Intelligence tests

Here's a lineage worth memorizing cold, because examiners trace it like a family tree. Modern IQ testing begins in 1905 in France, when Alfred Binet (with Théodore Simon) built the first practical intelligence scale to identify schoolchildren needing extra help. That's the root of everything that follows.

Binet's scale crossed the Atlantic and was revised at Stanford by Lewis Terman, producing the Stanford-Binet in 1916. This is where the classic IQ formula lived: mental age divided by chronological age, times 100 (the ratio IQ). The problem? That formula breaks down in adulthood, since mental age stops climbing while chronological age keeps going.

Enter David Wechsler, who solved that with the deviation IQ — the concept most likely to appear on your exam. Instead of a ratio, Wechsler scored each person against the average performance of their same-age peers, using a normal distribution with a mean of 100 and a standard deviation of 15. So an IQ of 115 is one SD above the mean; 85 is one SD below. Deviation IQ is now standard across essentially all modern intelligence tests, including current editions of the Stanford-Binet.

Wechsler's tests are the workhorses: the WAIS (Wechsler Adult Intelligence Scale) for adults and the WISC for children, both yielding a Full Scale IQ plus index scores. They're individually administered — one examiner, one client — which is exactly the point of appropriate use. Individual intelligence tests are given by trained examiners for high-stakes decisions (disability determination, giftedness, cognitive impairment), not group-administered or self-scored, and results must be interpreted within cultural and linguistic context.

Clinical Pearl: Know the lineage — Binet (1905, France) → Terman/Stanford-Binet (1916, ratio IQ) → Wechsler (deviation IQ). The deviation IQ uses mean 100, SD 15, and it replaced the outdated mental-age ratio because that ratio fails in adulthood.