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-to-threshold competition. Twenty-five 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. v9 changelog (May 2026) is a substantial editorial revision in response to feedback received on v8. The revision sharpens the manuscript's positioning against neighbouring frameworks and clarifies the scope of substrate-universal claims, while leaving the core architecture unchanged. Principal changes: (1) §5.6 Speculative coda. The physical-foundation material previously presented as load-bearing in §2.1.2 (Bell's theorem and probabilistic reality) and §2.1.3 (life as a consequence of the friction architecture) is consolidated into a new §5.6, explicitly labelled as a speculative coda doing no load-bearing work. The Bell reference is sharpened: Bell's theorem rules out local hidden-variable theories; deterministic interpretations of quantum mechanics remain viable, and BFT's argument is shown not to depend on which interpretation is correct. The Meaning-vs-Effort distinction previously orphaned in §2.1.2 is rehomed to §2.1.1. (2) §5.2 Control-theory positioning. A new anticipated-objection paragraph addresses control-theory accounts of self-regulation (Carver & Scheier 1982), articulating three load-bearing differences between BFT and the discrepancy-reducing feedback-loop tradition. (3) §5.3 Friction measurability. A new paragraph clarifies that the friction construct is non-circular: the artificial-substrate operationalisation (competing-routes measurement on transformer logprob distributions, per the companion work) provides an independent, behaviour-prior measure; the open biological-substrate task is an instrumentation problem rather than a conceptual one. (4) §2.6 Reactance citation. The reactance reframing now explicitly cites Brehm (1966; Brehm & Brehm 1981) where the social-psychology construct is referenced. (5) §2.1.1 Exhaustiveness softened. The four-field exhaustiveness claim is reframed as a defended-but-not-proven position — the standing-challenge framing is preserved. (6) §2.3 Cross-substrate scaling softened. The temporal-horizon scaling argument across primate intertemporal-choice and mammalian mutation-rate data is reframed from a scaling law to a suggestive convergence. (7) P20 Etiological equivalence — scope clause added. "Equivalent friction profile" is construed richly to include the bandwidth and learning-rate parameters that govern how a profile moves under intervention; the proposition claims that, once the full friction-state is matched, origin carries no additional predictive weight — not the broader claim that aetiology is clinically irrelevant. (8) §1.4 Substrate-scope statement. An explicit sentence states that BFT's primary domain is the biological behavioural substrate; substrate-invariance claims to artificial substrates are cross-referenced to companion work and are not load-bearing. (9) Citation consistency. All in-text companion-paper citations standardised from bare 'Lund 20XX' to 'Pødenphant Lund 20XX' (matching the References convention). Paper 5 (Pødenphant Lund 2026f) added as a cross-citation in §3.2 and §3.5b where the emotion-mapping companion is referenced. Paper 10's cite-letter and title updated to the current form. Companion DOIs migrated to their concept DOIs. (10) Bibliography. Four new entries: Brehm (1966); Brehm & Brehm (1981); Carver & Scheier (1982); Pødenphant Lund (2026f). The cosmological and astrobiological implications of the friction architecture are preserved in the speculative coda but are no longer presented as part of the load-bearing argument. The cognitive-mechanism, behavioural, and clinical claims of Sections 2–4 stand independently of the speculative material. Companion papers in the friction-theory series: Paper 1 — Friction as the cost of probabilistic computation (concept DOI 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 5 — A field-theoretic taxonomy of emotions (10.5281/zenodo.20058825); Paper 10 — Race all the way down/up (10.5281/zenodo.20014567); Paper 13 — Operational Friction Theory (10.5281/zenodo.20059876).