Spearman's name is almost synonymous with the term 'general intelligence' (otherwise known as psychometric 'g'). While still a student (albeit a rather old one -- Spearman was 48 when he received his Ph.D.), he published his famous two-factor theory of intelligence. Spearman speculated that all intellective functioning was underpinned by an overall mental ability accompanied by specific abilities for differing mental tasks.

One of the great achievements of psychology evolved from Spearman's efforts to operationalize his theory ­ the statistical procedure we now know as 'factor analysis'. The procedure he developed (the method of tetrad differences) indicated to Spearman that his theory was in fact correct. Unfortunately, not everyone agreed with him. Objections ranged (and still do) from the simplistic nature of 'g', through to the relative stability of the mathematical procedures adopted. On the other hand, many eminent (especially British) psychologists (e.g., Sir Cyril Burt) concurred and 'g' became an established psychological construct. Proponents of general intelligence, or 'g', still exist amongst prominent psychologists -- examples include Arthur Jensen and Philip Vernon. It took almost 30 years before Thurstone developed the technique of multiple factor analysis. This procedure questions the derivation, nature, and importance of 'g'.

Controversy still surrounds the question of whether 'intelligence' is best conceived of as a unitary entity (Spearman) or as a multi-faceted collection of abilities. Still "horses for courses" as the saying goes. Perhaps Spearman's dogmatic determination in promoting and maintaining his position lay in his 15 years of military service (as a British army officer) in India. Like many successful professionals, famous psychometricians appear to come from extraordinarily disparate backgrounds.