A conceptual model for the prediction of sexual intercourse in permanent couples.

The problem of the frequency of sexual intercourse in couples was investigated for the first time with a purely conceptual model. The model, based on a few axioms involving the notions of sexual appetite and erotic potential, was composed of two ordinary differential equations which turn out to be the same as those proposed almost one century ago in epidemiology. The model can be used to discuss the possibility of estimating strategic parameters from real data, as well as to criticize the rule of "the beans in the jar" proposed by Martin (1970).