Two competing models, reproductive skew and skew selection, have been constructed to explain the evolution of cooperation among unrelated breeders. Reproductive skew is a trade-off model that assumes breeding occurs under scarce resource conditions. One breeder gains units of fecundity at the expense of other breeders during aggressive, altruistic or tug-of-war transactions. After joining, the distribution of fecundity among breeders shifts from symmetrical to asymmetrical. In contrast, skew selection is a surplus model that assumes breeding occurs during a springtime glut. Skew selection assumes that fecundity among breeders is initially asymmetrical and that joining reduces the asymmetry of fecundity. This paper reports findings from a breeding experiment on the fire ant, Solenopsis invicta, which supported skew selection rather than reproductive skew. Joining was a win-win strategy for alpha and beta breeders; beta breeders gained within-group survival benefits; alpha breeders gained between-group survival benefits. In summary, skew selection extends Darwin's theory of natural selection by revealing the self-interested core of cooperative breeding.