David Wechsler, a psychologist who worked with World War I veterans, developed a significant IQ test in 1939 called the Wechsler-Bellevue Intelligence Scale. This test was innovative because it combined several subtests that measured both verbal and nonverbal skills, reflecting Wechsler's belief that intelligence is a global capacity involving purposeful action, rational thinking, and effective interaction with the environment. This test later evolved into the Wechsler Adult Intelligence Scale (WAIS), which has been revised multiple times but still serves the same core purpose.

Today, Wechsler's legacy includes three major intelligence tests: the WAIS-IV for adults, the Wechsler Intelligence Scale for Children (WISC-V), and the Wechsler Preschool and Primary Scale of Intelligence (WPPSI-IV). These tests are extensively used in educational and clinical settings across the globe. They are periodically recalibrated to ensure their accuracy, which involves administering them to large, representative samples of the population.

The WISC-V, for instance, consists of 14 subtests organized into five indices: Verbal Comprehension, Visual Spatial, Fluid Reasoning, Working Memory, and Processing Speed. These indices provide a comprehensive view of a child's cognitive abilities, resulting in both individual scores for each index and a Full Scale IQ score. This approach acknowledges that intelligence is multi-faceted, involving various cognitive processes.

Wechsler's tests, alongside the Stanford-Binet scales, remain central to the measurement of intelligence and are often linked to Spearman's concept of "g" (general intelligence). These tests have proven reliable over time and are correlated with important life outcomes, including academic success and career achievements.