and novel stimuli is caused by an interaction of the three dimensions below:

1. Novelty Seeking (NS) – degree to which people are impulsive, correlated with low dopamine activity.
2. Harm Avoidance (HA) – degree to which people are anxious, correlated with high serotonin activity.
3. Reward Dependence (RD) – degree to which people are approval seeking, correlated with low norepinephrine activity.

In one MRI study,^[14] Novelty Seeking correlated with increased grey matter volume in regions of the cingulate cortex, Harm Avoidance correlated with decreased grey matter volume in the orbitofrontal, occipital, and parietal cortex. Reward Dependence correlated with decreased grey matter volume in the caudate nucleus.

Five factor model of personality

The five factor model is widely used personality assessment that describes five core traits that a person possesses:

1. Openness – degree to which people enjoy experiencing new stimuli
2. Conscientiousness – degree to which people are dutiful and goal-oriented
3. Extraversion – degree to which people seek stimuli outside of themselves
4. Agreeableness – degree to which people aim to cooperate and please others
5. Neuroticism – degree to which people are emotionally unstable

Using an MRI, one study^[15] found correlation between the volumes of certain brain areas with each of the five traits in the Five Factor Model. Their results found that Openness/Intellect did not have any significant correlation with the volume of any brain structures. Conscientiousness was associated with increased volume in the lateral prefrontal cortex, a region involved in planning and the voluntary control of behavior. Extraversion was associated with increased volume of medial orbitofrontal cortex, a region involved in processing reward information. Agreeableness was associated with increased volume in regions that process information about the intentions and mental states of other individuals. Neuroticism was associated with increased volume of brain regions associated with threat, punishment, and negative emotions.

See also

• Hypostatic model of personality
• Neural basis of self
• Personality Psychology
• Psychophysiology

References

1. 1 2 LeDoux, J. (2003). "The Self". Annals of the New York Academy of Sciences. 1001 (1): 295–304. doi:10.1196/annals.1279.017. PMID 14625368. 
2. ↑ Davidson, R. J. (2001). "Toward a biology of personality and emotion" (PDF). Ann N Y Acad Sci. 935: 191–207. doi:10.1111/j.1749-6632.2001.tb03481.x. 
3. 1 2 3 4 Corr, Philip J.; Perkins, Adam M. (2006). "The role of theory in the psychophysiology of personality: From Ivan Pavlov to Jeffrey Gray". International Journal of Psychophysiology. 62 (3): 367–376. doi:10.1016/j.ijpsycho.2006.01.005. ISSN 0167-8760. 
4. ↑ Fowles, Don (2006). "Chapter 2: Jeffrey Gray's Contributions to Theories of Anxiety, Personality, and Psychopathology". In Canli, Turhan. Biology of personality and individual differences. Guilford Press. ISBN 1593852525. .
5. 1 2 Canli, Turhan (2006). "Chapter 5: Genomic Imaging of Extraversion". In Canli, Turhan. Biology of personality and individual differences. Guilford Press. ISBN 1593852525. .
6. 1 2 3 4 Canli, Turhan (2006). "Chapter 1: Introduction". In Canli, Turhan. Biology of personality and individual differences. Guilford Press. ISBN 1593852525. .
7. 1 2 3 4 5 6 7 DeYoung, Colin G. (2010). "Personality Neuroscience and the Biology of Traits". Social and Personality Psychology Compass. 4 (12): 1165–1180. doi:10.1111/j.1751-9004.2010.00327.x. ISSN 1751-9004. 
8. 1 2 Zuckerman, Marvin (2006). "Chapter 3: Biosocial Bases of Sensation Seeking". In Canli, Turhan. Biology of personality and individual differences. Guilford Press. ISBN 1593852525. .
9. ↑ Mehta, Pranjal; Gosling, Samuel (2006). "Chapter 20: How Can Animal Studies Contribute to Research on the Biological Bases of Personality". In Canli, Turhan. Biology of personality and individual differences. Guilford Press. ISBN 1593852525. .
10. ↑ Bell, A.M., & Aubin-Horth, N. (2010). What can whole genome expression data tell us about the ecology and evolution of personality?. Philosophical Transactions of the Royal Society B: Biological Sciences. Retrieved September 10, 2014, from http://rstb.royalsocietypublishing.org/content/365/1560/4001.full.pdf+html.
11. 1 2 3 Ebstein, Richard P.; Auerbach, Judith G. (2002). "Dopamine D4 receptor and serotonin transporter promoter polymorphisms and temperament in early childhood". Molecular Genetics and the Human Personality: 137–149. 
12. ↑ Whittle, S., Allen, N. B., Lubman, D. I., & Yucel, M. (2006). Neurobiological basis of temperament: Towards a better understanding of psychopathology. Neuroscience and Biobehavioral Reviews, 30(4), 511-525.
13. 1 2 LeDoux, J. E. (2003). Synaptic Self: How Our Brains Become Who We Are: Penguin Books.
14. ↑ Gardini, Simona; Cloninger, C. Robert; Venneri, Annalena (2009). "Individual differences in personality traits reflect structural variance in specific brain regions". Brain Research Bulletin. 79 (5): 265–270. doi:10.1016/j.brainresbull.2009.03.005. ISSN 0361-9230. PMID 19480986. 
15. ↑ DeYoung, C. G.; Hirsh, J. B.; Shane, M. S.; Papademetris, X.; Rajeevan, N.; Gray, J. R. (2010). "Testing Predictions From Personality Neuroscience: Brain Structure and the Big Five". Psychological Science. 21 (6): 820–828. doi:10.1177/0956797610370159. PMC 3049165. PMID 20435951.