Personality is a term that describes traits a person shows consistently. These traits are shown at different times and in different situations.  Understanding someone’s personality may allow you to predict their behavior in many situations.  Predictability makes it possible to explain and understand behavior.  A person's personality can often suggest their internal thoughts, feelings, and behaviors.  Intrapersonal functioning is a term used to describe the stable processes that underlie these thoughts, feelings, and behaviors. Gordon Allport defined personality as a "dynamic organization inside a person, of psychophysical systems that create the person's characteristic patterns of behavior, thoughts and feelings."

Hippocrates' ideas

Many theorists like Hippocrates had their own opinions on what affected the personality.  Hippocrates believed that the body contains four “humors” (body fluids): yellow bile, black bile, blood, and phlegm.  When one of the fluids was too much or too little, it affected the personality.  Hippocrates associated each of the humors with different elements, and temperaments:
 Blood was associated with air and resulted in sanguine, or hopefulness.
 Black bile, associated with earth, resulted in a temperament that was melancholic and resulted in a person feeling sad.
 Yellow bile was associated with fire and resulted in what he called a choleric temperament, resulting in irritability and aggression.
 Phlegm, associated with water, resulted in a phlegmatic temperament, associated with being apathetic.  
If any of the four humors were at irregular levels, Hippocrates believed the person would display the temperaments or show signs of sickness.

Different perspectives of personality research
When studying personality, one must consider all the different perspectives in the approach to understand how personality is created.  Some of these perspectives include things like trait theory, the influence of motives, evolutionary theories, and the social learning approach.  Each of these theories try and explain how the personality is created and what influences its development.

Trait theory 
Trait theory suggests that there are both traits and types within people that create the personality.  Types are categories that have measurable differences. Traits are stable qualities in people that have measurable differences.  Examples of traits are things like fairness, intelligence, confidence, and helpfulness. Individual differences are shown in the amount that a trait is seen.  Within trait theory, there are many different approaches to how traits operate and to what extent.  A Nomothetic (shared) view of traits suggests that traits apply to everyone and it is possible to compare traits among individuals.  It also states that individuals show a unique combination of traits. This means that while everyone has the same basic traits, not all individuals have the same levels of each trait.  An Ideographic (individual) view of traits suggests that traits are unique to people.  This view explains that comparisons are not possible among individuals because not all traits are shared. It says that traits may be different in importance for different groups of people.

Motives 
Motives are another important thing to consider when studying the personality.  Motives are forces that are influenced by something that someone needs. When a need is not met, an individual will have a motive to fulfill that need.  For example, every human being has a need for water.  The motive would be thirst.  The need for water influenced the motive to no longer be thirsty.  A press is an external event that acts as a trigger for motives.  Using the same example as before, a thirsty person may want to drink water by seeing a cold cup of water, which serves as a press.

An incentive (something that gives motivation to do something) value is another important concept when looking at theories on motives.  An incentive value is the degree to which a given behavior can satisfy a need.  This accounts for the differences in individuals.  What may be more satisfying to one person may not be the same thing to another person.  Each individual has a different incentive value.

Evolutionary theories 
Another theory about personality is the inheritance or evolutionary theories.  These theories suggest that personality is genetically-based.  Evolutionists believe that the personality has adapted over time to ensure survival and reproduction.

Social learning theory 
Social learning theory suggests that the personality is created by our experiences with relationships, our environment, and the social world in general.

Personality types

Through experimentation and research, psychologists have been able to identify five major personality traits.  These are known as the 'Big Five':

Neuroticism 
Neuroticism refers to how much negative emotion a person experiences.  Someone high in neuroticism has low emotional control and many changes in emotions.  This may cause them to exhibit signs of nervousness, anxiety, and excitability.  A neurotic person may also display hostility, depression, self-consciousness, impulsiveness, and vulnerability to stress

Extroversion 
Extroversion is also known as sociability.  This means their social adaptability, assertiveness, and energy level.  A person high in extroversion would be extremely warm, positive, and prone to excitement-seeking. A person low in extroversion would be quiet, and reserved. They are known as introverts.

Openness 
Openness is also known as the intellect factor. Openness is associated with culture and being open to ideas and experiences.  A person high in openness would be labeled creative, knowledgeable, and imaginative. A person low in openness would be traditional and close-minded.

Agreeableness 
Agreeableness is associated with conformity, friendliness, and likeability.  A person high in agreeableness would be considered friendly, kind, considerate, and good-natured. A person low in agreeableness would be argumentative, and cynical.

Conscientiousness 
Conscientiousness is also known as responsibility and having a strong will to achieve.  A person high in conscientiousness would be considered cautious, planful, serious, and hardworking. A person low in conscientiousness would be lazy, disorganized, and irresponsible.

Personality systems 
Sometimes personalities are categorized by having different types of them. These types are determined by traits, behaviors, motivations, or ways of thinking. Such personality systems include Myers-Briggs Type Indicator, and The Enneagram of Personality. They are often considered pseudoscience because of low reliability.

Personality disorders

While studying the healthy components of the personality, psychologists began to expand knowledge on the strange aspects of the personality.  In the Diagnostic and Statistical Manual of Mental Disorders (DSM-5), ten personality disorders were organized into three clusters: Cluster A, Cluster B, and Cluster C.  Cluster A consists of the odd and eccentric disorders.  These are paranoid, schizoid, and schizotypal.  Cluster B are the dramatic, emotional, and erratic personality disorders.  These are antisocial, borderline, histrionic and narcissistic.  Cluster C is characterized by personality disorders dealing with anxiety or fear.  This cluster are avoidant, dependent, and obsessive-compulsive.

There are many ways to understand why and how personality disorders are created.  One idea says that personality disorders are just extreme traits that are caused by being different. It views personality disorders as changing and ruining a person's life, as well as the lives of others.  The biological idea explains that personality is genetically-determined, so, that our behavior is the product of a complex biological organism.  It states that genetics and biology influence all the processes within a personality.

Specific modern studies

An article from 2011 describes the explanation of personality psychology that is mixed with the Big 5 model. The article begins discussing how personality psychology has many factors mixed within. It discusses cognition within the brain and also discusses personality traits and links between the two topics. Personality psychology is further explained through conceptual and logical aspects. Then the article continues to explain the logical requirements for coherency within the personality.

In 2006, self-regulatory behavior was analyze and compared with personality science and health behavior. It is understood from this, that an individuals personality is a link to an individuals self regulated behavior as long as an individuals health behavior. All of these are related to one another and contribute to personality.  A persons  health, and their behavior are a result of their personality science.

A survey was conducted in 2007 in order to better comprehend the reasons why students choose their major, and specifically looked within the major of psychology. The students were curious as to what it was that was influencing students to make the decision to have psychology as their major. The survey created was based of the Big Five Personality Inventory. The results found from the surveys that people who choose psychology as their major were not picking this field for a large salary or anything to have to do with the money. The results continued to express that psychology majors were dedicated to the major because of their personal past experiences. This leads to show how the Big Five contributes to major choices of individuals.

Adaptive personality was compared with the model of emotional intelligence by Meyer and Salovey. A few of the relationships expected did arise within the correlations resulting. The article emphasized the significance of emotional knowledge within emotional intelligence. This provides further information when it comes to counseling, and just how much emotional knowledge can be a bigger impact than previously expected within personality psychology.

Most traditional models of personality traits emphasize biology of the person to be very relevant to their personality traits, and even emphasize that this aspect of the person is unchangeable. But, within this article from 2008, the idea of sociogenomic biology is brought up for thought. This is introduced within the article and contradicts everything about biology and that biology is in fact changeable in certain circumstances. The article stresses that DNA can be changed through the environment in which one lives. This new thinking of biology is claimed to be the new modern way of looking at personality traits.

In 1977 there was a study done about sleep positions relating to personality traits. There were main points within the study that strongly urged that sleep positions reflected a person's personality. In contrast, a personality study conducted in the year 2012, suggested that the position a person sleeps in is related to specific traits of their personality. Their study consisted of 332 participants that were psychology majors. The results found did not match what was previously concluded. There were no strong points leading to prove that there was such a relation, and in fact, the results were found to be quite weak between the association.

References

Human behavior
Branches of psychology