Sexual selection traditionally involves male-male competition
and female choice, but in some species, including humans, sexual
selection can also involve female-female competition and male
choice. The degree to which one aspect of sexual selection or
another is manifest in human populations will be influenced by
a host of social and ecological variables, including the operational
sex ratio. These variables are discussed in connection with the
relative contribution of sexual selection and the division of labor
to the evolution of human sex differences.