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 partnerís sexual, rather than emotional infidelity, whereas women showed the reverse response displaying greater distress to a partnerís emotional infidelity rather than their sexual infidelity. Physiological measures of autonomic arousal corroborate the subjectís 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 partnerís youth and physical attractiveness. In fact, the peak levels of mate retention also correlated to the peak reproductive years for women in general. Partnerís age and physical attractiveness did not effect womenís mate retention; for women, their partnerís 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.