Epelman, M.A., S. Pollock, B. Netter, and Bobbi Low. 2005. "Anisogamy, expenditure of reproductive effort, and the optimality of having two sexes." Operations Research, 53(3): 560-567.
No good formal arguments exist for a central question in biology: Why, in species that have sexual reproduction, are there usually only "males" and "females"? We present a nonlinear optimization model that supports the conclusion that having only two sexes maximizes long-run viability.