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Monogamous for the Moment: Science of Sex

Eliot Spitzer recently joined the long line of male politicians—Bill Clinton, Jessie Jackson, Newt Gingrich, Gary Hart, Larry Craig—that can’t seem to keep a certain body part in their pants. The perennial question asked of these cuckolds is, why’d he do it? But for the biologist, a more apropos inquiry might be when it comes to the science of sex why are we surprised that he did?

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Who’s Been Sleeping in My Bed?
After all, monogamy isn’t something normally observed in nature. Whether it’s polygny (more than one female at one time) or polyandry (more than one male) the animal kingdom is rife with examples of polygamy. Only about 3 percent of mammals mate for life, making monogamy the exception, not the rule.

Adultery is prevalent even in species once considered the stalwarts of monogamy. Many birds, for instance, practice social monogamy—they pair off and stay with one partner for extended periods, or even the course of a lifetime. This led observational scientists to believe that they did not have sex outside the couplet. However, the advent of genetic fingerprinting proved otherwise: the offspring’s DNA didn’t always match that of the male nest mate. Extra pair copulations (mating outside of the social pair) were unexpected and common. For birds and other animals, social monogamy does not necessarily mean sexual monogamy.

Mutual Philandering
So who’s cheating, and why? Originally, it was thought that in polygamous partnerships, it was the male who was doing all the philandering. In 1948, A.J. Bateman noted that the male fruit fly, Drosophila melanogaster, would mate whenever and with whomever possible; the female was discriminatory and bit less receptive to sex. This led him to conclude that because a male’s genetic lineage—sperm—was small, easy to make, and readily available, he could and would expend it whenever the mood struck. A female’s eggs, on the other hand, are large and biologically expensive to make; they needed to hold out for the best genetic partner possible. From an evolutionary standpoint, males benefited from numerous partners and were therefore naturally promiscuous; females did not benefit, making them naturally more sexually conservative.

This theory turned out to be wrong. Promiscuity is not limited to males, nor does it only benefit them. In her book Dr. Tatiana’s Sex Advice to All Creation, evolutionary biologist Olivia Judson writes, “From stick insects to chimpanzees, females are hardly ever faithful.” Rather, she asserts, females benefit from promiscuity. A greater number of partners mean higher rates of conception for rabbits and prairie dogs, a higher number of eggs for lizards, and more eggs fertilized for fish.

The point of all this bed jumping is, of course, reproductive fitness, making sure offspring get the best genes on the market. While both sexes may be promiscuous in an attempt to make this happen, there is a slight difference between males and females. Studies show that females mated with the most dominant or genetically successful male do not seek extra pair copulations—if they already have the best, why mess with rest? Males, however, will mate with females of varying status; they lean more toward spreading the seed than aiming high (note our politician’s choices, all of “lower” stature than their own: intern, aide, escort, bathroom stall mate, staffer).

If Mouse, Then Man?
But does this animal evidence tell us anything about human behavior? According to behavioral scientist David P. Barash and psychiatrist Judith Eve Lipton, it may give us clues to why so monogamy is hard. In their book Myth of Monogamy: Fidelity and Infidelity in Animals and People, the authors state that monogamy “goes against some of the deep-seated inclinations with which biology has endowed most creatures, including humans.”

In addition, the size difference between men and women indicates that monogamy isn’t necessarily what we’re programmed to do. In humans and other animals, if males are larger than females, it shows that men had to compete for women; the big and tall males won multiple mating opportunities. The degree of polygyny correlates with the size difference in sexes and since men are larger than women are, this indicates we have been mildly polygynous in our not too distant past.

Till Death Do Us Part?
Yet, socially, it appears humans favor monogamy. Helen Fischer, a professor of anthropology at Rutgers University looked at ninety-seven societies, and found that 92 percent of men and 93 percent of women marry, and most take only one spouse. There is even brain wiring that sets us up for this long-term attachment. While the first stages of romantic courtship involve lust, ruled by the powerful hormone testosterone, followed by love, ruled by the feel good hormones serotonin and dopamine, we eventually settle into long-term attachment. This is thought to be fueled by oxytocin and vasopressin, which make us feel connected to both mates and children.

This connection, however, may not last. Divorce rates peak at four years of marriage; Fischer theorizes four years was the time it took early humans to make sure a child was going to survive. In other words, some people and animals pair off for a convenient amount of time—breeding and rearing—but seem to diverge after that.

Of course, one of the things that makes us most distinctly human is our ability to rise above animal instincts. We’re not dogs humping legs at dinner parties; for the most part, we have rational thought and discretion on our side. Social monogamy is more than a biological pursuit; it is a complex union involving our brains and hearts. Sexual monogamy naturally follows from long term partnerships, lest our primal inclinations of jealousy, anger, and competitiveness overtake our relationships. For most humans, social and sexual monogamy are in fact, one in the same. And unlike the philandering bird or the indiscriminate primate, humans make verbal, legal, and social promises not to nest jump. Multiple mates may be a biological instinct, but it’s certainly no excuse.

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