any theory of intelligence postulating that the abilities constituting intelligence are arranged in a series of levels (of a hierarchy) ranging from general to specific. Many of these theories are based on recognizing three levels of factors, first proposed by U.S. psychologist Karl J. Holzinger (1892–1954): (a) the general factor, applying to all intellectual tasks; (b) group factors, which apply to some but not all intellectual tasks; and (c) specific factors, applying to individual tasks. Examples of such theories are the three-stratum model of intelligence and the Cattell–Horn theory of intelligence.