Isospin relations by counting

A method is given for finding the relations between reaction cross sections or decay branching fractions that result from isospin conservation. The method was discovered long ago by Shmushkevich but is not widely known. It makes no call on the usual machinery of amplitude expansions and Clebsch–Gordan coefficients, but works instead by apportioning certain populations of particles according to a simple counting rule, the charge-uniformity rule. A number of examples, including several that require lengthy calculation to solve using Clebsch–Gordan coefficients, are here solved by inspection, usually without any equations at all.