Amygdala  -  A small structure in the medial temporal lobe of the brain whose activity has been associated with emotions, particularly fear.

Atheoretic  -  Having no basis in theory; empirical.

Big Five  -  As culled from a wealth of experiments and observations, the five major accepted factors of Trait Theory: extroversion, neuroticism, agreeableness, conscientiousness, and openness to experience.

Categorical Approach  -  Approach to studying personality that posits qualitatively different types of people, instead of continuous trait dimensions along which people can vary. Jerome Kagan suggests that a categorical approach may be most helpful in understanding psychology.

Dimensional Approach  -  Approach to studying personality that posits continuous trait dimensions along which people can vary, instead of qualitatively different types. Cattel and Eysenk follow a dimensional approach to psychology.

Empirical  -  Based on observed facts.

Extroversion-Introversion  -  A trait dimension that plays an important role in Eysenck's two-factor theory and in the Big Five. Refers to the tendency to seek stimulation.

Factor Analysis  -  Statistical technique used to reduce large amounts of data (e.g. answers to personality questionnaires given to large numbers of people) into groups of items, or factors, that correlate highly with each other but not with other items.

Heritability  -  The extent to which variance in a trait is attributable to genes.

Neuroticism-Stability  -  An important trait dimension in Eysenck's two- factor theory and in the Big Five. Refers to tendency to become emotionally upset.

Temperament  -  Genetically influenced behavioral type; similar to a trait, but more suited to a categorical approach and more explicitly biological.

Trait Theory  -  The branch of personality psychology that attempts to identify a small number of stable traits that can be used to distinguish between different people.