In personality psychology, the lexical hypothesis (also known as the fundamental lexical hypothesis, lexical approach, or sedimentation hypothesis) generally includes two postulates:

1. Those personality characteristics that are important to a group of people will eventually become a part of that group's language.

and that therefore:

2. More important personality characteristics are more likely to be encoded into language as a single word.

With origins during the late 19th century, use of the lexical hypothesis began to flourish in English and German psychology during the early 20th century. The lexical hypothesis is a major basis of the study of the Big Five personality traits, the HEXACO model of personality structure and the 16PF Questionnaire and has been used to study the structure of personality traits in a number of cultural and linguistic settings.

Sir Francis Galton was one of the first scientists to apply the lexical hypothesis to the study of personality, stating:

I tried to gain an idea of the number of the more conspicuous aspects of the character by counting in an appropriate dictionary the words used to express them... I examined many pages of its index here and there as samples of the whole, and estimated that it contained fully one thousand words expressive of character, each of which has a separate shade of meaning, while each shares a large part of its meaning with some of the rest.

— Francis Galton, Measurement of Character, 1884