Who propounded the two-factor theory of intelligence?

The two-factor theory of intelligence explains a person's performance on any intellectual task using exactly two kinds of ability: a general factor ('g'), which contributes to performance on every mental task, and specific factors ('s'), each unique to one particular task or activity.

Charles Spearman formulated this theory in 1904 after applying factor analysis to the correlations between scores on many different mental-ability tests. He found that every test loaded heavily onto one common factor plus a smaller factor specific to that test alone -- exactly the general-plus-specific structure the question describes.

Contrast with the other names offered:

• Thorndike proposed a multi-factor model made of three independent kinds of ability -- abstract, social, and mechanical.

• Vernon proposed a hierarchical model with a general factor at the top, broad group factors below it, and several minor/specific factors beneath those -- more than two levels.

• Stern is remembered for the intelligence quotient (mental age divided by chronological age, multiplied by 100) -- a way of scoring intelligence, not a factor structure.

Only one of the four is credited with a strict two-component (general + specific) model of intelligence, which is why that name is the answer.