Thurstone, Louis Leon (1887-1955) was an American psychologist who was instrumental in the development of psychological tests. His statistical techniques enabled psychologists to study such psychological characteristics as ability and personality.

Thurstone was born in Chicago in 1887. In 1912, he received a degree in engineering from Cornell University, New York. He worked for one summer for Thomas Alva Edison, then accepted a teaching position with the College of Engineering at the University of Minnesota. In 1915, he accepted an assistantship in psychology at the Carnegie Institute of Technology in Pittsburgh, Pennsylvania, and became head of the department. While there, his primary responsibilities included designing tests, including oral trade-aptitude tests for the army during World War I (1914–1916) and college entrance examinations for the American Council of Education.

In 1917, he received a Ph.D. degree in psychology from the University of Chicago. In 1924, he became associate professor of psychology, and later became full professor. He was interested in mental and intelligence testing, which was in its infancy then, and in 1930 he developed a psychometric laboratory. Psychometrics is the quantitative measurement of abilities and personality using statistical data. Thurstone conducted a major study in 1934 that demonstrated seven primary mental abilities: numerical ability, spatial visualization, perceptual speed, rote memory, verbal meaning, verbal fluency, and reasoning.

Thurstone developed new statistical techniques to evaluate large quantities of data. He used matrix algebra to develop the theory and procedures of multiple-factor analysis. The method is still widely used in studies of such characteristics as personality, creativity, and temperament. The Vectors of the Mind (1935) and Multiple-Factor Analysis (1947) are considered his most important works in that field. He expanded the field of psychophysics, which deals with the relation between physical stimulus and mental response. He extended it to include mental response to mental stimuli, and established a basis for the quantitative study of social attitudes.