Explication of the Cultural Transmission Model

I n "Cultural Transmission, Disproportionate Prior Expos ure, and th e Evolu ti on of Cooperation," Mark (2002) claims to have "condueted a formal theoretical analysis that isolated the logic of cultural transmission (to] reveal an evolutionary force toward cooperation under conditions previously thought to make the evolution of cooperation impossible" (p. 324). The most striking aspect of Mark's work is not that hi s simple model converges to cooperative behavior, but that it works under initial conditions severely biased against the evolution of cooperation: randomly matched interaction in a large population with only I % initial coopera· tors (99% initial defectors). Although this result is compelling at first glance, we show that it is not robust to minor changes in the model: coop· erative behavior evolves only when the partic· ular model specifications chosen by Mark are selected and does not evolve when the model is generalized. In particular, we show that the result relies on a decoupling between actors' fitness and behaviors. According to Mark's theo· retical model, this decoupJing between fitness and behaviors should be associated, depends on simplifications of his own proposed mech· anism, and is not robust in small populations. We conclude that the role of Mark's fonnulation of disproportionate prior exposure (OPE) for the evolution of cooperation is unclear, though it