While fashions in female beauty can vary across culture and throughout history, there are certain constant features that are attractive to men. Signs of youth, health, and fertility always form the basis of notions of female beauty. For example, round breasts and a low waist-to hip ratio are universal signs of femininity and fertility. Smooth, unwrinkled skin and the lack of gray hair signify youth. The beauty industry makes an enormous amount of money enhancing women’s signals of fitness and fertility. By enhancing the red color in her lips and cheeks, a woman can appear to have greater blood flow to her face, a sign of physical health and vigor. Likewise, millions of women dye their hair to maintain a youthful appearance.

The anthropologist Helen Fisher has conducted a series of studies on the neurobiology of love. In a 1998 article, she proposed a three-part system. This system is composed of the sex drive, attraction, and attachment. The sex drive is associated with sex hormones, specifically estrogens and androgens. Attraction is similar to romantic infatuation. This euphoric preoccupation with the desired mate is mediated by two kinds of neurotransmitters, dopamine and norepinephrine. Dopamine is associated with the reward circuitry and is involved with many kinds of desire. Norepinephrine is involved with arousal, as well as attention and focus. Attachment is associated with the hormones oxytocin and vasopressin.

These hormones, known as neuropeptides, are evolutionarily ancient chemicals that are involved in a broad range of social behaviors. Oxytocin is released during nursing and childbirth and is also released in both males and females during sexual intercourse and after orgasm. Dr. Fisher speculates that these three systems are relatively independent of each other. Combining these systems in different ways allows human beings to engage in a broad repertoire of mating and reproductive strategies. In other words, people can highlight sex, attachment, or infatuation at different times or with different people.