Intelligence

Charles Spearman (1904) proposed the existence of a general intelligence factor (g) underlying all cognitive abilities. According to his model, people who perform well on one type of cognitive task tend to perform well on others, suggesting a common factor. Raymond Cattell (1963) refined this idea by distinguishing between fluid intelligence (gf) — the ability to reason and solve new problems, independent of experience — and crystallized intelligence (gc) — knowledge and skills acquired through education and experience. Fluid intelligence tends to decline with age, while crystallized intelligence is maintained or increases.

Howard Gardner (1983) challenged the unitary view of intelligence by proposing the theory of multiple intelligences: linguistic, logical-mathematical, spatial, musical, bodily-kinesthetic, interpersonal, intrapersonal, and naturalistic. Robert Sternberg (1985) proposed the triarchic theory, distinguishing analytical (academic), creative (ability to generate novel solutions), and practical (adaptation to the real world) intelligence. Both theories expanded the conception of intelligence beyond traditional academic abilities, though they have been criticized for lacking rigorous empirical evidence.

The measurement of intelligence through IQ tests (Wechsler, Stanford-Binet) has been both one of psychology's most influential contributions and one of its most controversial. IQ tests reasonably predict academic and occupational performance, but have been criticized for cultural biases that may disadvantage minority groups. The Flynn effect — the sustained increase in IQ scores over decades in all industrialized countries — raised fundamental questions about what the tests actually measure.

The debate over the heritability of intelligence has been particularly intense. Twin studies suggest substantial heritability, but heritability does not imply genetic determinism: the environment exerts a crucial influence, especially under conditions of deprivation. Nisbett et al. (2012) comprehensively reviewed the evidence, concluding that both genetic and environmental factors (nutrition, education, cognitive stimulation) contribute significantly to individual differences in intelligence, and that group differences primarily reflect environmental inequalities.