Human violence may not be as “natural” or as fixed in evolution as some long-standing theories suggest, according to new research that argues aggression is not one simple trait. Instead, researchers found that mild aggression and deadly violence follow different patterns across primates, including humans.
The study, led by Bonaventura Majolo of the School of Psychology at the University of Lincoln in the United Kingdom, was published in Evolution Letters.
It examined five forms of aggression in 100 group-living primate species. Those included mild aggression within groups and between groups, killings of adults within groups and between groups, and infanticide.
New doubts over whether human violence is natural
The findings challenge a common assumption in debates over the origins of human aggression. For years, some theories have treated aggression as a single inherited tendency, asking whether humans are naturally violent or shaped mainly by culture and social conditions. This study took a different approach. It separated aggression into distinct forms and tested how closely those forms are linked through evolution.
Researchers found that mild aggression was only weakly connected to lethal violence. In other words, species that show frequent nonlethal aggression do not necessarily show a greater tendency to kill. That suggests aggression does not simply rise along a straight line from minor conflict to deadly attacks.
Instead, the strongest links appeared among different forms of lethal aggression. Killings of adults within a group and between groups were positively related, and both also showed links with infanticide. Those patterns were strongest when the attacker was male.
The role of evolutionary history was also uneven. Phylogeny, or shared ancestry, had a weaker effect on mild aggression than on lethal aggression. The effect was especially strong for some forms of killing, including male infanticide.
Different forms of aggression tell different stories
Researchers said the results point to a more complicated picture than many existing models allow. Mild aggression may serve different purposes, carry different risks and happen under different conditions than lethal aggression. That means a species can be aggressive in everyday social conflict without being more likely to escalate to killing.
The study also warns against grouping all forms of lethal aggression into one broad category. While killings of adults and infants were related, the links were moderate, not absolute. The pattern also differed between males and females.
Among the species in the dataset, infanticide was far more common than adult killing. Researchers found infanticide in 65% of species, while between-group adult killing appeared in 22% and within-group adult killing in 19%. That gap may reflect both the lower risk of attacking infants and the difficulty of observing rare lethal attacks in the wild.
The study does not argue that lethal violence is absent in primates. It shows the opposite: deadly aggression exists across primate species. But it also suggests humans and other primates should not be described as simply violent or peaceful by nature. Different kinds of aggression appear to have different evolutionary roots.
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