While the Big Five model is robust, it doesn't replicate in all languages when derived from local dictionaries. The most cross-culturally stable structure is a two-factor model consisting of "Dynamism" (extroversion, competence) and "Social Propriety" (dependability, reliability).

The Hierarchical Taxonomy of Psychopathology (HITOP) model reveals that symptoms of mental health problems cluster into five major dimensions that closely correspond to the Big Five personality traits. This suggests mental illness can be understood as an extreme expression of normal personality variation.

At the Big Five level, gender differences in personality appear small. However, breaking down the traits into sub-facets reveals more pervasive differences. For example, within Extraversion, men score higher on Assertiveness while women score higher on Enthusiasm, effects that cancel each other out at the broader level.

Psychology is moving away from a firm distinction between personality and mental health. A persistent mental health issue, by definition, is a stable pattern of experience and behavior, which fits the scientific definition of a personality trait. The two concepts are fundamentally intertwined.

Personality stability isn't just one concept. 'Rank order' stability measures if the most extraverted person in a group remains the most extraverted over time. 'Mean level' stability tracks how the average trait level for an entire population changes with age, such as dips in conscientiousness during adolescence.

Most people pick their identities from pre-canned societal boxes like "hipster" or "redneck." High-agency individuals, however, build their lives deliberately by questioning every choice. This creates a unique combination of traits that makes them interesting and authentic.

fMRI research revealed that averaging multiple brain scans creates a composite image that represents no single individual's brain activity. This fallacy of averages extends across society, from education to medicine, proving that systems designed for the 'average' fail to serve the individual.

While major life events can alter personality, most do so unpredictably. Across large populations, only two events have been found to reliably predict personality shifts: getting a first job and entering a first serious romantic relationship. Both tend to increase conscientiousness and agreeableness.

Based on levels of positive and negative emotion, individuals fit into four quadrants. Mad Scientists (high/high), Cheerleaders (high positive/low negative), Judges (low/low), and Poets (low positive/high negative). Each has unique strengths and weaknesses.