The mechanical sufficiency gap: A diagnostic framework for engineering-origin inferences in archaeology, with application to the walking-moai hypothesis

The leap from showing that a feature could perform a function to claiming that it was engineered for that function — the mechanical sufficiency gap — is made routinely across archaeology, from megalithic transport to obelisk tipping to handaxe symmetry, without a diagnostic framework specific to the claim type. This paper provides one. Archaeological reasoning frequently treats three distinct claims as interchangeable: that a feature is mechanically sufficient for a function, that it was archaeologically involved in that function, and that it originated as an engineering adaptation for it. We propose a five-test diagnostic framework derived from the logical entailments of the engineering-origin hypothesis as distinguished from four named alternatives: stylistic convention, coincidental sufficiency, multi-purpose adaptation, and selection from a heterogeneous population. The tests examine task-scale calibration, feature distinctiveness, production-stage transitions, variance compression, and convergence with independent markers, and are derived prior to any specific empirical case. Applied to the walking-moai transport debate using a publicly available moai morphometric database (n = 222 analytic sample), the framework yields a mixed diagnostic outcome: Tests 4 and 5 are satisfied (variance compression; Cramér’s V = 0.944 for eye-socket convergence), supporting distinct operational stages, while Tests 1, 2, and 3 are not (size scaling null; 71% distributional overlap; no observable D-form transition pre-transport). The walking-transport mechanism is preserved; the more specific inference that D-form is its engineering signature is not uniquely supported by the morphometric record. Stylistic convention combined with selection remains the most parsimonious alternative.