Binet’s two principles of test construction were age differentiation and general mental ability. Age differentiation refers to the fact that with increasing age, children develop their abilities. Thus, older children have greater abilities than do younger ones. Spearman developed his own theory of general mental ability, or g, based on the idea that a single general factor underlies all intelligence. Modern theorists have taken this concept further in gf-gc theory, in which there are two basic types of intelligence: fluid (gf) and crystallized (gc).