The models discussed so far have been, effectively, asexual.We have considered fertility in terms of females and their daughters, but we have used pooled survivorship data to derive model mortalities. In so doing, we have assumed an equal sex ratio at birth and equal sex-specific mortality rates. This introduces some error, and the direction is probably known: pre-industrial males have generally a higher adult survivorship than females, although the sex ratio may vary from age to age and from group to group (Weiss 1972b). Given our purpose and the quality of data available to anthropologists it is doubtful that the error introduced in this way is significant. A fieldworker who insists on constructing 2-sex models must have a larger data sample, so that the stochastic variations in age-class representation are minimized. But if the data seem sufficient and a 2-sex demographic model is desired, this chapter will present the methods by which such a model can be constructed.