Galton reasoned that superior intelligence would be a reflection of superior physical development of brain and body and to investigate this possibility he set about measuring a variety of physical variables, such as reaction time and grip strength, and looked for a correlation between these measures and measures of success in endeavors thought to reflect intellectual ability.
He was the inventor of the first usable intelligence test, known at that time as the Binet test and today referred to as the IQ test. His principal goal was to identify students who needed special help in coping with the school curriculum.
Henry H. Goddard
He was the first to translate the Binet intelligence test into English in 1908 and distributing an estimated 22,000 copies of the translated test across the United States and was the leading advocate for the use of intelligence testing in societal institutions including hospitals, schools, the legal system and the military.
He was the inventor of the concept of the intelligence quotient, or IQ, later used by Lewis Terman and other researchers in the development of the first IQ tests, based on the work of Alfred Binet.
Terman promoted his test, known colloquially as the "Stanford-Binet" test, as an aid for the classification of developmentally disabled children.
World War I
During World War I a way was needed to evaluate and assign recruits which led to the rapid development of several mental tests.
He argued for a model of intelligence that included seven unrelated factors (verbal comprehension, word fluency, number facility, spatial visualization, associative memory, perceptual speed, reasoning, and induction).
Spearman's most notable contribution to intelligence testing is the idea that all aspects of intelligence, to a certain extent, are correlated with each other and he believed that only two factors are measured by intelligence tests, a general intelligence factor common to all tests and a specific factor that is distinctive in each test
He produced the first version of his test in 1939 which gradually became more popular and overtook the Binet in the 1960s and has been revised several times, as is common for IQ tests in order to incorporate new research.
He proposed two types of cognitive abilities in a revision of Spearman's concept of general intelligence. Fluid intelligence (Gf) was hypothesized as the ability to solve novel problems by using reasoning and crystallized intelligence (Gc) was hypothesized as a knowledge-based ability that was very dependent on education and experience. In addition, fluid intelligence was hypothesized to decline with age while crystallized intelligence was largely resistant.
John L. Horn
He argued that Gf and Gc were only two among several factors and he eventually identified 9 or 10 broad abilities but the theory continued to be called Gf-Gc theory.
His Structure of Intellect model used three dimensions which when combined yielded a total of 120 types of intelligence.
John B. Carroll
He proposed the Three Stratum Theory, which is a hierarchical model with three levels.
It is a merging of the Gf-Gc theory of Cattell and Horn with Carroll's Three-Stratum theory.