Mixing matrices: Necessary constraints in populations of finite size

This paper analyses the constraints used to define the elements of mixing matrices employed in deterministic models of the transmission dynamics of sexually transmitted infections. These matrices define the degree to which the choice of sexual partners in a population is assortative (like with like) or disassortative (like with unlike) in populations stratified by sexual activity (i.e. the number of different sexual partners per unit of time). An expanded set of constraints is described which defines patterns of mixing on the assortative to disassortative spectrum in populations of finite size. The relevance of these new constraints to past work in this area is assessed.