The biological basis of personality is a 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. In the context of the biological body, neuroscience evidence suggests that the brain is modular, meaning that the mental state is biologically structured and that personality is composed of distinct components. 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.
History of biology-based personality research 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.
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. 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. The significance of Gray's work and theories was his use of biology to define behavior, which stimulated a lot of subsequent research..
In 1951, Hans Eysenck and Donald Prell published an experiment in which identical (monozygotic) and fraternal (dizygotic) twins, ages 11 and 12, were tested for neuroticism. It is described in detail in an article published in the Journal of Mental Science in which Eysenck and Prell concluded that "The factor of neuroticism is not a statistical artifact, but constitutes a biological unit which is inherited as a whole ... neurotic predisposition is to a large extent hereditarily determined."The Journal of Mental Health, July 1951, Vol. XCVII, "The Inheritance of Neuroticism: An Experimental Study", H. J. Eysenck and D. B. Prell, p. 402. The study concluded that the neuroticism trait was a result of up to eighty percent of genetics. There was a stronger correlation among identical twins rather than fraternal twins.
The idea of biology-based personality research is relatively new, but growing in interest and number of publications.. In August 2004, there was a conference specifically on the topic, called The Biological Basis of Personality and Individual Differences.. 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. 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." Furthermore, a journal devoted to cultivating research investigating the neurobiological basis of personality has recently been established and is called "Personality Neuroscience."
Personality theories with biological basis There are many theories of personality that centre 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.
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.
Extraversion (E) – degree to which people are outgoing and are interactive with people, which is mediated by the activation of the reticular formation. Neuroticism (N) – degree of emotional instability, which is associated with the limbic system. 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.
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. 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. 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 model of personality
This model of personality is based on the idea that different responses to punishing, rewarding, and novel stimuli the main characteristics of the human mind is caused by an interaction of the three dimensions below:
Novelty Seeking (NS) – degree to which people are impulsive, correlated with low dopamine activity. Harm Avoidance (HA) – degree to which people are anxious, correlated with high serotonin activity. Reward Dependence (RD) – degree to which people are approval seeking, correlated with low norepinephrine activity.
Five factor model of personality The five factor model (also known as the Big Five) is a widely used personality assessment that describes five core traits that a person possesses:
Openness – degree to which people enjoy experiencing new stimuli Conscientiousness – degree to which people are dutiful and goal-oriented Extraversion – degree to which people seek stimuli outside of themselves Agreeableness – degree to which people aim to cooperate and please others Neuroticism – degree to which people are emotionally unstable
There is large body of research relating the Big Five traits to individual differences in the brain's structure and function, as measured by MRI-based techniques. A selection of these findings are outlined in the "Brain imaging basis of personality" section below.
Two factor model of personality A higher-order factor structure can be derived from the Big Five traits, as these traits have often been found to be correlated. Agreeableness, Conscientiousness, and Neuroticism (reversed) can be distilled into a single factor α, or the Stability factor. On the other hand, Extraversion and Openness can be distilled into a single factor β, or the Plasticity factor. These two meta-traits have been shown to be significantly heritable using behavior genetic analysis, which suggests a neurobiological basis that is unique and specific to these meta-traits. Indeed, a growing body of evidence demonstrates that serotonin is associated with Stability and dopamine is associated with Plasticity.
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. 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,. 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..
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."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.