Short answers, pulled from the story.
Personality describes the behavioral, cognitive, and emotional patterns that make up a person's unique adjustment to life. It is relatively stable but can change over time through experiences and developmental processes. Most theories focus on traits, motivation, skills, and identity.
The Big Five are openness to experience, conscientiousness, extraversion, agreeableness, and neuroticism, often memorized as "OCEAN". It is the approach with the most support in the field. About half of the variance in these factors appears attributable to a person's genetics rather than environment.
Personality can be measured through objective tests and projective measures, including the Big Five Inventory, the Minnesota Multiphasic Personality Inventory, the Rorschach Inkblot test, the Neurotic Personality Questionnaire KON-2006, and Eysenck's Personality Questionnaire. Sound tests rely on reliability and validity, the two factors that make a test accurate. The 16PF measures personality using Cattell's 16-factor theory.
Two families of explanation address this difference: instrumental theories, which say extraverts choose more positive situations, and temperamental theories, which say extraverts are disposed toward higher positive affect. Lucas and Baird found no statistically significant support for the instrumental theory but did find that extraverts experience a higher level of positive affect. Mood maintenance, stronger in extroverts, also lets their positive moods last longer.
The biological basis of personality is the theory that anatomical structures such as genes, hormones, and brain areas underlie individual differences. The frontal lobes handle foresight and anticipation while the occipital lobes process visual information, and testosterone is important for sociability, affectivity, aggressiveness, and sexuality. Studies show the expression of a personality trait depends on the volume of the brain cortex associated with it.
Cultural norms, beliefs, and practices shape how people interact and behave, which influences personality development. Western cultures value individualism, independence, and assertiveness, reflected in extraversion, while Eastern cultures value collectivism, cooperation, and social harmony, reflected in agreeableness. Despite these differences, the Big Five have shown clear cross-cultural applicability across many languages and nations.
William James, who lived from 1842 to 1910, argued that the temperament of philosophers influences their philosophy even when they seek only impersonal reasons. In his 1907 Pragmatism lectures he described rationalists as "tender-minded" and going by principles, and empiricists as "tough-minded" and going by facts. He treated temperament as tantamount to a bias.