The sociologist, Davis (1948) defined jealousy as a fear and rage reaction fitted to protect, maintain, and prolong the intimate association of love. In a pair-bonding species like our own that lives in social groups, jealousy is a logical prediction from evolutionary theory. In fact, if jealousy did not exist as a universal human characteristic, it would represent an oddity that demanded scientific explanation.
The function of jealousy is
somewhat different between the two sexes. In males, jealousy revolves around
the issue of uncertainty of paternity. Whereas women have always known if an
infant is hers or not, until the advent of modern DNA testing techniques men
could never be certain that a child was the product of their loins.
Although paternal
uncertainty is a problem in all primate species, true jealousy may be unique to
the evolution of the human line. The fossil record indicates that our
Australopithecine ancestors were probably polygynous based on the fact that
adult males were much larger than adult females. Polygamy is a general term
referring to one individual with many mating partners. Polygyny refers to one
male with many females mating partners and polyandry refers to one female with
many male partners. Monogamy refers to a one to one mate pairing of male and
female. In a polygynous species, where one male monopolizes the reproduction of
many females, the males are typically much larger than the females (Alexander,
Howard, Noonan, & Sherman, 1979). This is because males compete directly
with other males for control of a harem of females. For example, a male
silverback gorilla may weigh about 450 pounds while the adult females in his
harem weigh an average of about 200 pounds. The monogamous gibbon species are
virtually identical in body size between male and female. In these animals the
pair-bonded adults and their offspring vigorously defend their territory
against the intrusion of any other members of their species. Male and female
chimpanzees are also very similar in body size, but their mating system is of a
promiscuous nature. When a female chimpanzee goes into estrous, she is mated
with by numerous male partners. Male chimpanzees do not appear to display any
kind of behavior resembling human sexual jealousy. Male chimps do compete with
each other but mainly for dominance status, not directly for access to females.
In chimpanzees mating competition occurs at the level of the sperm, which will
be discussed in the next section.
Jealousy probably arose from
other drives involving the proprietary defense of resources. Three factors that
lead to the evolution of jealousy in the human line were: 1) group living, 2)
pair-bonding, and 3) gender-based division of labor. Living in social groups is
a trait that we share with our closest living relatives, the chimpanzees.
Therefore, it was probably displayed by our common ancestor 5 to 7 million
years ago. Group living serves a protective function and helps facilitate the
procurement of resources. Because of the prolonged dependency and vulnerability
of hominid infants, and possibly because of the harsh conditions of the Ice
Ages, pair-bonding became increasingly important to the survival of our
ancestors. Infant survival may have been highly tenuous without the
provisioning provided by a pair-bonded male. A pair-bonding system meant that
males were investing a great deal of energy into one particular female. Because
the pair-bonded male lived in a social group there was always the possibility
that the child or children that he was investing in were not his own. This
problem is further exacerbated by a gender-based division of labor. For tens of
thousands, if not hundreds of thousands of years, humans have followed a
hunter-gatherer lifestyle. Gathering is typically done by females within a
fairly close proximity of the base camp; whereas hunting is a male activity
which may involve great distances and days or weeks of being away. The chronic
separation between pair-bonded males and females necessitated by this division
of labor provided ample opportunity for infidelity. Any hominid males lacking a
tendency toward jealousy may have spent all of their time rearing the offspring
of other males and genes coding for such tendencies were long ago eliminated.
Similarly, female hominids who failed to show a jealous response to their
pair-bonded male being involved with other females, were likely to lose that
pair bond and consequently their offspring were at risk for perishing.
Based on evolutionary logic,
it was predicted that male jealousy would be more concerned with sexual
infidelity and female jealousy would be more concerned with emotional
infidelity. Buss, Larson, Westen, & Semmelroth (1992) used a series of
forced choice experiments to demonstrate that men indicated greater distress to
a partner’s sexual, rather than emotional infidelity, whereas women showed the
reverse response displaying greater distress to a partner’s emotional
infidelity rather than their sexual infidelity. Physiological measures of
autonomic arousal corroborate the subject’s self-reported weighting of these
different conditions of fidelity. For example, when men were asked to imagine
either the scenario of their significant other being engaged sexually with
another partner, or emotionally with another partner, their heart rate and
galvonic skin responses were greatly elevated by the idea of sexual infidelity
much more so than the idea of emotional infidelity. In interviews with men and
women, sexual involvement with another party was the most mentioned situation evoking
jealousy among men. But, in women their partner spending time socially with
another party was the most frequently mentioned cause of jealousy (Francis,
1997). A cross-cultural comparison of the Netherlands, Germany, and the United
States made the same finding: that men find sexual infidelity a more salient
trigger for jealousy and women find emotional infidelity a stronger trigger
(Buunk, Angleitner, Oubaid, & Buss, 1996).
Once the emotion of jealousy has been triggered in an individual, it elicits any of a number of behaviors that can be classified under the category of mate guarding or mate retention. The most overt forms of mate guarding include physical intimidation and violence directed toward the perceived rival or toward the mate. Much more subtle forms of mate guarding would be tactics such as simply making oneself more attractive to the mate or acting in a subordinate fashion to the mate and promising to do better in the future. Buss & Shackelford (1997) investigated a number of mate retention tactics used by married couples. They found that for men, the number of acts of mate retention was positively correlated with their partner’s youth and physical attractiveness. In fact, the peak levels of mate retention also correlated to the peak reproductive years for women in general. Partner’s age and physical attractiveness did not effect women’s mate retention; for women, their partner’s income and level of ambition was positively correlated with their level of mate guarding. Women responded to perceived competition for their mate by striving to enhance their physical appearance and by telling others that their man was committed to them in a long-term relationship. Men were likely to respond to the perceived threat posed by a rival by increasing their display of resources. Men were also more likely than women to use submission to their partner as a mate retention tactic. Men were much more likely than women to use threats or physical violence to deal with perceived rivals, and the likelihood of this was highly correlated with the attractiveness and youthfulness of their mates.