Behavioural Friction Theory: Toward a Common Currency for Behavioural Science

This paper introduces Behavioural Friction Theory (BFT), a formal integrative framework for behavioural science. BFT proposes that all behaviour is regulated by friction across four fields (Safety, Meaning, Ability, Effort) and five layers (Biological, Emotional, Inner, Cognitive, External), resolved through a race competition. Thirty formal propositions are derived, each with specification of existing empirical support and minimal test designs. BFT is presented as a theoretical framework inviting empirical scrutiny — not a replacement for existing theories, but the architecture within which they can be compared, connected, and extended. Companion papers in the friction-theory series (all Zenodo-live): Paper 1 — Friction as the cost of probabilistic computation (10.5281/zenodo.20012654); Paper 2 — Capacity scaling of encoding-through-loading (10.5281/zenodo.20013491); Paper 3 — Friction-guided inference (10.5281/zenodo.20014121); Paper 4B — Substrates encode experience, not information (10.5281/zenodo.20059861); Paper 5 — A field-theoretic taxonomy of emotions (10.5281/zenodo.20058825); Paper 6 — Matched-friction (10.5281/zenodo.20059863); Paper 7 — Forward-modelling in bounded race substrates (10.5281/zenodo.20449154); Paper 10 — Race all the way down/up (10.5281/zenodo.20014567); Paper 13 — Operational Friction Theory (10.5281/zenodo.20059876); Paper 14 — Logic as Reactance (10.5281/zenodo.20217712); Paper 16 — The Physics of Learning (10.5281/zenodo.20416959). v13 changelog (June 2026) spans the v11 → v13 jump (an intermediate v12 was prepared but not separately released). It consolidates a substantive reworking of the four-field section and a refinement of the core commit mechanism: (1) §2.1.1 — capacity-relative friction law (friction density rises with task demand but its consequence is capacity-gated: constructive struggle inside the capacity envelope, destructive crash outside, with a variance-spike and inverted-U at the capacity edge), anchored in three established human-cortex results (expected value of control; dorsal-anterior-cingulate conflict monitoring; the neural-efficiency effect); plus the Effort field's affective half and a Safety-field limit-case pointer. (2) §2.2 — the Net Friction Rule's capacity condition is reframed as a pressure-realised race outcome rather than a separate deliberative gate. (3) §2.3 — a pipe-diameter analogy (plastic effective-capacity) for the pressure-gated race. (4) §2.4 — a revision of the central commit mechanism bringing it into alignment with the companion Operational Friction Theory: the commit-threshold is removed; race-opening is the operative threshold; commit is structurally obligatory and the most-resolved route manifests at the forced deadline. (5) §3.3 / §3.3b / §3.7 / §3.8 — mirror-friction as an object-general forecast; an all-or-nothing collapse bias as a consequence of the race architecture; the endocrine layer as the setter of momentary field-states; and a new falsifiable empirical programme section. (6) §2.1 — the term "common currency" is credited to its established neuroeconomic use (a neural value signal) and BFT's distinct proposal — a friction (cost) currency with an integrative scope — is made explicit. Each substantive addition cleared external dual review before release.