Measuring the Mind
Francis Galton made the first scientific attempt to measure intelligence in the late 19th century. He hypothesized that intelligence stemmed from sensory discrimination and reaction time, but these measures correlated poorly with real mental ability and were abandoned.
Alfred Binet and Théodore Simon created the first practical intelligence test in 1905 to identify French schoolchildren needing academic support. Binet introduced mental age, an age-equivalent score comparing performance to the typical average.
| Figure | Contribution | Outcome |
|---|---|---|
| Galton | Sensory discrimination | Unsuccessful |
| Binet & Simon | First practical test | Introduced mental age |
| Terman | Stanford-Binet | Introduced ratio IQ |
Common pitfall: Assuming Binet's test was designed to rank children. It was strictly created to identify students needing extra academic support.
The Ratio IQ Formula
Lewis Terman adapted Binet's test at Stanford, producing the Stanford-Binet test and the intelligence quotient (IQ) ratio:
In this formula, Mental age is the tested performance level, and Chronological age is the actual physical age. For example, a 10-year-old with a mental age of 12 scores .
This ratio IQ formula worked well for children, but it broke down for adults because mental age growth eventually plateaus while chronological age keeps climbing. Modern tests solved this limitation with deviation IQ scores.