The 1955 WAIS is often credited with inventing deviation-IQ scoring. The sources here say Wechsler's Bellevue scales already used it: Form II was the first to swap the mental-age formula for standard scores, and the 1939 scale already used deviation IQ.1,5 The timeline entry on this site follows that record.
Why dividing by age fails for adults
A ratio IQ divides mental age by actual age. If test performance stops rising in the mid-teens, a perfectly average adult's ratio IQ keeps falling every birthday. A deviation IQ compares each person with others their own age, so the average is always 100.6,5
Illustration only: it assumes measured "mental age" stops rising at 16 for an average person. It's arithmetic, not WAIS data.
Built for a hospital's patients
Wechsler was chief psychologist at Bellevue Hospital in New York from 1932 to 1967, and the scales grew out of assessing his patients there.7 The 1939 Wechsler-Bellevue drew its subtests from existing tests, including Robert Yerkes's army testing programme and the Binet-Simon scale.3
The Stanford-Binet dominated at the time. Wechsler insisted on testing everyone on both verbal and performance scales, which went against the professional opinion of the day. He also introduced subtest profiles and three IQ scores instead of one.4,1
"…the global capacity of a person to act purposefully, to think rationally, and to deal effectively with his environment."David Wechsler's definition of intelligence7
Eleven subtests, three IQs
The 1955 WAIS kept the verbal/performance split and eleven subtests covering the same ground as the 1939 scale.6 The subtests below are as listed for the 1981 WAIS-R, which kept the same structure.3,5
Gaps between a person's Verbal and Performance IQ drew wide clinical interest.1 The size of the 1955 standardization sample isn't given in these sources.
Seventy years of the same idea
After Wechsler's death in 1981, the Psychological Corporation, now part of Pearson, carried on the revisions.5 The current test is for ages 16 and over, and is given one-to-one by a trained examiner.2
References
- [1]Development of the WAIS-III: A brief overview, history, and description. Graduate Journal of Counseling Psychology, Marquette University (PDF).epublications.marquette.edu
- [2]Wechsler Adult Intelligence Scale (WAIS-5). Cogn-IQ (2026).cogn-iq.org
- [3]Wechsler Adult Intelligence Scale. Psynso (2017).psynso.com
- [4]Wechsler Adult Intelligence Scale (WAIS): psychological testing. MentalHealth.com (2026).mentalhealth.com
- [5]Wechsler Adult Intelligence Scale: full WAIS guide. Practice Test Geeks (2026).practicetestgeeks.com
- [6]Jouve, X. History of the WAIS: Wechsler-Bellevue to WAIS-V. Psychologic (2023, reviewed 2026).psychologic.online
- [7]Wechsler Adult Intelligence Scale. EPFL Graph Search.graphsearch.epfl.ch
- [8]The evolution and evaluation of the WAIS: a historical and scientific perspective. Cogn-IQ Research Papers (2023).cogn-iq.org
Related historical tests
The 1939 adult test that the WAIS re-standardized. It had already introduced the verbal/performance split and deviation scoring; the WAIS gave it better items and national norms.
Wechsler's version for ages 5 to 15, published six years earlier. It also comes from the Bellevue subtests, and the two scales share the same VIQ, PIQ and FSIQ structure.
A school test that could be given to a whole class with booklets. It shows the group-testing alternative to the one-to-one, examiner-led WAIS.
The WWII recruit test, also reported as standard scores rather than mental ages. It gave test-takers a standing relative to others rather than a mental age.
A test built entirely from matrix puzzles. It is the opposite of Wechsler's approach of sampling many different abilities and combining them.
Cattell's personality questionnaire, also read as a profile of scores. It measures personality rather than ability, but it too is read as a shape across many scales rather than as a single number.