John Leonard Horn (September 7,1928 – August 18, 2006) was a scholar, cognitive psychologist and a pioneer in developing theories of multiple intelligence.

For his Ph.D. research at the University of Illinois, Horn identified other broad intellectual abilities to supplement fluid reasoning ability and crystalized ability postulated by Raymond Catrell along with J. B. Carroll in 1999. The test publisher, Riverside Publisher suggested combining theories so they created the Cattell – Horn – Carroll theory of cognitive abilities which is the theory that is the basis for many modern IQ tests.

Horn started his career as a lecturer of Educational Psychology at the University of California, Berkeley in 1967. He was an Associate Professor of Psychology at the University of Denver from 1970 to 1986. Meanwhile, he was also a Research Associate at the Institute of Psychiatry for the University of London in England (1972), and a Research Associate of Psychiatric Clinic for the University Hospital in Lund, Sweden (1982). He was a Professor of Psychology and the head of Adult Development and Aging at the University of Southern California from 1986 – 2006.

He received numerous awards, including: Research Career Development Awards through the National Institute of Health (1968-1972) the annual prize for Distinguished Publications in Multivariate Psychology (1972), Lifetime Achievement Award, SMEP, (1992). Horn also served as a president of the National Association for the Advancement of Colored People and The American Civil Liberties Union.