Majority Vote Model — Sociophysics

Majority Vote Model (de Oliveira 1992): Each agent adopts the majority opinion of its neighbors with probability (1−q); does opposite with noise probability q.
Phase transition: q_c ≈ 0.075 on 2D square lattice. For q < q_c: ordered phase (consensus). For q > q_c: disordered (no consensus).
Universality class: Same as 2D Ising model (β=1/8, γ=7/4, ν=1) — remarkable given non-equilibrium dynamics.
Susceptibility χ = N(⟨m²⟩−⟨|m|⟩²) peaks at q_c. Binder cumulant U = 1 − ⟨m⁴⟩/(3⟨m²⟩²) → 2/3 for ordered, 0 for disordered.