One of the more persistent assumptions embedded in debates about human violence is that aggression is essentially one thing, existing on a spectrum from minor disputes to lethal outcomes. Pick up almost any popular treatment of the subject — or many academic ones — and you will find this framing lurking beneath the surface. Species that fight a lot are portrayed as closer to the violent end of nature. Species that rarely squabble are presented as peaceful. Humans get placed somewhere on this line, and the location chosen tends to tell you more about the author’s priors than about the biology.
A new comparative analysis published in Evolution Letters1 challenges that framing at its root. Working across 100 primate species including Homo sapiens, researchers from the University of Lincoln found that how frequently a species engages in everyday, non-lethal aggression tells you remarkably little about whether it kills conspecifics. Mild aggression and lethal violence appear to have followed different evolutionary trajectories. They are not the same trait in different intensities. They are, in a meaningful phylogenetic sense, different things.
The team, led by Bonaventura Majolo and Samantha Wakes, assembled data on five distinct types of aggression: between-group mild aggression, within-group mild aggression, between-group adulticide, within-group adulticide, and infanticide. The data covered wild, non-provisioned, group-living primates only — captive populations excluded — and drew on published literature spanning 1950 to mid-2022, supplemented by unpublished data gathered through a survey of primatologists. Thirty-nine researchers contributed data on 28 species. The final dataset represents 43 genera across 11 primate families.
The analytical framework was Bayesian phylogenetic modeling, which allowed the team to test the strength of evolutionary correlations between each pair of aggression types while controlling for shared ancestry and five socio-ecological variables: sexual dimorphism, within-group sex ratio, group size, dietary folivory, and degree of territoriality.
What phylogeny explains, and what it doesn’t
The first notable result is structural. Phylogenetic relatedness explains more variance in lethal aggression than in mild aggression. Infanticide shows the strongest phylogenetic signal among the five types; within-group mild aggression shows the weakest. This means the distribution of lethal behaviors across primate lineages maps more predictably onto evolutionary history than the distribution of everyday squabbling does. Species that kill tend to come from lineages where killing has occurred before. Species that bicker don’t show the same taxonomic clustering.
When the team looked at correlations between aggression types, a clear pattern emerged. The three lethal types — between-group adulticide, within-group adulticide, and infanticide — were positively and moderately correlated with one another. Species where adults kill members of rival groups are also more likely to kill members of their own group, and more likely to engage in infanticide. The correlation between within- and between-group adulticide was particularly robust, with around 97–98% of the posterior distribution pointing in the same direction across both the null and full models.
Mild aggression told a different story. Neither type of mild aggression showed a meaningful evolutionary relationship with any of the lethal forms. The credible intervals for those pairwise correlations straddled zero. In the full model controlling for socio-ecological variables, the relationship between between-group mild aggression and infanticide, for instance, dropped to near nothing. The team ran five different versions of the dataset — varying how mild aggression was calculated — and obtained consistent results throughout.
The sex-specific analyses added another layer. When lethal aggression was split by the sex of the attacker, the evolutionary correlations among male lethal behaviors were strong: male between-group adulticide, male within-group adulticide, and male infanticide were tightly linked. In females, the picture was murkier. Female lethal aggression types were weakly correlated in the null model, with only between- and within-group female adulticide showing a stronger relationship when socio-ecological controls were added. Male infanticide had the highest phylogenetic signal of any category tested, with phylogeny accounting for roughly 59% of variance.
What escalation models get wrong
Animal contest theory generally predicts that mild aggression can escalate to lethal violence under the right conditions: high-value resources, closely matched opponents, or situations where the cost of continued conflict falls below the cost of backing down. This is a reasonable prediction. But it implies that the two types of aggression share an underlying logic, that mild conflict and lethal conflict sit on the same decision tree.
The Lincoln study’s findings suggest the decision tree model may be misleading, at least at the evolutionary level. Primates that engage in frequent mild aggression are not simply primates running a higher baseline risk of eventual killing. The socio-ecological conditions that would need to align for escalation to happen are, in mildly aggressive species, rarely present. Alternatively, those species may have developed conflict management behaviors — reconciliation, third-party intervention, social constraints — that function as effective circuit breakers. The result is evolutionary decoupling: lineages that argue a lot did not produce lineages that kill a lot.
This has practical implications for how socio-ecological models are built. Classical frameworks for understanding primate social structure incorporate predictions about competitive intensity but say little about when, or whether, that competition tips into lethality. The new findings suggest those transitions operate under different logic than the mild aggression preceding them.
The data on infanticide versus adulticide also bear consideration. Across the full dataset, infanticide was present in roughly 65% of species, adulticide in about 20%. The gap is large. Part of the explanation is methodological: infanticide has been studied more systematically in primates, so the literature is more likely to capture its presence. But risk asymmetry also matters. Killing an infant carries a much lower cost to the attacker than killing a conspecific adult, where both parties face genuine injury risk. Adulticide, when it occurs, tends to require numerical advantage, often attacking with multiple individuals against a single target. The team notes that planning-intensive adulticide, involving coordination, advance positioning, and target selection, may require levels of proactive aggression that are rare outside Pan and Homo sapiens. For most non-human primates, lethal attacks on adults are more likely stochastic: an isolated individual caught in the wrong encounter at the wrong time.
There is a broader implication here for how scholars interpret violence statistics, including the kind of cross-cultural historical comparisons that underpin arguments about whether modernity has made humans less violent. Treating adulticide, infanticide, and inter-group killing as a single undifferentiated measure of violence may obscure more than it reveals. The three types are correlated, but moderately, and the correlation is weaker in females than in males. Rolling them together into one “violence score” risks producing conclusions that reflect the amalgamation rather than any underlying biological reality.
The study cannot address rates, only presence or absence of lethal aggression. Whether a species kills once across decades of observation, or kills regularly, might matter enormously for understanding adaptive value and evolutionary trajectory. That data simply does not exist for most species at the scale required. The number of primate species with continuous long-term research across multiple populations is small, and adulticide is precisely the kind of rare event most likely to be missed or underreported. The figures for how many species show adulticide should be read as conservative.
What the study does establish is that the question “are humans violent?” is poorly formed if it treats violence as a single trait. Whether Homo sapiens has an evolved tendency toward mild aggression is a separate question from whether we have an evolved tendency toward lethal aggression, which is a separate question from whether we kill adults versus infants, within our social groups or outside them. Those distinctions are not just semantic. They have different phylogenetic histories, different socio-ecological triggers, and apparently different evolutionary dynamics. A species can be argumentative without being murderous. A species can be murderous without being especially argumentative. The relationship between the two is not a gradient. It is, as the data suggest, close to zero.
Further Reading
Gómez, J.M., Verdú, M., González-Megías, A., & Méndez, M. (2016). The phylogenetic roots of human lethal violence. Nature, 538(7624), 233.
Gómez, J.M., Verdú, M., & González-Megías, A. (2021). Killing conspecific adults in mammals. Proceedings of the Royal Society B, 288(1955), 20211080.
Lukas, D., & Huchard, E. (2014). The evolution of infanticide by males in mammalian societies. Science, 346(6211), 841–844.
Lukas, D., & Huchard, E. (2019). The evolution of infanticide by females in mammals. Philosophical Transactions of the Royal Society B, 374(1780), 20180075.
Wrangham, R.W. (1999). Evolution of coalitionary killing. American Journal of Physical Anthropology, 110(S29), 1–30.
Wrangham, R.W. (2018). Two types of aggression in human evolution. Proceedings of the National Academy of Sciences, 115(2), 245–253.
Wilson, M.L., et al. (2014). Lethal aggression in Pan is better explained by adaptive strategies than human impacts. Nature, 513(7518), 414–417.
Pinker, S. (2011). The Better Angels of Our Nature: The Decline of Violence in History and Its Causes. Penguin.
Dwyer, P., & Micale, M. (2021). The Darker Angels of Our Nature: Refuting the Pinker Theory of History & Violence. Bloomsbury Publishing.
Majolo, B., Wakes, S.J., & Ruta, M. (2026). Origins of violence: evolutionary decoupling between mild and lethal conspecific aggression in primates. Evolution Letters. https://doi.org/10.1093/evlett/qrag002
