 The justification for these transition rules is as follows: if \(W_C \lt \overline{W}\), then the expected fitness of Cooperate is lower than the average fitness of the population. That means it is more advantageous to Defect than Cooperate, and so we would expect some proportion of the population to switch. The rate at which individuals switch is proportional to how much worse Cooperate does than the population average. (We are being deliberately ambiguous in terms of whether we are thinking in terms of biological or cultural evolution, but at this level of abstraction it makes little difference.) Since \(\frac{W_C}{\overline{W}} \lt 1\), it follows that \(p_c' \lt p_c\), as would be expected.