Mammal study suggests man may not be 'cruelest animal,' after all

A study has found that many animals kill members of their own species at rates higher than those among human populations. What has caused the murder rate among humans to drop?

Michael Rubinkam/AP/File
Police exhume an unidentified homicide victim in Hanover Township, Pa. A new study finds that humans are among a group of mammals who murder members of their own species, a connection which could have evolutionary ties.

Humans have carried out their fair share of violence in recorded history, but a new study shows that killing rates among humans has dropped compared to those among our fellow murderous mammals, making man far from Friedrich Nietzsche's characterization as “the cruelest animal.”

Examining data from more than 1,000 species, the study, led by José María Gómez from the University of Granada, creates parameters for how violent an animal should be based on the actions of those of its close relatives. The research found that primates ranked high in terms of interspecies killings: While only 0.3 percent of mammals die at the hands of another member of their species, primates at one time murdered those within their group at a rate of 1.8 percent, lending a nod to the origins of humanity’s sometimes inhumane nature.

While such findings link human violence to evolutionary origins, other reasons behind violent human behavior, and explanations for its decrease, remain up for debate.

Today, the world’s most violent animal is the meerkat, with nearly one in five being killed by another member of its species, the study found. Other surprising contenders include horses, squirrels, and gazelles, all ranking among the 50 most murderous animals.  

Meanwhile, human tendency to engage in violent behavior has abated throughout history. Some philosophers have argued that the construction of civilizations and societal norms has led to a dramatic drop in homicides and other killings, while modern researchers argue that decreasing testosterone levels in humans in the past 80,000 years led to behavioral changes, and that those who cooperated with others rather than resorting to violent means saw greater survival success.

“Consensus does not exist, and positions are polarized,” Dr. Gómez told The Atlantic. “We hope that our study will shed light to the role that both evolution and culture have played in human lethal violence.”

Some experts take issue with the study’s findings, arguing that the types of violence humans and related animals engage in takes different forms at varying rates. Gómez’s study tackled infanticide, adult deaths, and other types of murder as a single phenomenon, but connecting evolutionary ties between humans and other animals could require a more nuanced analysis of violence.

“In the primates, infanticide is undoubtedly the commonest type,” Richard Wrangham, a professor of biological anthropology at Harvard University, told The Atlantic. “[Humans] belong to a club of species that kill adults at an exceptionally high rate — a small club that includes a few social and territorial carnivores such as wolves, lions, and spotted hyenas. That’s worth stressing in order to avoid readers leaping to the conclusion that there is nothing surprising about human violence. Humans really are exceptional.”

But humans have become exceptional in other ways — including the ability to care for people outside of their families and create complex, governing bodies to solve crises and uphold mutually beneficial guidelines.

Researchers have found that humans, even children just over a year old, engage in “prosocial behavior,” or, voluntary behavior that benefits others, at high rates, hinting that some innate aspects of humanity guide our more “civilized” interactions. At the same time, violence at the state level has increased while interpersonal homicides have dropped, lending evidence to the argument that societal structures have reshaped the very violence they sought to prevent.

Still, researchers have yet to pinpoint just what caused humanity as a whole to become less violent, and what remaining factors account for the different ways humans carry out interspecies violence when compared to closely related animals. While Gómez’s study acknowledges “that monopolization of the legitimate use of violence by the state significantly decreases violence in state societies," it’s hard to argue that a decrease in interpersonal violence due to state constructions is true progress when a large number of humans still perish due to the effects of state actions, like war and other international conflict.

As Steven Pinker, a psychology professor at Harvard, wrote in his book, “The Better Angels of Our Nature,” the connection between a society’s level of violence and its success in other areas is clear, but it’s difficult to determine the underlying causes in such a relationship.

“Everything in human affairs is connected to everything else, and that is especially true of violence. Across time and space, the more peaceable societies also tend to be richer, healthier, better educated, better governed, more respectful of their women, and more likely to engage in trade. It’s not easy to tell which of these happy traits got the virtuous circle started and which went along for the ride, and it’s tempting to resign oneself to unsatisfying circularities, such as that violence declined because the culture got less violent.”

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