Boy meets girl. Boy falls in love with girl. Boy courts girl. Boy marries girl. Ah, love is such a wonderful thing. Which begs the question: is love--or sexual desire in any form--a factor of society, or a factor of evolution?

Experts once believed that nurture, not nature, begets love, romance, jealousy, and sex. Emile Durkheim, the patriarch of modern sociology, claimed that such deeply felt emotions as sexual jealousy, a father's love of his child, or the child's love of the father are "far from being inherent in human nature." The new and emerging field of evolutionary psychology, however, tells us that love and sex are in our evolutionary history and our genes. Great is the power of nature over nurture.

Evolutionary psychology theorizes that behavior is mostly inherited and that every organism acts (consciously or not) to enhance its inclusive fitness--increasing the frequency and distribution of its genes in future generations. Those genes are shared in one’s identical twin (100%), siblings (on average, 50%), cousins (on average, 25%) and so on down the kinship line. (Helping your relatives now makes evolutionary, not just moral, sense.)

What does evolutionary theory predict you should expect from your mates? Differences between males and females are explicable in terms of differential parental investment; the male contribution to reproduction (lots of sperm and a few minutes of work) is cheap, short and pleasurable; the female reproductive experience, however is long, dangerous, and painful. The best way for a male to maximize his inclusive fitness is to diversify his genetic portfolio (play the field); while the best way for a female to insure the survival of the baby she has invested so much in is to try and get that one guy to make as many contributions as possible.

In The Evolution of Human Sexuality (p. 27, 1979), anthropologist Donald Symons elaborates on this theory’s consequences, in the light of evolutionary psychology's findings:

1. Men are inclined to multiple partners, whereas women are more flexible and may be equally satisfied in polygynous [one male--multiple females], monogamous, or polyandrous [one female--multiple males] marriages.
2. Men are much more likely to be sexually aroused by the sight of women and the female genitals than women are by the sight of men and the male genitals.
3. Men base a woman's sexual attractiveness on her physical characteristics, especially youth. Women, on the other hand, find political and economic prowess sexier in men; youth is relatively unimportant.

The evolution of desire may also determine why we look the way we do, and what turns us on. The human breast consists of fat, not milk glands, and breast size varies greatly among human females without affecting their ability to nurse. Thus, the explanation cannot be based on the need to nurse infants. Rather, human female breasts are secondary sexual characteristics that evolved to attract mates. According to Desmond Morris (The Naked Ape, 1967), this took place along with the switch from dogstyle to missionary mating, the pendulous shape and cleavage of the breasts mimicking the previous attractiveness of the female buttocks. This also explains why men find other pendulous shapes (like ear lobes) and other cleavages (like toes in low-vamped shoes) such a turn-on.

And while we're at it, what other female attributes turn men on? All together now--"big breasts, silky skin, red lips, long legs – all on a young, nubile babe." Evolutionary theory posits that these features have served as cues to a female’s reproductive and sexual viability over the course of time. In short, men have evolved to seek Porn Heaven--where "sex is sheer lust and physical gratification, devoid of more tender feelings and encumbering relationships, in which women are always aroused, or at least easily arousable, and ultimately are always willing" (Symons, p. 171). Evolution has insured the continuing success of the cosmetics, fashion, and pornography industries.

Finally, evolutionary psychology provides explanations for why we split up too. Monogamy is seen from the evolutionary perspective not as the norm but as the exceptional result of an increased level of social pressure. Anthropologist Helen Fisher has concluded that "human beings in a variety of societies tend to divorce between the second and fourth years of marriage, with a divorce peak during the fourth year" (The Anatomy of Love: The Natural History of Monogamy, Adultery, and Divorce. New York: Norton, 1992, p. 360). This four-year peak conforms to the traditional period between human successive births--four years. Like pair-bonding in species that mate only through a breeding season, human paired bonds originally evolved to last only long enough to raise a single dependent child through infancy, the first four years, unless a second child was conceived.

It seems to be a disappointment--that generations of love poems, songs, and epic plays all come down to evolution. But evolution’s conclusions on love and sex aren’t written in stone just yet. Harmon Holcomb, a philosopher of science at the University of Kentucky, skeptically examines the theories of evolutionary psychology and finds that for the most part, at this point, they are neither pseudoscience, nor hard science, but protoscience--science in the making. To become a true science, evolutionary psychology must put forth hypotheses that are capable of being critically disproven, rather than just reinforced or reconfirmed. Until that day comes, hang on to your copy of Shakespeare’s Love Sonnets.