  The main focus of this paper is to introduce a new method to control
perturbative calculations of CP asymmetric reaction rates in the Boltzmann
equation. CP asymmetries in particle reactions are traditionally calculated in
terms of complex couplings, Feynman integrals, and Cutkosky rules. We use an
expansion of the $S$-matrix unitarity condition instead, obtaining a general
expression for the asymmetries without reference to the imaginary part of the
loops. Asymmetry cancelations implied by CPT and unitarity are manifested in a
diagrammatic way and easy to track at any order of perturbation theory. We
demonstrate the power of this general framework within the right-handed
neutrino and top-quark scattering asymmetries in seesaw type-I leptogenesis.
