R. A. Fisher's 1936 statistical re-analysis found Mendel's pea-ratio data fit the expected 3:1 ratio implausibly closely

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In a 1936 paper, statistician and geneticist R. A. Fisher reviewed the segregation ratios Gregor Mendel had reported from his 19th-century pea-breeding experiments and found they matched the theoretically predicted ratios (such as 3:1) far more closely than random sampling should produce. Fisher's chi-squared analysis put the probability of a genuinely random dataset landing at least as close to the prediction at roughly 0.99993 — meaning a dataset this "clean" should almost never occur by chance. Fisher did not conclude outright fraud; his own words leave the explanation open: "it remains a possibility among others that Mendel was deceived by some assistant who knew too well what was expected."

The charge has stayed genuinely contested since. Candidate explanations include unconscious researcher bias in judging borderline plant classifications, selective reporting of the cleanest trials, small effective sample sizes that understate normal variance, and outright fabrication by an assistant — with no settled consensus verdict. (A 2010 statistical reconciliation — Pires & Branco, Statistical Science 25(4):545–565, posted to arXiv as 1104.2975 in April 2011 — argues that a probability model of unconscious experimenter bias can fit Mendel's data without offending Fisher's analysis; abstract read, body not, so a further lead rather than a claim asserted here. [Promotion wording, corrected 2026-09-12 audit: "A 2010 Bayesian reconciliation, arXiv:1104.2975, argues a probability model can explain the fit without conceding fraud — an unread further lead, not asserted here." The arXiv abstract describes a probability model, not a Bayesian one; 2010 is the journal year, the arXiv posting is 2011.])

This is the founding instance, in the vault, of "fraud-by-overfit": data too well-behaved to be real measurement is itself treated as evidence of a defect in its provenance — later reprised in the retraction forensics of claim-hirsch-forensics-drove-dias-superconductivity-retraction and formalized statistically in claim-simonsohn-fabrication-flagged-via-excessive-similarity-to-random-sampling. See observation-suspicious-perfection-independence-absence-signals-defect for the general law these three instances share.

Correction history.

• 2026-09-05 — Source upgrade, not a substantive correction. The claim and its quote are unchanged; what changed is their grounding. Until now the note rested on The Grand Locus's blog restatement of Fisher (Tier 2). The 2026-09-01 batch capture fetched Fisher's own paper — "Has Mendel's Work Been Rediscovered?", Annals of Science 1(2):115–137 (1936), mirror at genepi.qimr.edu.au, source_sha ce3f431b1de09091db2dbaa908ef5b12e1d89eca8ef3fe936338732c9a2e1110 — and confirmed both the pooled figure against Table V directly (84 df, X²=41.6056, "Probability of exceeding deviations observed" = .99993) and the "deceived by some assistant" sentence against Fisher's own wording. The frontmatter source_url is intentionally kept at the Grand Locus carrier, because it is the copy verified_verbatim on 2026-08-07 and it matches the clean quote; the archived Fisher PDF's OCR renders "that" as "t hat" throughout, so the literal cached string is "…among others t hat Mendel was deceived…" and a naive verbatim grep of the clean quote against the primary will not match. Found in the promotion of the 2026-09-01 suspicious-perfection verification capture.