Ancient Ideas of Intelligence
How Plato, Aristotle and early thinkers understood reason, the soul and the faculties of the mind.
Galton and the Birth of Mental Testing
Francis Galton pioneered measuring human differences β and helped launch the troubling eugenics movement.
Binet and the First Intelligence Test
Alfred Binet created the first practical intelligence test in 1905 β to help children, not to rank them.
Spearman and the Discovery of g
Charles Spearman used factor analysis to argue for a general intelligence factor, g, behind all mental abilities.
Wechsler and the Modern IQ Scale
David Wechsler introduced deviation IQ and the WAIS and WISC scales that define modern intelligence testing.
The Flynn Effect: Why IQ Scores Rose
IQ scores rose dramatically across the 20th century β the Flynn effect β and may now be reversing.
The History of Fluid and Crystallized Intelligence
Raymond Cattell and John Horn split intelligence into fluid reasoning (Gf) and crystallized knowledge (Gc).
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Last updated: 2026-06-18 Β· β Reviewed by the All-Lifes editorial team Β· About Β· Methodology