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Genes may help predict infidelity, study reports

Nov. 30, 2006
Special to World Science  

The chance that in­fi­del­i­ty will in­trude on a ro­man­tic re­la­tion­ship may be part­ly writ­ten in the cou­ple’s genes, a stu­dy has found. The re­sults sug­gest a DNA test could tell a man the rough chances his fe­male part­ner will cheat on him, though it would­n’t nec­es­sar­i­ly work the op­po­site way.

The In­fi­del­i­ty dis­cov­ered by Au­gus­tus Le­o­pold Egg, 1858. A wife col­lapses af­ter her hus­band finds a letter re­veal­ing be­tray­al. (Tate Gal­lery, Lon­don)


The study found that wom­en act less pas­sion­ate­ly toward—and are like­li­er to cuck­old—part­ners who share genes with them in a spe­cial part of the ge­nome. This may in part re­flect an ev­o­lu­tion­ary mech­a­n­ism to re­duce in­breed­ing, the in­ves­ti­ga­tors spec­u­lat­ed.

In­fi­del­i­ty touches about half of all coup­les, mar­ried or not, ac­cord­ing to Not Just Friends, a 2002 book by psy­chi­a­trist and in­fi­del­i­ty re­search­er Shir­ley Glass. And last year, sci­ent­ists re­ported that one in 25 dads may be un­know­ing­ly rais­ing ano­ther man’s child. 

In the new stu­dy, re­search­ers with the Uni­ver­si­ty of New Mex­i­co, Al­bu­quer­que, fo­cused on a set of genes that past stud­ies have im­p­li­cat­ed in a link be­tween sex­u­al at­trac­tion and ge­net­ic sim­i­la­ri­ty. 

The clus­ter of genes is termed the ma­jor his­to­com­pat­i­bi­li­ty com­plex, or MHC. The genes, on hu­man Chro­mo­some 6, are in­volved in im­mune re­sponses. The study is the first “to test the hy­poth­e­sis that MHC sim­i­larity pre­dicts as­pects of ac­tu­al hu­man sex­u­al re­la­tion­ships,” the re­search­ers wrote. The find­ings ap­peared in the Oc­to­ber is­sue of the re­search jour­nal Psy­cho­log­i­cal Sci­ence.

MHC genes pro­duce mo­le­cules that en­a­ble cells to rec­og­nize in­tru­sive par­a­sites. The mo­le­cules and the genes are ex­treme­ly di­verse and fast-evolv­ing. Bi­ol­o­gists think these traits may be ev­o­lu­tion­ary mech­an­isms to help or­gan­isms stay a step ahead in the arms race with par­a­sites. 

This may al­so ex­plain past stud­ies sug­gesting that hu­mans and an­i­mals pre­fer mates with dissi­m­i­lar MHC genes, ac­cord­ing to some sci­en­tists. Such a pre­ference might help as­sure that off­spring have a wide range of im­mu­ni­ty genes in the hol­ster, giv­ing them an edge over pa­th­o­gens.

Studies have even point­ed to a pos­si­ble route by which peo­ple sub­con­scious­ly as­sess potential mates’ MHC com­pat­i­bil­ity: smell. In the mid-1990s, re­search­ers found that peo­ple sniff­ing T-shirts worn by oth­ers tended to pre­fer the odor of those whose wear­ers were least like them in this ge­netic re­gion.

Sev­er­al years lat­er, sci­en­tists linked si­m­i­lar pre­ferences to sex­u­al fi­del­i­ty in birds. Il­le­git­i­mate chicks in three spe­cies of typ­i­cal­ly mo­nog­a­mous shore­birds showed up most­ly in the nests of gen­ti­cally si­m­i­lar par­ents, in­ves­ti­ga­tors found—al­though it was­n’t clear wheth­er the ma­jor his­to­com­pat­i­bi­li­ty genes spe­ci­fi­cal­ly played a role. The study ap­peared in the Oct. 10, 2002 is­sue of the re­search jour­nal Na­ture.

The new study ech­oes el­e­ments of both pre­vi­ous stud­ies. 

The researchers stud­ied 48 male-fe­male cou­ples who were ei­ther dat­ing “ex­clu­sive­ly,” by their de­scrip­tion, or mar­ried or liv­ing to­geth­er.

As the pro­por­tion of MHC genes the cou­ple shared in­creased, “wom­en’s sex­u­al re­spon­siv­ity to their part­ners de­creased, their num­ber of [out­side] sex­u­al part­ners in­creased, and their at­trac­tion to men oth­er than their pri­ma­ry part­ners in­creased,” the re­search­ers wrote in a pa­per de­scrib­ing their find­ings. 

Two quantities were almost equal on av­er­age, ac­cord­ing to Chris­tine Garver-Apgar, the study’s lead au­thor: the frac­tion of MHC genes shared, and the wo­man’s num­ber of extra part­ners. In oth­er words, if the man and woman had half the genes in com­mon, the woman would have on av­er­age near­ly half a lov­er on the side. 

But these ten­den­cies were found on­ly for wom­en; men’s at­trac­tion and like­li­hood of cheat­ing ap­peared un­re­lat­ed to the genes, the re­search­ers wrote. Nor did these mol­ecu­lar sta­tis­tics seem to af­fect other aspects of re­la­tion­ships. “MHC shar­ing,” the sci­ent­ists wrote, “does not broad­ly pre­dict re­la­tion­ship sa­tis­fac­tion.”


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The chance that infidelity will intrude on a romantic relationship may be partly written in the couple’s genes, researchers have found in a new study. The results suggest DNA tests might reveal the approximate likelihood that a man’s female partner will cheat on him, though not necessarily the other way around. The study found that women are less fond of men whose genes are similar to their own in a specific region of the genome. And if romantically involved with such men, women seem more prone to cheat on them than on other partners. The tendency may in part be an evolutionary mechanism to forestall inbreeding, the investigators speculated. The research focused on a set of genes implicated through past studies in a link between sexual attraction and genetic similarity. The genes are termed the major histocompatibility complex, or MHC, a cluster of genes on human Chromosome 6. These produce molecules that aid in immune responses. It was the first “to test the hypothesis that MHC similarity predicts aspects of actual human sexual relationships,” the researchers wrote. The findings appeared in the October issue of the research journal Psychological Science. The genes they studied molecules that enable cells to recognize intrusive parasites. These molecules and genes are noted for being highly diverse. Biologists think this diversity may be an evolutionary mechanism to help organisms stay a step ahead in the arms race with parasites. This may also explain past studies suggesting that humans and animals prefer mates with dissimilar MHC genes, according to some scientists. Such a preference might help assure that offspring have a wide range of immunity genes in the holster, giving them an edge over pathogens. People’s odor, studies have found, may help them subconsciously judge other people’s genetic compatibility with them with respect to these genes. In the mid-1990s, researchers found that people smelling T-shirts worn by others tended to prefer the odor of those whose wearers were most unlike them in the major histocompatibility genes. Several years later, scientists found evidence that similar preferences influence sexual fidelity in birds. “Illegitimate” chicks in three species of typically monogamous shorebirds showed up mostly in the nests of gentically similar parents, the investigators found, although it wasn’t clear whether the major histocompatibility genes played a role in this case. The study appeared in the Oct. 10, 2002 issue of the research journal Nature. The new study echoes elements of both previous studies. The researchers with the University of New Mexico, Albuquerque, studied 48 male-female couples who were either dating “exclusively,” by their description, or married or living together. As the proportion of the genes the couple shared increased, “women’s sexual responsivity to their partners decreased, their number of [outside] sexual partners increased, and their attraction to men other than their primary partners increased,” the researchers wrote in a paper describing their findings. They did not specify, though, how many additional partners were associated with a given increase in genes shared. Attempts to reach members of the research group for comment were unsuccessful. The researchers did write that the findings were all statistically significant—with less than 5% probability that such results would occur by chance—and the results were even stronger when women were ovulating. However, these tendences were found only for women: men’s likelihood of cheating appeared to be unrelated to the similarity of the genes.