David Wechsler defined intelligence globally as 'the aggregate or global capacity of the individual to act purposefully, to think rationally, and to deal effectively with his environment.' Unlike Alfred Binet, whose earlier scales were heavily weighted toward verbal-academic tasks and relied on the concept of 'Mental Age' divided by 'Chronological Age' multiplied by 100, Wechsler conceptualized intelligence as a multi-faceted and multi-directional trait composed of distinct yet interacting cognitive abilities.
- The Birth of Deviation IQ: Wechsler discarded the Mental Age concept in adult testing because intelligence does not grow linearly across the lifespan. Instead, he introduced Deviation IQ, which compares an individual's raw score against the performance of a normative standardization sample of the same age group, yielding a standard normal distribution with a predetermined mean (typically 100) and standard deviation (typically 15).
- Evolution of the Batteries: David Wechsler developed the Wechsler-Bellevue Intelligence Scale in 1939. This evolved into the WAIS (1955), WAIS-R (1981), WAIS-III (1997), and WAIS-IV (2008). Parallel versions were constructed for children—the WISC (Wechsler Intelligence Scale for Children, 1949, currently WISC-V) for ages 6 to 16, and the WPPSI (Wechsler Preschool and Primary Scale of Intelligence) for ages 2 years 6 months to 7 years 7 months.