In psychology and psychometrics, the big five personality trait model or five-factor model (FFM)—sometimes called by the acronym OCEAN or CANOE—is the most common scientific model for measuring and describing human personality traits. The framework groups variation in personality into five separate factors, all measured on a continuous scale:

The five-factor model was developed using empirical research into the language people used to describe themselves, which found patterns and relationships between the words people use to describe themselves. For example, because someone described as "hard-working" is more likely to be described as "prepared" and less likely to be described as "messy", all three traits are grouped under conscientiousness. Using dimensionality reduction techniques, psychologists showed that most (though not all) of the variance in human personality can be explained using only these five factors.

Research into personality inventories found five broad dimensions could explain most variation in human personality and temperament, with more-detailed analyses typically dividing the traits into more specific subfactors. For example, extraversion is typically associated with qualities such as gregariousness, assertiveness, excitement-seeking, warmth, activity, and positive emotions. Other models, like HEXACO, supplement the Big 5 traits with additional variables.

The five-factor model has been described as one of the major breakthroughs of quantitative behavioural science^{[citation needed]}. The five-factor structure has been confirmed by many subsequent studies across cultures and languages, which have replicated the original model and reported largely similar factors. The traits have predictive validity for objective metrics other than self-reports: for example, conscientiousness predicts job performance and academic success, while neuroticism predicts self-harm and suicidal behavior.

The Big Five model originated from the lexical hypothesis, which suggests that the most important personality traits are encoded in language. Raymond Cattell built upon earlier lexical work by reducing thousands of descriptors to 16 personality factors, later clustered into five global traits, which some consider the "original Big Five". Ernest Tupes and Raymond Christal then analyzed peer ratings of U.S. Air Force officers and derived five core dimensions: Surgency, Agreeableness, Dependability, Emotional Stability, and Culture—an approach later popularized by Warren Norman. In the 1980s, John M. Digman and colleagues consolidated evidence from previous studies and reaffirmed five major traits, while Paul Costa Jr and Robert R. McCrae developed the NEO model, starting with three factors and expanding it into the widely accepted Five Factor Model (FFM). These four sets of researchers used somewhat different methods in finding the five traits, making the sets of five factors have varying names and meanings. However, all have been found to be strongly correlated with their corresponding factors.

In 1884, British scientist Sir Francis Galton became the first person known to consider deriving a comprehensive taxonomy of human personality traits by sampling language. The idea that this may be possible is known as the lexical hypothesis.

British-American psychologist William McDougall of Duke University, writing in 1932, put forward a conjecture observing that "five distinguishable but separable factors" could be identified when looking at personality. His suggestions, "intellect, character, temperament, disposition and temper", have been seen as "anticipating" the adoption of the Big Five model in subsequent years. The model was built on understanding the relationship between personality and academic behaviour.

In 1936, American psychologists Gordon Allport of Harvard University and Henry Odbert of Dartmouth College implemented the lexical hypothesis. They organised for three anonymous people to categorise adjectives from Webster's New International Dictionary and a list of common slang words. The result was a list of 4504 adjectives they believed were descriptive of observable and relatively permanent traits.