Research conducted over the last fifteen years has placed in question many of the traditional conclusions about the evolution of human female sexuality. Women have not lost estrus, as earlier researchers thought, but it is simply concealed, resulting in two functionally distinct sexualities with markedly different ends in each phase.
At the fertile phase of the cycle, women prefer male traits that may mark superior genetic quality, and at infertile phases, they prefer men willing to invest resources in a mate. Thus, women's peri-ovulatory sexuality functions to obtain a sire of superior genetic quality, and is homologous with estrus in other vertebrates.
This model sheds light on male human sexuality as well: men perceive and respond to women's estrus, including by increased mate guarding. Men's response is limited, compared to other vertebrate males, implying coevolutionary history of selection on females to conceal estrus from men and selection on men to detect it. Research indicates that women's concealed estrus is an adaptation to copulate conditionally with men other than the pair-bond partner. Women's sexual ornaments-the estrogen-facilitated features of face and body-are honest signals of individual quality pertaining to future reproductive value.