Foundations of Intelligence

Measuring the Mind, From the Beginning

Francis Galton, a cousin of Charles Darwin, was among the first to attempt a scientific study of intelligence, in the late 19th century. Galton hypothesized that intelligence could be measured through basic sensory discrimination (reaction time, sensory acuity), reasoning that "smarter" nervous systems should process sensory information more efficiently — but these measures turned out to correlate poorly with more meaningful measures of mental ability, and his approach was largely abandoned.

The first genuinely practical intelligence test came from Alfred Binet and Théodore Simon in France in 1905, commissioned by the French government to identify schoolchildren who needed additional academic support. Binet introduced the concept of mental age: a child's test performance was compared to the average performance of children at different ages, yielding an age-equivalent score. A child who performed like a typical 8-year-old, regardless of their own actual age, had a mental age of 8.

Lewis Terman at Stanford University adapted and extended Binet's test for American use, producing the Stanford-Binet test, and introduced the now-familiar intelligence quotient (IQ) — originally a simple ratio:

$$IQ = \frac{\text{Mental age}}{\text{Chronological age}} \times 100$$

Under this ratio formula, a 10-year-old with a mental age of 12 would score $12/10 \times 100 = 120$. This ratio IQ formula worked reasonably well for children but broke down for adults, since mental age growth naturally plateaus while chronological age keeps climbing — a problem that later, more modern tests solved with a different approach (deviation IQ, covered later in this course).

Common pitfall: assuming Binet's original test was designed to rank children from smartest to least smart for its own sake. It was explicitly practical and applied — commissioned to IDENTIFY students needing extra academic support, not to create a general-purpose ranking of intelligence.