Personality is 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 one's 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.

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