"Long before it's in the papers"
January 27, 2015


Is warfare linked to evolution?

Dec. 30, 2014
Courtesy of Pro­ceed­ings of the 
Na­tio­n­al Aca­de­my of Sci­en­ces
and World Science staff

A­mong an east­ern Af­ri­can farm­ing peo­ple, males who par­ti­ci­pate in live­stock raids in youth may en­joy great­er long-term “re­pro­duc­tive suc­cess,” a study has found.

The re­sults could feed in­to a long­stand­ing de­bate over wheth­er war­fare is a prod­uct of ev­o­lu­tion. Ev­o­lu­tion­ary the­o­ry holds that char­ac­ter­is­tics of a spe­cies take root be­cause they en­hance sur­viv­al and reprod­uction. Over genera­t­ions, this causes ad­van­ta­geous fea­tures for an in­di­vid­ual to spread through­out a popula­t­ion, while un­help­ful char­ac­ter­is­tics die out.

In the new stu­dy, pub­lished in the jour­nal Pro­ceed­ings of the Na­tio­n­al Aca­de­my of Sci­en­ces this week, Luke Glo­wacki and Rich­ard Wrang­ham of Har­vard Uni­vers­ity traced the num­ber of wives and chil­dren of 120 male mem­bers of the pas­tor­al­ist Nyan­gatom peo­ple of Ethi­o­pia and South Su­dan. 

In the short term, the au­thors found, men who en­gaged in live­stock raids did­n’t have more wives or chil­dren than non-raiders. This sug­gests that cap­tured live­stock aren’t di­rectly used as lev­er­age for mar­riage op­por­tun­i­ties—rather they’re used by ex­ist­ing family mem­bers, the re­search­ers said.

On the oth­er hand, “elders who were iden­ti­fied as pro­lif­ic raiders in their youth have more wives and chil­dren than oth­er el­ders,” the au­thors wrote. “Our re­sults sug­gest that in this cul­tur­al con­text raid­ing pro­vides op­por­tun­i­ties for in­creased reprod­uctive suc­cess over the life­time.”

“The causes of war­fare in small-scale so­ci­eties con­tin­ue to be de­bated,” the au­thors added. “Most an­thro­po­log­i­cal ex­plana­t­ions have fo­cused on causes that ig­nore the in­di­vid­ual ben­e­fits war­riors some­times re­ceive for par­ticipa­t­ion.”

How­ev­er, they added, “ev­o­lu­tion­ary an­thro­po­l­o­gists have com­monly ar­gued that war­riors may re­ceive fit­ness ben­e­fits,” or ad­van­tages that en­hance their reprod­uctive op­por­tun­i­ties. “This hy­poth­e­sis has a con­ten­tious his­to­ry, in part be­cause of con­cerns that a pos­i­tive as­socia­t­ion be­tween war­fare and reprod­uctive suc­cess may sug­gest bi­o­log­i­cal” tend­encies to­ward vi­o­lence.

Glo­wacki and Wrang­ham al­so examined wheth­er raid­ing and reprod­uction among the Nyan­gatom might be only co­in­ci­den­tly re­lat­ed be­cause both are re­lat­ed to a third fac­tor—the man’s num­ber of old­er sib­lings, which may en­hance re­sources to lev­er­age mar­riage op­por­tun­i­ties. But the researchers con­clud­ed that this was­n’t the case.

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Among an eastern African farming people, male members who participated in livestock raids in youth may enjoy greater long-term “reproductive success,” a study has found. The results could feed into a longstanding debate over whether warfare among humans was a product of evolution. Evolutionary theory holds that characteristics of a species take root because they enhance survival and reproduction. Over generations, that causes advantageous features for an individual to spread throughout a population, while unhelpful characteristics die out. In the new study, published in the journal pnas this week, Luke Glowacki and Richard Wrangham of Harvard University traced the number of wives and children of 120 male members of the pastoralist Nyangatom people of Ethiopia and South Sudan. In the short term, the authors found, men who engaged in livestock raids didn’t have more wives or children than non-raiders. This suggests that captured livestock aren’t directly used as leverage for marriage opportunities—rather they’re used by existing family members, the researchers said. On the other hand, “elders who were identified as prolific raiders in their youth have more wives and children than other elders,” the authors wrote. “Our results suggest that in this cultural context raiding provides opportunities for increased reproductive success over the lifetime.” “The causes of warfare in small-scale societies continue to be debated,” the authors wrote. “Most anthropological explanations have focused on causes that ignore the individual benefits warriors sometimes receive for participation.” However, they added, “evolutionary anthropologists have commonly argued that warriors may receive fitness benefits,” or advantages that enhance their reproductive opportunities. “This hypothesis has a contentious history, in part because of concerns that a positive association between warfare and reproductive success may suggest biological proclivities to engage in violence.” The authors also studied whether raiding and reproduction among the Nyangatom might be only coincidentally related because both are related to a third factor—the man’s number of older siblings, which may enhance resources to leverage marriage opportunities—but concluded that this wasn’t the case.