Louis Leon Thurstone

Louis Thurstone proposed that intelligence is not a single factor but a set of distinct primary mental abilities, including verbal comprehension, word fluency, number facility, spatial visualization, associative memory, perceptual speed, and reasoning. He identified these through factor analysis of intelligence test data in the 1930s.

The Thurstone scale is a method for measuring attitudes using the equal-appearing intervals technique. Respondents are presented with statements that have been pre-rated by judges along a continuum, and their agreement or disagreement with specific items yields a numerical attitude score on the topic.

Thurstone's Law of Comparative Judgment is a mathematical model for constructing psychological scales from pairwise comparison data. Published in 1927, it allows researchers to convert the proportion of times one stimulus is preferred over another into scale values on a psychological continuum.

Charles Spearman argued that a single general intelligence factor, g, underlay all cognitive performance, while Thurstone argued that multiple independent primary mental abilities existed. This debate between unitary and multifactor models of intelligence shaped psychometrics for decades and influenced the development of modern intelligence tests.