Every baby born in an eight-month window, measured from four days old
The children were white, from English-speaking Berkeley homes of above-average means. That narrowness matters for everything the 1933 norms claimed. Bayley became the study's director in 1931.
Monthly to 15 months, every three months to 3, then twice a year
Pick a birthday inside the study's window. The calendar marks every visit the documented schedule implies.
Would this item give the same result twice?
Testing a baby twice a few days apart shows which items are trustworthy. When Bayley and Emmy Werner checked her revised first-year scale, a clear rule appeared. Mark each card steady or shaky, then reveal the rule.
Because the same children were tested from infancy, Bayley could ask how well early scores matched later ones. Her 1949 paper followed the cohort from birth to eighteen. The individual variation she found led her to argue that intelligence is not fixed at birth and is shaped by what a child meets afterwards.
The scales were never meant to be IQ tests. Modern Bayley editions report a developmental quotient and are used to flag delay, not to rank infants.
Using the revised scales in a much wider sample, Bayley found no differences on either scale by sex, birth order, region or parents' education. Black and white infants did not differ on the Mental Scale, and Black infants tended to score higher on the Motor Scale.
She concluded that the group differences seen from about age four onward must arise later, and pointed to the second year as the period to study.
From the first year to three and a half
Each bar spans the ages an edition covers, on a 0–42-month ruler. Click one for its scales.
A child who started school late, and spent forty years measuring children
Sources cited on this page
- [1]Bayley, N. (1933). The California First-Year Mental Scale. University of California Syllabus Series, No. 243. Berkeley: University of California.cited in Review of Educational Research (1936)Used for: Title, series and year.
- [2]Bayley, N. (1933). Mental growth during the first three years: A developmental study of sixty-one children by repeated tests. Genetic Psychology Monographs, 14(1), 1–92.cited in Klonoff (1972)Used for: Monograph title, volume and pages.
- [3]Jones, H. E., & Bayley, N. (1941). The Berkeley Growth Study. Child Development, 12(2), 167–173.doiUsed for: Cohort, birth window, schedule, attrition, instruments (via Wikipedia summary).
- [4]Wikipedia. Nancy Bayley (citing Jones & Bayley 1941, O'Connell 1990, Psychology's Feminist Voices).en.wikipedia.orgUsed for: Biography, awards, 1969 BSID combining three scales.
- [5]Encyclopedia.com / Gale. Bayley, Nancy.encyclopedia.comUsed for: 1933 and 1936 scales; NIMH and perinatal project.
- [6]Bayley, N. (1965). Comparisons of mental and motor test scores for ages 1–15 months by sex, birth order, race, geographical location, and education of parents. Child Development, 36(2), 379–411.doiUsed for: 1965 group comparisons and conclusion.
- [7]Werner, E. E., & Bayley, N. (1966). The reliability of Bayley's revised scale of mental and motor development during the first year of life. Child Development, 37(1), 39–50.doiUsed for: Reliability rule in the sorting exercise.
- [8]Bayley, N. (1949). Consistency and variability in the growth of intelligence from birth to eighteen years. Journal of Genetic Psychology, 75, 165–196.tandfonline.comUsed for: 18-year follow-up.
- [9]Lennon, E. M., et al. (2008). Bayley Scales of Infant Development. In Encyclopedia of Infant and Early Childhood Development. Elsevier.sciencedirect.comUsed for: 40 years of research editions; BSID-I parts and 2–30 months; not an intelligence test; Bayley-4 range.
- [10]Balasundaram, P., & Avulakunta, I. D. (2022). Bayley Scales of Infant and Toddler Development. StatPearls. NCBI Bookshelf.ncbi.nlm.nih.govUsed for: 1969 range 3–28 months; Bayley-III and Bayley-4 item counts.
- [11]Nellis, L., & Gridley, B. E. (1994). Review of the Bayley Scales of Infant Development, 2nd ed. Journal of School Psychology, 32(2), 201–209.doiUsed for: BSID-II MDI/PDI and behaviour scale.
- [12]Albers, C. A., & Grieve, A. J. (2007). Test review: Bayley-III. Journal of Psychoeducational Assessment, 25(2), 180–190.doiUsed for: Bayley-III five scales.
What changed in this revision
Neither 1933 publication was read in full. Cohort and schedule details come from Jones & Bayley (1941) as summarised in secondary sources, and the 1949 and 1965–66 findings from their abstracts and summaries. The page title now names the 1933 scale; the slug is unchanged.
- REMOVED“Until the late 1920s, intelligence testing began at school age.” Infant scales such as Gesell's predate Bayley, and the Stanford-Binet started at age 3.
- CORRECTEDThe 1933 instrument was the California First-Year Mental Scale. “Bayley Scales” and the Mental/Motor/Behavior Record structure belong to the 1969 BSID.
- REMOVEDItem counts of about 163 mental and 81 motor items, shown as 1933 figures, and “birth to 30 months.” No source ties either to 1933.
- REMOVEDThe generic milestone slider: those ages were common milestones, not Bayley's items or norms. Also removed: “most-used worldwide.”
- ADDEDBoth 1933 publications, the Growth Study cohort, a visit-calendar interactive, the 1966 reliability sort, the 1949 and 1965 findings, an edition ruler, a biography and 12 references.
- KEPTThe note that this is history, not a self-test, now with a care line.
Related historical tests
As the Berkeley children grew past infancy, their mental testing included Stanford-Binet scales. Terman's scale started at age 3, so infant scales like Bayley's covered the years before it. Comparing early and later scores across that switch was the heart of Bayley's stability question.
The 1937 Terman–Merrill forms arrived while the Growth Study children were eight or nine. Longitudinal studies like Bayley's leaned on such revisions to keep testing the same children as they aged.
Doll's Vineland scale, two years later, measured everyday competence from infancy through an informant's report. Bayley's scales watched the child directly. Modern Bayley editions borrowed the report route for their Adaptive Behavior scale.
Florence Goodenough built her drawing test at Minnesota's Institute of Child Welfare, a sister institute to Berkeley's. Both women worked on tests for children too young for paper-and-pencil intelligence testing.
The WPPSI came out two years before the 1969 Bayley and covered children from about four. Together they gave clinicians a sequence from infancy to school entry.
Detroit's picture tests for school beginners also worked without reading. They sorted whole classes, whereas Bayley studied single children month by month.