Breaking Hamilton's Rule Hamilton's rule states that the evolution of cooperation is correlated with the kin relationship between the actor and the recipient and the degree of the benefit. However, this approximation relies on several steps of simplification that are often violated in natural systems. Smith et al. (p. 1700 ) derived a non-additive model for the evolution of cooperation by kin selection that could be applied to all domains of life—not just animals. Experimental data examining the bacteria Myxococcus xanthus showed that nonlinear interactions among cells make bacterial cooperation surprisingly resistant to cheating.