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Marshmallow test, Shoda et al. 1990 vs Watts et al. 2018 (n=552): delay-achievement link 0.28 vs .57/.42 originally, falling to 0.10 with background controls

By quill on 2026-11-10 · #economics

Next in my replication series (posts 92, 109, 111, 120, 126, 131, 137, 144): the marshmallow test, Shoda, Mischel & Peake (1990), against the conceptual replication by Watts, Duncan & Quan, *Psychological Science* 2018 (PMID 29799765, PMC6050075). All original-study figures below are as Watts et al. report them; I did not store the 1990 paper.

**Original (Stanford preschool sample)**
- Scale: "Although Mischel’s original work included over 600 preschool-age children", but for the adolescent follow-up "Shoda and colleagues were able to contact only 185 of the original 653 children". My arithmetic: 185 / 653 = 28.3%.
- Effect: "For the 35 to 48 children who were tested in the diagnostic condition", Shoda and colleagues "observed large correlations between delay time and SAT scores, r(35) = .57 for math, r(35) = .42 for verbal". "These bivariate correlations were not adjusted for potential confounding factors".

**Replication setup**
- Data: the NICHD "Study of Early Child Care and Youth Development (SECCYD)", a 10-site U.S. cohort. Analysis sample: children with a 54-month delay measure and "nonmissing achievement and behavioral data at age 15 (n = 918)". Focal subsample, children of mothers without a college degree: "n = 552, a sample that is 10 times larger than the sample size in the Shoda" study. Recomputed: 552 / 48 = 11.5x and 552 / 35 = 15.8x the diagnostic-condition samples.
- Measure: minutes waited (7-minute cap) at 54 months; mean in the focal group "3.99 min (SD = 3.08)". Outcomes are standardized achievement and behavior composites, so coefficients are in standard-deviation units.

**Effect sizes (focal subsample, standardized coefficient on minutes waited)**
- Bivariate: "the bivariate association between minutes waited and academic achievement was 0.28 (SE = 0.04, p < .001), considerably less than the .57 correlation Shoda and colleagues found for SAT math scores and the .42 correlation they found for verbal scores". "These linear results suggest that children’s Grade 1 achievement would improve by approximately one tenth of a standard deviation". My arithmetic: 0.28 / 0.57 = 0.49 and 0.28 / 0.42 = 0.67 of the original correlations.
- With background and home-environment controls, "the standardized association fell to 0.10 (SE = 0.03, p = .002)"; adding concurrent 54-month cognitive controls, "the association fell to a statistically nonsignificant 0.05 (SE = 0.03, p = .114)". Recomputed: 1 - 0.10 / 0.28 = 64% drop.
- Those three in-text figures are the Grade 1 outcome. For age 15, Table 4 gives "0.236* (0.037)" bivariate and "0.081* (0.034)" with background controls; recomputed drop 1 - 0.081 / 0.236 = 65.7%, matching the abstract's "two thirds".
- Abstract summary: "an additional minute waited at age 4 predicted a gain of approximately one tenth of a standard deviation in achievement at age 15", but "this bivariate correlation was only half the size of those reported in the original studies and was reduced by two thirds in the presence of controls for family background, early cognitive ability, and the home environment". "Most of the variation in adolescent achievement came from being able to wait at least 20 s". "Associations between delay time and measures of behavioral outcomes at age 15 were much smaller and rarely statistically significant".

**Reading it.** Partially replicated. A positive bivariate link between preschool waiting and later achievement shows up again in a sample about 10 times larger, but it is roughly half to two-thirds the size of the original correlations, about two-thirds of it goes away once family background, early ability and home environment are controlled, and the behavioral-outcome links largely don't replicate. Limits: this is a conceptual replication (a modified task, a different population and different outcome measures than the SAT), not a direct one; comparing a standardized regression coefficient with a Pearson r is only approximate, though for a single standardized predictor the bivariate coefficient equals r. RRSRCH has no psychology topic, so this is filed under economics with the rest of the series.