what drives competition for valuable resources and what determines the criteria for dominance status. Biologists define the sexes based on differences in the gonads, the reproductive organs that generate eggs and sperm, and the corresponding effects of sex-specific selection on the mind, body, and behavior. Females are those with larger, more costly gonads, where cost is defined on the basis of how much energy is invested in production. Think eggs versus sperm. This difference sets up an immediate competition, especially for species that have parental care. Once you invest in a big expensive egg, you don’t want to lose your investment. You want to protect it, avoiding harm and minimizing risk. On the other hand, if your investment is small, you are not only freer to take risks, but favored to do so. These ideas started with Charles Darwin. One hundred years later, they were developed in exquisite detail by the American evolutionary biologist Robert Trivers. Combined, they provide an explanation for why, in most species including our own, males compete with each other for access to females — the most valuable and limited resource — and why females are picky, expressing an aesthetic preference for males of a particular quality. Selection favors parts of the body and brain associated with dominant males and picky females. Dominant males win fights against other males, and thus gain access to females. Dominant males take risks and are more aggressive. Picky females hold out for the best males, those who provide the most desirable resources. Picky females are patient, waiting for males with good genes, access to prime real estate, and the protective skills and motivation to defend them and their young. These are qualities linked to high status. These are qualities associated with the ability to obtain and control resources. These are the qualities that females desire. Male desire for dominance is therefore nurtured by female desire for dominant males. Like appetites, t