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

In Horn (1988) he reported a full list of such broad level abilities: The Cattell-Horn model was, more or less, replicated by Carroll's (1993) analysis of 450+ intelligence measures, which also yielded a higher order factor similar to Spearman’s ‘g’.

McGrew (2005) reported that in 1999 the test publisher "Riverside Publishing met with Horn and Carroll privately in Chapel Hill, North Carolina, to seek a common, more meaningful  umbrella term that would recognise the strong structural similarities of their respective theoretical models, yet also recognize their differences.

This sequence of conversations resulted in a verbal agreement that the phrase “Cattell-Horn-Carroll theory of cognitive abilities” made significant practical sense, and appropriately recognized the historical order of scholarly contribution of the three primary contributors.

While obtaining his doctorate at the University of Illinois, Horn was involved with local chapters of the National Association for the Advancement of Colored People and the American Civil Liberties Union.