Charles Spearman was the first to develop techniques that measured "intercorrelations" between different tests of intelligence. The development of these theories lead to the evolution of the two-factor theory of intelligence, in which he postulated that the existence of a general intellectual ability factor that can and is tapped by all other mental abilities such as linguistic, mechanical, and arithmetic abilities. (238) Spearman instituted tests that measured the magnitude of this general intelligence and concluded that the higher the general intelligence, the greater a subject's overall intelligence would be. (237)
Spearman's work led directly to the development of multi-intelligence models, like those of Guilford (1967), that attempt to explain the varied types of identifiable intelligence witnessed throughout the general population. Evolving from the discussion of intelligence as a general idea accepted by the lay population but without real definition, psychologists from Sternberg to Spearman attempted to decipher the popular idea of intelligence, capture it within an acceptable definition, and understand its origins, growth, possibilities, and application. Each scientist presented a new way to examine the nebulous idea, mollifying its uncertain nature with an infusion of definition and standards, shedding light on the wholly inconspicuous concept of brilliance.
Cohen and Swerdlik, Jay and Mark E. Psychological Testing and Assessment. 6th Ed. New York: McGraw-Hill, 2005.