This paper introduces the ARCH model—Behavior = Archetype × Drive × Culture—as a biologically grounded framework for understanding emotion and behavior across species. Rooted in Tinbergen’s ethological paradigm, the model formalizes emotion as the emergent outcome of conserved behavioral scripts (archetypes), motivational systems (drives), and sociocultural modulation (culture). Neurobiologically anchored in systems such as the amygdala, hypothalamus, and mesolimbic pathways, the model accounts for affective expression across phylogeny, from bacteria to primates, and includes applications in psychiatric diagnosis, comparative ethology, and cognitive neuropsychology. The ARCH model reframes psychopathology as misalignment within the triad, offering explanatory value for conditions such as dissociation, addiction, and violent extremism. By unifying neural substrates, motivational states, and symbolic environments, ARCH provides a scalable, testable, and evolutionarily conserved law of behavior, with implications for psychiatry, AI, and behavioral science.