The Dangerous Passion: Why Jealousy Is as Necessary as Love and Sex (4 page)

To produce a single child, women bear the burdens and pleasures
of nine months of pregnancy—an obligatory form of parental investment that men cannot
share. Men, to produce the same child, need only devote a few hours, a few
minutes, or even a few seconds. Wide is the gulf between men and women in the
effort needed to bring forth new life. Over time, therefore, a strategy of
casual mating proved to be more reproductively successful for men than for
women. Men who succeeded in the arms of many women out-reproduced men who
succeeded with fewer. An ancestral woman, in contrast, could have had sex with
hundreds of partners in the course of a single year and still have produced
only a single child. Unless a woman’s regular partner proved to be infertile,
additional sex partners did not translate into additional children. As a
consequence, men evolved a more powerful craving for sex with a variety of women.

This sex difference in desire creates an intriguing puzzle.
Sexual encounters require two people. Mathematically, the number of
heterosexual encounters must be identical for the sexes. Men cannot satisfy
their lust for sex partners without willing women. Indeed, men’s passion for
multiple partners could never have evolved unless there were some women who
shared that desire. Is casual sex a recent phenomenon, perhaps created by the
widespread prevalence of birth control devices that liberated women from the
previous risks of pregnancy? Or did ancestral women do it too?

Three scientific clues, when taken together, provide a
compelling answer. Men’s sexual jealousy provides the first clue, the ominous
passion that led us to this mystery. If ancestral women were naturally inclined
to be flawlessly faithful, men would have had no evolutionary catalyst for
jealousy. Men’s jealousy is an evolutionary response to something alarming: the
threat of a loved one’s infidelity. The intensity of men’s jealousy provides a
psychological clue that betrays women’s desire for men other than their regular
partners.

Second, affairs are known in all cultures, including tribal
societies, pointing to the universal prevalence of infidelity. Prevalence rates
vary from culture to culture (high in Sweden and low in China), but affairs
occur everywhere. Sexual infidelity causes divorce worldwide more than any
other marital violation, being closely rivaled only by the infertility of the
union. The fact that women have affairs in cultures from the Tiwi of northern
Australia to the suburbs of Los Angeles reveals that some women refuse to limit
themselves to a single partner despite men’s attempts to control them and
despite the risk of divorce if discovered.

A third line of evidence comes from new research on human sperm
competition. Sperm competition occurs when the sperm from two different men
inhabit a woman’s reproductive tract at the same time. Human sperm remain
viable within the woman’s tract for up to seven days, not merely one or two
days as scientists previously believed. Indeed, my colleagues have discovered
hundreds of “crypts” recessed within the vaginal walls of women in which they
store a man’s sperm and then release it several days later to enter a marathon
race to her egg. If a woman has sex with two men within the course of a week,
sperm competition can ensue, as the sperm from different men scramble and
battle for the prize of fertilizing the egg. Research on sperm competition
reveals that men’s sperm volume, relative to their body weight, is twice that
which occurs in primate species known to be monogamous, a clue that hints at a
long evolutionary history of human sperm competition.

Human sperm, moreover, come in different “morphs,” or shapes,
designed for different functions. Most common are the “egg getters,” the
standard government-issue sperm with conical heads and sinewy tails designed
for swimming speed—the Mark Spitzes of the sperm world. But a substantial
minority of sperm have coiled tails. These so-called kamikaze sperm are poorly
designed for swimming speed. But that’s not their function. When the sperm from
two different men are mixed in the laboratory, kamikaze sperm wrap themselves
around the egg getters and destroy them, committing suicide in the process.
These physiological clues reveal a long evolutionary history in which men
battled with other men, literally within the woman’s reproductive tract, for
access to the vital egg needed for transporting their genes into the next
generation. Without a long history of sperm competition, evolution would have
favored neither the magnitude of human sperm volume nor the specialized sperm
shapes designed for battle.

All these clues—the universality of infidelity, men’s sexual
jealousy, and the hallmarks of sperm competition—point to a disturbing answer
to the question of ancestral women’s sexual strategies. They reveal the
persistent expression of women’s passion for men other than their husbands, a
phenomenon that must have occurred repeatedly over the long course of human evolution.
Modern women have inherited this passion from their ancestral mothers.

Because scientists have focused primarily on the obvious
reproductive benefits of men’s desire for sexual variety, the potential
benefits to women of short-term sexual passion languished for years unstudied.
The puzzle is compounded by the fact that a woman’s infatuation with another
man comes laden with danger. An unfaithful woman, if discovered, risks damage
to her social reputation, the loss of her partner’s commitment, physical
injury, and occasionally death at the hands of a jealous man. Undoubtedly, many
women weigh these risks, and choose not to act on their sexual desires. The
benefits to women who do act on their passion for other men, given the possibility
of catastrophic costs, must be perceived as sufficiently great to make it worth
the risk.

For the past seven years, Heidi Greiling and I have been
studying why women have affairs. Our lab has focused on the benefits that are
so alluring that women from all walks of life are willing to take great risks
to pursue sex and love outside of marriage. Our research centered on three
questions: What
benefits
do women reap from affairs? What
circumstances
are most likely to drive a woman into another man’s arms? And
which women
are most prone to affairs?

Historically, women may have benefited from an affair in
countless ways. The first and most obvious benefit comes from the direct
resources that an affair partner may provide. A few expensive dinners may not seem
like much today, but an extra supply of meat from the hunt would have made the
difference between starving and surviving during ancestral winters when the
land lay bare, or between merely surviving and robustly thriving during more
plentiful times.

Women also can benefit from affairs in the currency of quality
genes. The puzzle of the peacock’s tail provided the telltale clue to this
benefit. A peahen’s preference for peacocks with brilliant plumage may signal
selection for genes for good health. When peacocks carry a high load of
parasites, their diminished health is revealed in duller displays. By selecting
for luminescence, peahens secure good genes for health that benefit their
offspring. Research by Steve Gangestad and Randy Thornhill of the University of
New Mexico reveals that women may be choosing affair partners with especially
healthy genes. Women who have sex with different men can also produce more
genetically diverse children, providing a sort of “hedge” against environmental
change.

Although genetic and resource benefits may flow to women who
express their hidden sexual side, our studies uncovered one benefit that
overshadowed the others in importance, a benefit we call “mate insurance.”
During ancestral times, disease, warfare, and food shortages made survival a
precarious proposition. The odds were not trivial that a husband would succumb
to a disease, become debilitated by a parasite, or incur injury during a risky
hunt or a tribal battle. The paleontological and cross-cultural records reveal
this clue—the skulls and skeletons show injuries mostly on males. A woman’s
husband, in short, stood a significant chance of suffering a debilitating or
lethal wound.

Ancestral women who failed to have mate insurance, a backup
replacement in the event that something happened to her regular partner, would
have suffered greatly compared to women who cultivated potential replacements.
Modern women have inherited the desires of their ancestral mothers for
replacement mates. In the words of one woman in our study, “Men are like
soup—you always want to have one on the back burner.” Mate insurance provides a
safeguard against reasonable risks of losing a partner.

And mate insurance remains relevant today, even though we’ve
conquered many of the hazards that felled our forebears. American divorce rates
now approach 67 percent for those currently getting married, up from the mere
50 percent figure that alarmed many over the past two decades. Remarriage is
rapidly becoming the norm.
The Dangerous Passion
explores how women’s
desire for additional partners is ancestral wisdom that, however alarming to
husbands, continues to serve a critical insurance function for women today.

Women’s attraction to lovers has another mysterious ingredient:
the puzzle of concealed ovulation. Unlike chimpanzees, women’s genitals do not
become engorged when they ovulate. Women have “lost estrus” and engage in sex
throughout their ovulatory cycle. Conventional scientific wisdom has declared
that a woman’s ovulation is cryptic, concealed even from the woman herself. But
have the urges associated with ovulation totally vanished?

In the most extensive study of ovulation and women’s sexuality,
several thousand married women were asked to record their sexual desires every
day for a period of twenty-four months. The methods were crude but
straightforward: women simply placed an
X
on the recording sheet on
each day that they experienced sexual desire. Basal body temperature was
recorded to determine the phase of the menstrual cycle. These thousands of data
points yielded a startling pattern. On the first day of a woman’s period,
practically no women reported experiencing sexual desire. The numbers rose
dramatically across the ovarian cycle, peaking precisely at the point of maximum
fertility, and then declining rapidly during the luteal phase after ovulation.
Women, of course, can experience sexual desire at any phase of their cycle.
Nonetheless, they are five times more likely to experience sexual desire when
they are ovulating than when they are not.

Women sometimes act on their passions. A recent survey of 1,152
women, many of whom were having affairs, revealed a startling finding. Women
who stray tend to time their sexual liaisons with their affair partners to
coincide with the peak of their sexual desire, when they are most likely to
conceive. Sex with husbands, in sharp contrast, is more likely to occur when
women are
not
ovulating, a strategy that may be aimed at keeping a man
rather than conceiving with him. None of this is conscious, of course. Women do
not think “I’ll try to time sex with my affair partner when I’m ovulating so
that I’ll bear his child and not my husband’s.” Psychologically, women simply
experience sexual desire more when they are ovulating, and if they have an
affair partner, have urges to have sex with him during this phase. Ovulation
may seem concealed to outside observers, but women appear to act on the
impulses that spring from it. And when that desire for men other than their
husbands occurs, it’s difficult for most men to tell when their mates are
straying or may be likely to stray. I call this the signal detection problem.

Across cultures, people have affairs that are specifically
designed to avoid detection. In Arizona, one motel marquee boasts that it is
the “No-Tell Motel.” In states across America, you can rent some hotel rooms at
an hourly rate. The woman returning from a business trip does not make her
brief fling on the road the first topic of conversation. The husband who
conceals his finances from his wife may be funneling resources to support a
mistress on the side.

Spouses experience a signal detection problem. Consider camping
in the woods at night and hearing a sound somewhere in the dark. Was that the
sound of a twig snapping, merely the wind blowing, or the unfamiliar night
sounds playing tricks on your ears? Assuming that you have correctly detected
the signal as a twig snap, the possible causes of this event are many, but they
are not infinite. It could be a rock that somehow got dislodged. But it could
also be a dangerous animal or a hostile human. The signal detection problem is
not merely about picking up accurate signals in the face of an uncertain and
ambiguous welter of information. It is also about making correct inferences
about the cause of the signal.

Since sexual infidelities are almost invariably secret, the
signals they might emit are intentionally muted. An unfamiliar scent, the
purchase of a sharp new jacket, the running of a yellow light, a new interest
in Beethoven or the Beastie Boys, an unexplained absence—all of these can be
signals, but they can originate from many causes other than infidelity. The
jealous person experiences an elevated sensitivity to signals of infidelity:
“He may see a red flush on his wife’s cheek, she may appear to be standing
awkwardly, or sitting sideways on a chair, she has put on a clean dress, there
is a cigarette-end in the fireplace . . . the jealous man sees a handkerchief
on the floor, a wet cloth in the bathroom, newspapers in a ditch, and attaches
to all the same import.”

Consider the case of a European psychiatrist who counseled many
couples referred to him in which one of the spouses experienced “morbid
jealousy.” Most cases were husbands who had delusions that their wives were
sexually unfaithful, and these delusions destroyed the fabric of trust required
for harmonious marriage. Because he believed that extreme jealousy was a
psychiatric illness that could not be cured, his most common recommendation was
that the couples separate or divorce. Many couples followed his
recommendations. Because he was keenly interested in the subsequent fate of his
patients, he routinely contacted them after a number of months had passed. To
his astonishment, he discovered that many of the wives of his patients had
subsequently become sexually involved with the very men about whom their
husbands had been jealous! Some of these women actually married the men who
were the objects of their husbands’ suspicions. In many cases, the husbands
must have been sensing signs of infidelity. But because the wives proclaimed
innocence and declared that their husbands’ jealousy was irrational, the
husbands ended up believing that the problem was in their heads. The problem of
signal detection is how to identify and correctly interpret a partner’s
betrayal in an uncertain social world containing a chaos of conflicting clues.