Personality is usually defined as the set of habitual behaviors, cognitions and emotional patterns that evolve from biological and environmental factors. While there is no generally agreed upon definition of personality, most theories focus on motivation and psychological interactions with ones environment.  Trait-based personality theories, such as those defined by Raymond Cattell define personality as the traits that predict a person's behavior. On the other hand, more behaviorally based approaches define personality through learning and habits. Nevertheless, most theories view personality as relatively stable.
The study of the psychology of personality, called personality psychology, attempts to explain the tendencies that underly differences in behavior. Many approaches have been taken to studying personality, including biological, cognitive, learning and trait based theories, as well as psychodynamic, and humanistic approaches. Personality psychology is also divided among the first theorists, with a few influential theories being posited by Sigmund Freud, Alfred Adler, Gordon Allport, Hans Eysenck, Abraham Maslow, and Carl Rogers.
The trait based approach has yielded multiple conceptions of personality, including a number of five factor models, Eysenck's traits, Cattel's traits and Cloninger's temperament and character traits.
Personality can be determined through a variety of tests. However, dimensions of personality and scales of personality tests vary and often are poorly defined. Examples of such tests are the: Big Five Inventory (BFI), Minnesota Multiphasic Personality Inventory (MMPI-2), Rorschach Inkblot test, Neurotic Personality Questionnaire KON-2006, or Eysenck's Personality Questionnaire (EPQ-R).
Personality is often broken into statistically-identified factors called the Big Five, which are openness to experience, conscientiousness, extraversion, agreeableness, and neuroticism (or emotional stability). These components are generally stable over time, and about half of the variance appears to be attributable to a person's genetics rather than the effects of one's environment.
Some research has investigated whether the relationship between happiness and extraversion seen in adults can also be seen in children. The implications of these findings can help identify children that are more likely to experience episodes of depression and develop types of treatment that such children are likely to respond to. In both children and adults, research shows that genetics, as opposed to environmental factors, exert a greater influence on happiness levels. Personality is not stable over the course of a lifetime, but it changes much more quickly during childhood, so personality constructs in children are referred to as temperament. Temperament is regarded as the precursor to personality. Whereas McCrae and Costa's Big Five model assesses personality traits in adults, the EAS (emotionality, activity, and sociability) model is used to assess temperament in children. This model measures levels of emotionality, activity, sociability, and shyness in children. The personality theorists consider temperament EAS model similar to the Big Five model in adults; however, this might be due to a conflation of concepts of personality and temperament as described above. Findings show that high degrees of sociability and low degrees of shyness are equivalent to adult extraversion, and also correlate with higher levels of life satisfaction in children.
Another interesting finding has been the link found between acting extraverted and positive affect. Extraverted behaviors include acting talkative, assertive, adventurous, and outgoing. For the purposes of this study, positive affect is defined as experiences of happy and enjoyable emotions. This study investigated the effects of acting in a way that is counter to a person's dispositional nature. In other words, the study focused on the benefits and drawbacks of introverts (people who are shy, socially inhibited and non-aggressive) acting extraverted, and of extraverts acting introverted. After acting extraverted, introverts' experience of positive affect increased whereas extraverts seemed to experience lower levels of positive affect and suffered from the phenomenon of ego depletion. Ego depletion, or cognitive fatigue, is the use of one's energy to overtly act in a way that is contrary to one's inner disposition. When people act in a contrary fashion, they divert most, if not all, (cognitive) energy toward regulating this foreign style of behavior and attitudes. Because all available energy is being used to maintain this contrary behavior, the result is an inability to use any energy to make important or difficult decisions, plan for the future, control or regulate emotions, or perform effectively on other cognitive tasks.
One question that has been posed is why extraverts tend to be happier than introverts. The two types of explanations attempt to account for this difference are instrumental theories and temperamental theories. The instrumental theory suggests that extraverts end up making choices that place them in more positive situations and they also react more strongly than introverts to positive situations. The temperamental theory suggests that extraverts have a disposition that generally leads them to experience a higher degree of positive affect. In their study of extraversion, Lucas and Baird found no statistically significant support for the instrumental theory but did, however, find that extraverts generally experience a higher level of positive affect.
Research has also been done to uncover some of the mediators that are responsible for the correlation between extraversion and happiness. Self-esteem and self-efficacy are two such mediators. Self-efficacy has been found to be related to the personality traits of extraversion and subjective well-being. Self-efficacy is one's belief about abilities to perform up to personal standards, the ability to produce desired results, and the feeling of having some ability to make important life decisions. However, the relationship between extraversion (and neuroticism) and subjective happiness is only partially mediated by self-efficacy. This implies that there are most likely other factors that mediate the relationship between subjective happiness and personality traits. Another such factor may be self-esteem. Individuals with a greater degree of confidence about themselves and their abilities seem to have both higher degrees of subjective well-being and higher levels of extraversion.
Other research has examined the phenomenon of mood maintenance as another possible mediator. Mood maintenance, the ability to maintain one's average level of happiness in the face of an ambiguous situation (meaning a situation that has the potential to engender either positive or negative emotions in different individuals), has been found to be a stronger force in extraverts. This means that the happiness levels of extraverted individuals are less susceptible to the influence of external events. Another implication of this finding is that extraverts' positive moods last longer than those of introverts.
Modern conceptions of personality, such as the Temperament and Character Inventory have suggested four basic temperaments that are thought to reflect basic and automatic responses to danger and reward that rely on associative learning. The four temperaments, harm avoidance, reward dependence, novelty seeking and persistence are somewhat analogous to ancient conceptions of melancholic, sanguine, choleric, phlegmatic personality types, although the temperaments reflect dimensions rather than distance categories. While factor based approaches to personality have yielded models that account for significant variance, the developmental biological model has been argued to better reflect underlying biological processes. Distinct genetic, neurochemical and neuroanatomical correlates responsible for each temperamental trait have been observed, unlike with five factor models.
The harm avoidance trait has been associated with increased reactivity in insular and amygdala salience networks, as well as reduced 5-HT2 receptor binding peripherally, and reduced GABA concentrations. Novelty seeking has been associated with reduced activity in insular salience networks increased striatal connectivity. Novelty seeking also correlates with dopamine synthesis capacity in the striatum, and reduced auto receptor availability in the midbrain. Reward dependence has been linked with the oxytocin system, with increased concentration of plasma oxytocin being observed, as well as increased volume in oxytocin related regions of the hypothalamus. Persistence has been associated with increased striatal-mPFC connectivity, increased activation of ventral striatal-orbitofrontal-anterior cingulate circuits, as well as increased salivary amylase levels indicative of increased noradrenergic tone.
It has been shown that personality traits are more malleable by environmental influences than researchers originally believed. Personality differences also predict the occurrence of life experiences.
There has been some recent debate over the subject of studying personality in a different culture. Some people think that personality comes entirely from culture and therefore there can be no meaningful study in cross-culture study. On the other hand, others believe that some elements are shared by all cultures and an effort is being made to demonstrate the cross-cultural applicability of "the Big Five".
Cross-cultural assessment depends on the universality of personality traits, which is whether there are common traits among humans regardless of culture or other factors. If there is a common foundation of personality, then it can be studied on the basis of human traits rather than within certain cultures. This can be measured by comparing whether assessment tools are measuring similar constructs across countries or cultures. Two approaches to researching personality are looking at emic and etic traits. Emic traits are constructs unique to each culture, which are determined by local customs, thoughts, beliefs, and characteristics. Etic traits are considered universal constructs, which establish traits that are evident across cultures that represent a biological bases of human personality. If personality traits are unique to individual culture, then different traits should be apparent in different cultures. However, the idea that personality traits are universal across cultures is supported by establishing the Five Factor Model of personality across multiple translations of the NEO-PI-R, which is one of the most widely used personality measures. When administering the NEO-PI-R to 7,134 people across six languages, the results show a similar pattern of the same five underlying constructs that are found in the American factor structure.
Similar results were found using the Big Five Inventory (BFI), as it was administered in 56 nations across 28 languages. The five factors continued to be supported both conceptually and statistically across major regions of the world, suggesting that these underlying factors are common across cultures. There are some differences across culture but they may be a consequence of using a lexical approach to study personality structures, as language has limitations in translation and different cultures have unique words to describe emotion or situations. For example, the term "feeling blue" is used to describe sadness in more Westernized cultures, but does not translate to other languages. Differences across cultures could be due to real cultural differences, but they could also be consequences of poor translations, biased sampling, or differences in response styles across cultures. Examining personality questionnaires developed within a culture can also be useful evidence for the universality of traits across cultures, as the same underlying factors can still be found. Results from several European and Asian studies have found overlapping dimensions with the Five Factor Model as well as additional culture-unique dimensions. Finding similar factors across cultures provides support for the universality of personality trait structure, but more research is necessary to gain stronger support.
The modern sense of individual personality is a result of the shifts in culture originating in the Renaissance, an essential element in modernity. In contrast, the Medieval European's sense of self was linked to a network of social roles: "the household, the kinship network, the guild, the corporation - these were the building blocks of personhood", Stephen Greenblatt observes, in recounting the recovery (1417) and career of Lucretius' poem De rerum natura: "at the core of the poem lay key principles of a modern understanding of the world." "Dependant on the family, the individual alone was nothing," Jacques Gélis observes.
The biological basis of personality is the theory that anatomical structures located in the brain contribute to personality traits. This stems from neuropsychology, which studies how the structure of the brain relates to various psychological processes and behaviors. For instance, in human beings, the frontal lobes are responsible for foresight and anticipation, and the occipital lobes are responsible for processing visual information. In addition, certain physiological functions such as hormone secretion also affect personality. For example, the hormone testosterone is important for sociability, affectivity, aggressiveness, and sexuality. Additionally, studies show that the expression of a personality trait depends on the volume of the brain cortex it is associated with.
There is also a confusion among some psychologists who conflate personality with temperament. Temperament traits that are based on weak neurochemical imbalances within neurotransmitter systems are much more stable, consistent in behavior and show up in early childhood; they can't be changed easily but can be compensated for in behavior. In contrast to that, personality traits and features are the product of the socio-cultural development of humans and can be learned and/or changed.
Personology confers a multidimensional, complex, and comprehensive approach to personality. From a holistic perspective, personology studies personality as a whole, as a system, but in the same time through all its components, levels and spheres.