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The biological basis of personality is the theory that personality is influenced by the biology of the brain. Though closely related to personality psychology, the biological basis of personality focuses on why or how personality traits manifest through biology, in addition to identifying personality traits. This is investigated by correlating personality traits with scientific data from experimental methods such as brain imaging and molecular genetics.

Much of the current understanding of personality from a neurobiological perspective places an emphasis on the biochemistry of the autonomic nervous system. This has led to a few biologically-based personality theories such as Eynsenck's Three Factor Model of Personality, Grey's Reinforcement Sensitivity Theory (RST), and Cloninger's Model of Personality. Some research in the differences in brain structures has also provided biological support for the Five Factor Model of Personality, which is not biologically based.

Defining Personality in 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. The concept of a biological basis of personality is no doubt up for debate, but this article will aim to present the current research and theories on the topic.

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.

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 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'.

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.

Personality Development: 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. This feature allows for neurons to learn from repeated experiences, retain memories, and ultimately maintain personality. 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.

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 the basics of 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 (RST)
Gray's RST in based on the idea that there are three brain systems that all differently respond to rewarding and punishing stimuli.
 * 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 is 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 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 study, Novelty Seeking correlated with increased grey matter volume in regions of the cingulate cortex, Harm Avoidance correlated with decreased grey matter volume in the orbito-frontal, 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 found correlation between the volume 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 (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.

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


 * Serotonin: Serotonin is a monoamine neurotransmitter has been found to promote avoidance behavior through inhibitory pathways. Specifically, serotonin has been associated with Neuroticism, Agreeableness, and Consciousness (traits defined by the Five Factor Model of Personality).


 * Dopamine: Dopamine is another monoamine neurotransmitter, and has been found to promote exploratory behavior. Dopaminergic pathways have been specifically correlated with and Extraversion dimension of the Five Factor Model of Personality. Cloninger asserted that

Varying polymorphisms and sequence repeats in the gene for dopamine receptor D4 and serotonin transporter gene 5-HTTLPR, both, are involved in trait variation of extraversion 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. This suggests that dopamine and serotonin interact to regulate the conflicting behavioral traits of careless exploration vs. cautious inhibition.


 * Monoamine oxidase (MAO) Enzyme: This enzyme is correlated with sensation seeking, and has a preferential affinity for dopamine.