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which falls as the residual pool of defectors, 1 - x - y , shrinks. Once a group happens to reach a state where $w_p < c$. punishing costs less at the margin than plain, unpunishing cooperation does, and ordinary within-group selection stops working against the punishing type in that group. If groups that reach this state also out-compete or out-reproduce groups that never do — amplified, the authors argue, by cultural transmission mechanisms such as conformist imitation of locally common strategies, which are not available to purely genetic evolution — group-level selection can carry punishment, and the large-scale cooperation it sustains, to frequencies individual selection alone would never reach [ 9 ] . This is an elegant resolution of the second-order free-rider problem on its own terms, and it is also exactly the kind of multi-level selection argument that a long tradition in evolutionary biology remains skeptical can operate strongly enough in real human populations, given realistic rates of migration and group extinction, to do the work the theory assigns it — a live theoretical objection the model’s internal consistency does not by itself settle.