 Another challenge in using evolutionary models to study social norms is that there is a potential problem of representation. In evolutionary models, there is no rigorous way to represent innovation or novelty. Whether we look at an agent-based simulation approach, or a straightforward game-theoretic approach, the strategy set open to the players, as well as their payoffs, must be defined in advance. But many social norms rely on innovations, whether they are technological or social. Wearing mini-skirts was not an option until they were invented. Marxist attitudes were largely not possible until Marx. The age at which one gets married and how many children one has are highly linked to availability of and education about birth control technologies. While much of the study of norms has focused on more generic concepts such as fairness, trust, or cooperation, the full breadth of social norms covers many of these more specific norms that require some account of social innovation.