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The Evolution of Jealousy

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  L ove wasn’t the only thing in the air onValentine’s Day 2003. A Texas jury had justfound C. Harris guilty of killing her husbandin a “sudden passion.” After encounteringhim at a hotel with a mistress, she had driventhe car over his body again and again. As oth-ers were exchanging tokens of love, the “Mer-cedes murderer” was sentenced to spend 20years in jail.Clara Harris was hardly the first woman tostand accused of murdering in a jealous rage.In various studies, jealousy is often rankedamong the top three motives for nonaccidentalhomicides where motive is known—alongwith rage arising from a quarrel and murderduring the commission of a crime. Across theages the confounding power of sexual jeal-ousy has inspired poetry, novels, drama, artand opera. It has also captured the attention of psychologists, who have used a variety of the-oretical approaches in their pursuit of scien-tific understanding.Early work focused on Freudian interpreta-tions, the influence of which can still be seen inthe psychiatric literature. As in other domainsof psychology, however, recent research hasfollowed a rather different direction. For someyears now, a small but persistent group of in-vestigators has attempted to uncover the na-ture and srcin of this painful and dangerouscounterpart of romantic love. Most of us know jealousy from experienceas a deeply negative emotion that arises whenan important relationship is threatened by a ri-val. Given the inherent intricacies of social re-lationships, a simple theory that adequatelycaptures the complexity of jealousy is unlikely.Hence research has focused on the interplay between social and cognitive factors in the in-cidence and expression of this emotion. Somepsychologists have explored cultural differ-ences and have found that jealousy is morepronounced in cultures that attach social im-portance to marriage and sanction sexual grat-ification only in the marriage bed and in cul-tures that place a premium on personal prop-erty. Others have tried to account for whysome individuals show strong jealousy at theslightest provocation and others seem less sus-ceptible. (Factors examined in these studiesrange from personality and parental-attach-ment styles to who has the better “deal” in arelationship.) One relatively straightforward idea about jealousy in romantic relationships is at the cen-ter of a hot debate among psychologists. Dur-ing the 1990s, as evolutionary psychologists began applying Darwin’s theories to human behavior in novel ways, a new theory of thesrcins of jealousy developed. Jealousy, it wassuggested, might have given a fitness advan-tage to men and women in our ancestral envi-ronment. But the selective pressures on malesand females struggling to survive and repro-duce in this environment were asymmetrical.Thus jealousy, like many of the emotions asso-ciated with mating, came to have a differentcharacter in men and women. The notion that jealousy evolved into an “innate module”—awired-in brain circuit that has different prima-ry triggers in men and women—is one of themost celebrated applications of an evolution-ary approach to psychology.Debate over this hypothesis continues. Infact, newer evidence raises questions aboutwhether there is a fundamental difference be-tween the male and the female experience of  jealousy in romantic relationships. After re-viewing the evidence in light of other theories,I believe an evolutionary explanation for thisemotion may turn out to be more subtle andcomplex than the recent view suggests. Jeal-ousy could certainly be an innate and adaptiveemotion, but its form may be better explained by social-cognitive approaches, as well as de-velopmental theory, than by theories based onproposed sex differences in our ancestors’ mat-ing strategies. 62American Scientist, Volume 92© 2004 Sigma Xi, The Scientific Research Society. Reproductionwith permission only. Contact perms@amsci.org. The Evolution of Jealousy  Did men and women, facing different selective pressures, evolve different“brands” of jealousy? Recent evidence suggests not Christine R. Harris Christine R. Harris receivedher Ph.D. in psychology in1998 at the University of California, San Diego, whereshe is currently assistant pro- fessor of psychology. Her pri-mary research interests are inhuman emotion, including theeffects of emotion on cognitive processes, social influences onemotional reactivity and specif-ic emotions—particularly jeal-ousy, embarrassment andhumor. Address: Department of Psychology-0109, UCSD, La Jolla, CA 92093-0109. Internet:charris@ucsd.edu  2004 January–February 63www.americanscientist.org Figure 1. Long a popular theme in literature, art, music and drama, jealousy is today the subject of debate among psychologists. An archetypeof female jealousy is the mythological character of Hera, wife of Zeus. After Semele was impregnated by the philandering Zeus, Hera set in mo-tion a series of events leading to Semele’s death by fire. As his mother was consumed by the flames, Dionysus (Bacchus) was born. Hera isshown observing the unfolding scene from above in the engraving The Birth of Bacchus  , made around the beginning of the 19th century by Jean-Baptiste Patas, after a 16th-century painting by Jules Romain (Guilio Romanus). Evolutionary psychologists have proposed that female jealousyis triggered by a mate’s emotional infidelity, whereas male jealousy is triggered by sexual infidelity. The author finds the evidence supportingthis view to be equivocal at best.    H  a  r  g  r  e   t   t   R  a  r  e   B  o  o   k  a  n   d   M  a  n  u  s  c  r   i  p   t   L   i   b  r  a  r  y ,   U  n   i  v  e  r  s   i   t  y  o   f   G  e  o  r  g   i  a © 2004 Sigma Xi, The Scientific Research Society. Reproductionwith permission only. Contact perms@amsci.org.  The Specific Innate Module Theory As I mentioned above, evolutionary psycholo-gists seek to explain the peculiarities of humanpsychology in terms of the selective pressuresthat operated on our ancestors in the Pleis-tocene Epoch—the pressures that determinedwhose progeny survived and whose did not.The wired-in emotions we have now, thesewriters maintain, are not necessarily ones thatincrease our inclusive evolutionary fitness (thesurvival of our genes) today. But they tendedto provide a payoff in the very different envi-ronment of our forebears. This framework has been used in attempts to understand such di-verse features of human nature as pregnancysickness and depression. According to David Buss at the Universityof Texas at Austin and several other evolu-tionary psychologists, a specific set of braincircuits guides our emotional reaction tothreats in the context of sexual relationships.This emotional-cognitive module, they argue,makes men innately predisposed to jealousyover a mate’s sexual infidelity. It makes womeninnately predisposed to jealousy over a mate’s emotional infidelity.This difference in the sexes’ response to spe-cific triggers is present now, according to thesetheorists, because people once faced differentinclusive fitness risks during the Pleistocene.According to the theory of natural selection,mutations that increase fitness are favored andsurvive, because future generations inheritthese mutations from successful individuals.Ancestral man purportedly faced a graveDarwinian threat from cuckoldry—a result of the fact that eggs are fertilized internally andpaternity is always somewhat uncertain.Should a man’s mate be impregnated by an-other man, he might easily expend his scarceresources on genetically unrelated children,thus making his own Darwinian fitnessplunge. Hence, natural selection shaped themale brain to respond specifically to sexual in-fidelity with intense jealousy—an emotion thatwould motivate actions to defend againstcuckoldry.Ancestral woman, knowing that she was themother of her children, faced no such risk, andthus was not under the same selection pres-sure to respond to sexual infidelity. Rather, shefaced the threat that a philandering mate mightdivert his resources to another woman and herchildren. Because human children requireyears of care, these resources were supposedlycritical to her inclusive fitness. Therefore, ac-cording to the theory, women developed an in-nate psychological module that is particularlysensitive to emotional infidelity (the assump-tion here being that men expend resources onthe women they love). This evolutionary theory of jealousy has re-ceived a great deal of general media attentionin the past few years . However, as this articlewill show, there are other accounts that would be entirely consistent with evolution by natur-al selection. Self-Report Studies The innate-module theory has come to have astrong following among psychologists largely because of an outpouring of research that relieson self-reports of college students. Subjects areasked to imagine a romantic relationship inwhich their partner is either having sex withsomeone else or is falling in love with someone 64American Scientist, Volume 92 sexual infidelityversus deep emotional infidelitysexual infidelity versuslove infidelity   p  e  r  c  e  n   t  a  g  e  r  e  p  o  r   t   i  n  g  m  o  r  e   d   i  s   t  r  e  s  s  r  e   l  a   t  e   d   t  o  s  e  x  u  a   l   i  n   f   i   d  e   l   i   t  y 506040302010070 malefemale   n  u  m   b  e  r  o   f  s   t  u   d   i  e  s 10128642014 difference between men and women (percent)difference between men and women (percent) 0–9 10–19 20–29 30–39 40–49 50–59 60–69 Figure 2. Jealousy’s nature has been examined through forced-choice studies in which subjects describe their responses to imaginary scenariosinvolving a mate’s infidelity. Imagined sexual infidelity might be expected to trigger jealousy in men because jealousy provided a payoff to an-cestral man, whose inclusive fitness was threatened by cuckoldry. In contrast, ancestral woman might have responded more strongly to emo-tional infidelity because it implied a threat to the resources needed for the care of her children. When David Buss and his colleagues asked 202college students for their reactions to infidelity scenarios involving sex, “deep emotional attachment” and “falling in love,” their responses fitthe predicted pattern (left) . Since that 1992 study, more than two dozen similar studies have been published. The difference between the pro-portions of heterosexual men and women choosing sexual infidelity as the stronger trigger has ranged in these studies from 8 percent, in a studyof Austrian adults, to 69 percent, in a United States college-student sample (right) . Only one study found a reverse effect: Sexual infidelity wasof greater concern to Dutch lesbians than © 2004 Sigma Xi, The Scientific Research Society. Reproductionwith permission only. Contact perms@amsci.org.  else, and then are required to choose which of the two types of infidelity would be more up-setting to them.This forced-choice method was first de-signed by Buss and his colleagues in 1992 andhas since been used in more than two dozenstudies. In the United States, the method al-most invariably produces a significant sex dif-ference: Most women (usually 70 percent ormore) indicate that emotional infidelity would be more disturbing, whereas more men (usual-ly between 40 and 60 percent) report that sexu-al infidelity would be worse. Recently I conducted an integrative review(or “meta-analysis”) surveying these data. Ifound that the sex effect is robust and moderatein size but tends to be smaller among older sub- jects or in samples that include homosexuals.A sex effect has also been found in samplesfrom other countries. However, in comparisonto their counterparts in the U.S., far fewer Eu-ropean or Asian men seem to choose sexual in-fidelity as worse (often as few as 25 to 30 per-cent). This is an effect of culture comparable insize to the effect of sex. Some of us have pointed out that such sexdifferences need not reflect innate modules.One possibility is that men and women maysimply draw different conclusions about thehypothetical infidelity and what other un-pleasantnesses it would likely imply. These in-ferences, then, produce the sex effect on theforced-choice scenario. According to one view(nicknamed the “double-shot” or “two-for-one” hypothesis), men tend to think sexual in-fidelity would be more distressing becausethey infer that if a woman has sex with anotherman, she is probably also in love with him.Women tend to believe that men can have sexwithout being in love. Hence, sexual infidelitydoes not necessarily imply emotional infidelity.Instead, women reason that a man in love islikely to be having sex, and therefore theychoose emotional infidelity as worse. Evidence supporting this explanation forthe sex difference in answers to the forced-choice question is somewhat mixed. Some in-vestigators have confirmed that these differ-ences in inferences mediate sex differences onforced-choice jealousy measures; others havenot found support. Thus, such inferencesprobably play some role but cannot complete-ly account for the difference.David DeSteno and his colleagues at North-eastern University used an additional ap-proach to exploring the causes of sex differ-ences on the forced-choice question. Theyreasoned that if sex differences reflect wired-in,sex-specific evolved modules, then deprivingpeople of the opportunity to reflect on thechoice should increase the sex difference, po-larizing the responses of men and women.They imposed a “cognitive load manipulation”on study subjects by asking them to remembera string of seven digits while answering ques-tions. The cognitive load did not change males’responses, but females’ responses shifted to-ward picking sexual infidelity as the morepowerful jealousy trigger. This suggests thatfemales’ responses to the forced-choice ques-tions may reflect inferences, along the lines of the double-shot hypothesis, or self-presenta-tion strategies—the natural tendency of sub- jects to give answers that present a desired im-pression of themselves.A number of investigators have tried pre-senting sexual and emotional infidelity scenar-ios separately, and assessing jealousy reactionswith continuous rating scales rather than theforced-choice measure. Curiously, this tends toget rid of the predicted sex differences alto-gether, and on occasion it has even shiftedthem in the opposite direction (with womenreporting stronger reactions to sexual jealousy).All in all, then, the forced-choice questionclearly reveals some sort of sex difference, andone superficially in line with the evolutionaryanalysis. However, it is far from clear that it re-ally reflects any sort of innate bias of the sortthat evolutionary psychologists have pro-posed, rather than some other, more cognitive-ly sophisticated kind of difference. Physiological Measures of Jealousy Scientifically minded readers will be wonder-ing whether this question couldn’t be studiedin a way that circumvents the issues raised byself-report measures. Buss and his colleagueshave used measures of autonomic nervous sys- 2004 January–February 65www.americanscientist.org© 2004 Sigma Xi, The Scientific Research Society. Reproductionwith permission only. Contact perms@amsci.org. no loadload   p  e  r  c  e  n   t  c   h  o  o  s   i  n  g  s  e  x  u  a   l   i  n   f   i   d  e   l   i   t  y  a  s  m  o  r  e  u  p  s  e   t   t   i  n  g 5060403020100701008090 malefemale Figure 3. To help determine whether the sex differ-ences in Figure 2 reflected “wired-in,” innate modulesof male and female jealousy, David DeSteno and hiscolleagues imposed a cognitive load. Subjects wereasked to retain in memory a string of seven digitswhile answering questions. The load had no effect onmales’ responses, but females’ responses shifted to-ward picking sexual infidelity as the more powerfuljealousy trigger. This shift suggests that women’s re-sponses to forced-choice scenarios may reflect infer-ences or self-presentation strategies.
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