DACS: From Historical Inertia to Forward Evidence for Behavioral Authentication

In behavioral biometric authentication under user drift, forward-only adaptation is not merely an optimization problem; it is an information-selection problem under a restricted forward budget (≤20 evaluations): should a drifted verifier be updated using historical training movement or measured forward responses? Testing historical movement reveals that trajectory transfer provides little benefit, yielding only +0.504 pp on H-MOG and severe negative transfer on HuMI (-5.940 pp), showing that historical movement is not a reliable adaptation signal. We introduce DACS, a response-grounded alternative that replaces trajectory inheritance with two complementary sources of forward evidence. Controlled Component Attribution (ComAtt) identifies responsive components from development interventions, while current-user search resolves independent signed edits through a disjoint validation gate. On BehaveFormer authentication cohorts, four-component DACS improves balanced accuracy by 3.037 pp on H-MOG and 1.646 pp on HuMI within twenty objective evaluations, with factorial controls confirming a positive attribution–direction interaction. Evaluated through dual lenses, DACS delivers: (a) a lightweight seventeen-evaluation discrete variant retaining 76.5–79.7% of continuous-search gains; and (b) a full-cohort intention-to-treat audit achieving 5.3×/10.3× larger population-level gains than Random-k while limiting observed severe deterioration to 2.82% on H-MOG and 6.75% on HuMI. These results support an information-anchor shift: exposing plasticity through measured component responses and directing it using present-user evidence.