Sometimes women may implicitly or explicitly encourage such violent behavior on the part of the men, particularly when it is directed against outsiders. Ethnologist Charles Hose writes that, “The Iban women urge on the men to the taking of heads; they make much of those who bring them home, and sometimes a girl will taunt her suitor by saying that he has not been brave enough to take a head; and in some cases of murder by Sea Dayaks, the murderer has no doubt been egged on in this way.” Similarly, anthropologist Bruce Knauft notes that among the Asmat of New Guinea, “women disparaged and were loath to marry men who had not proven themselves by taking heads in warfare.” In The Cattle Brings Us To Our Enemies (2010), anthropologist J. Terence McCabe writes that, “Young women will sing songs about young men who are successful raiders, and such individuals often receive the benefits of their adulation.”

The pursuit of sex can be a motivator to engage in violence, and successful application of force to obtain social or political power may provide extensive reproductive benefits. In their paper ‘The evolutionary foundations of revolution’, sociologist Joseph Lopreato and F.P.A. Green write that, “ruling powers, including successful revolutionaries and their entourage, typically have sexual access to a disproportionate number of mates and therefore contribute an extraordinary number of offspring, legitimate or otherwise, to the population pool.” The reproductive benefits that can be conferred to males through conquest and the acquisition of power are also reflected in genetic data. In his book Who We Are and How We Got Here (2016), geneticist David Reich notes a common trend of sex-biased admixture across many human populations, writing that, “This pattern of sex-asymmetric population mixture is disturbingly familiar... the common thread is that males from populations with more power tend to pair with females from populations with less.”

Of course, while fitness considerations are key to understanding why sex should be such a prime source of conflict—as I discussed here, here, here, and here—there is also a social process at work that requires explanation: namely why sexual violations should be so commonly met with an act of revenge.

Anthropologists have long known that reciprocity is an important mechanism governing cooperation in many traditional societies. In his classic work The Gift (1925), Marcell Mauss writes that,

In the economic and legal systems that have preceded our own, one hardly ever finds a simple exchange of goods, wealth, and products in transactions concluded by individuals. First, it is not individuals but collectivities that impose obligations of exchange and contract upon each other. The contracting parties are legal entities: clans, tribes, and families who confront and oppose one another either in groups who meet face to face in one spot, or through their chiefs, or in both these ways at once. Moreover, what they exchange is not solely property and wealth, movable and immovable goods, and things economically useful. In particular, such exchanges are acts of politeness: banquets, rituals, military services, women, children, dances, festivals, and fairs, in which economic transaction is only one element, and in which the passing on of wealth is only one feature of a much more general and enduring contract. Finally, these total services and counter-services are committed to in a somewhat voluntary form by presents and gifts, although in the final analysis they are strictly compulsory, on pain of private or public warfare

Of course, there is a corollary to this, which is that the logic of reciprocity will also, on occasion, demand retribution. Thus, “To refuse to give, to fail to invite, just as to refuse to accept, is tantamount to declaring war; it is to reject the bond of alliance and commonality.” Anthropologist Ernst Halbmayer notes that among the Yukpa horticulturalists of northwest Venezuela, the various subgroups, “generally see each other as enemies... Their relations have been characterized by negative reciprocity, war and wife-stealing.”

Revenge and fights over women and sex are the prime motives for warfare across Amazonia. Anthropologists Robert S. Walker and Drew H. Bailey write that,

In order of importance, the tallied motives for killings (including multiple responses) were revenge for previous killings or other wrong-doings like adultery or sorcery (n=63 or 70% of responses), jealousy over women (n=16 or 18% of responses), gain of captive women and children (n=6 or 7% of responses), fear or deterrence of an impending attack (n=3 or 3% of responses), and lastly the theft of material goods (n=2 or 2% of responses).

In 54% of the external raids recorded in Walker and Bailey’s sample, at least one woman was captured.

This social logic of negative reciprocity, which may perpetuate long-running feuds between groups, also often leads men to take extreme measures to protect their honor and status within their group. In his book Blood Revenge (1984), anthropologist Christopher Boehm writes that, “To fail to retaliate homicidally in many contexts used to result in severe damage to one’s honor, in that the disapproval of the tribal moral community was so intense that it became almost intolerable.” Boehm noted that most feuding between families among the Montenegrins began with an insult to honor, and that the commonly noted causes are “abduction of a maiden to marry her, seduction of maidens, adultery, runaway wives, and a breach of betrothal agreements, as well as disputes over pastures.”

Anthropologist E. Adamson Hoebel writes that, “A Comanche male who had suffered a legal wrong was under social obligation to take action against the offender. For a man not to do so was not looked upon as an act of social grace; indeed, such behavior was a social disgrace.” As if to drive home what one of the most consequential wrongs a man could suffer was, Hoebel adds that, “Adultery and taking another's wife were direct attacks upon the prestige of the wife's husband. Both acts were unmistakable challenges which could not be ignored by the man who would maintain enough face to make life livable.”

In his book on the Yanomami, anthropologist Napoleon Chagnon gives an example of the way a man unable to protect his honor by force might expect to be treated in some societies;

Although Rerebawä has displayed his ferocity in many ways, one incident in particular illustrates what his character can be like. Before he left his own village to take his new wife in Bisaasi-teri, he had an affair with the wife of an older brother. When it was discovered, his brother attacked him with a club. Rerebawä responded furiously: He grabbed an ax and drove his brother out of the village after soundly beating him with the blunt side of the single-bit ax. His brother was so intimidated by the thrashing and promise of more to come that he did not return to the village for several days. I visited this village with Kaobawä shortly after this event had taken place; Rerebawä was with me as my guide. He made it a point to introduce me to this man. He approached his hammock, grabbed him by the wrist, and dragged him out on the ground: “This is the brother whose wife I screwed when he wasn’t around!” A deadly insult, one that would usually provoke a bloody club fight among a more valiant Yanomamö. The man did nothing. He slunk sheepishly back into his hammock, shamed, but relieved to have Rerebawä release his grip (Chagnon 31).

Of course, we can also think of the ways the logic of negative reciprocity may successfully prevent some conflicts. If you know with a great deal of certainty that any social violation on your part will be met with extreme retribution, you may be less likely to act out in the first place.

Absent social norms and institutions that enable effective conflict prevention and resolution, a never-ending game of tit-for-tat becomes an increasingly likely outcome.