We derive the Born rule as the unique outcome statistics induced by a canonical symmetry projection beneath nonselective measurement and unobservable symmetry structure. Concretely, given a compact symmetry group acting by conjugation, we identify the quantum channel that removes symmetry-orbit dependence while preserving exactly the invariant operator algebra. In a standard von Neumann algebraic setting, we prove that Haar averaging yields a normal, unital, completely positive idempotent map whose range is the fixed-point algebra, and hence a conditional expectation onto the invariant subalgebra. Moreover, any normal channel that is invariant under the symmetry action and fixes the invariant algebra pointwise must coincide with this Haar-induced projection. Specializing to projective measurement, Haar averaging over the associated phase-rotation symmetry yields the Lüders (pinching) reduction as the unique normal update map satisfying the same invariance and fixpoint requirements. The induced outcome weights are characterized as the unique probabilities specified by the restriction of the quantum state to the commutative outcome algebra, and they are preserved by the canonical reduction. As a further structural consequence of this symmetry projection, the von Neumann entropy is monotone under the induced reduction. The channel-based derivation applies in all finite dimensions, including qubits, and addresses a structural question not resolved by Gleason-type and positive operator-valued measure (POVM)-based classification results: it identifies the unique normal symmetry-forced reduction channel associated to a fixed measurement context, rather than characterizing admissible global probability functionals. License note: Distributed under CC BY-NC-ND 4.0.
Salimah Meghani (Sat,) studied this question.