Is the world more or less violent than it used to be? It’s a huge question, one that anthropologists can’t seem to agree on. In his famous book The Better Angels of Our Nature, psychologist Steven Pinker proposed that violence has declined in modern history, suggesting that we have, as a species, outgrown our ancient, violent, caveman past. But some anthropologists disagree, arguing that history’s apparent decline in human violence has nothing to do with changes in our nature.

A new study, published Monday in Proceedings of the National Academy of Sciences, suggests that the level of human violence is linked the scale of our societies. By examining data from actual conflicts and human populations throughout history, the American anthropologists behind the study propose that population size is the most important factor shaping a society’s level of violence, arguing that this trend holds true across communities and centuries.

“In most of our large societies — India, China, the U.S., Russia — less than one percent of the population is involved in warfare (in the army), while in small societies you might have 20 or 30 percent of the society involved in warfare,” Notre Dame University anthropologist Rahul Oka, Ph.D., first author on the study, tells Inverse.

Vietnam War
Anthropologists argue that history has not made humans less violent. Larger societies simply can't afford the resources it takes to wipe each other out in battles.

In other words, the larger a society is, the smaller the percentage of people in that society who are involved in organized violence — a subset of people Oka and his team call the “war group.” Societies with proportionally smaller war groups lose a smaller portion of their population in the event of a conflict, suggesting that humans haven’t become any less violent over the years. It just appears that way because our societies have become so big that they can no longer maintain large war groups.

To illustrate this idea, Oka points to North Korea, a country with a relatively small population. In North Korea, a significant 20 percent of citizens are part of the society’s war group. Rather than take this skewed proportion as an indication that North Koreans are a more violent people, Oka says it illustrates that a smaller society can devote more resources to a military than a larger society can.

Modernity, his work has shown, has done little to stem the tendency toward violence, in North Korea or elsewhere. What limits this tendency today is the unprecedented size of our societies. By studying army size and number of casualties in more than 400 historical conflicts involving 295 societies going back to 2500 B.C., his team found that in small-scale societies, the percentage of people killed in a conflict is quite high, even if the number of people killed in total is numerically low. For large societies — like the majority of states today — it was the other way around.

“If you look at just the numbers, the number of people killed in World War I and World War II was extremely high, but when you look at them as proportions of the population, they’re actually pretty low,” says Oka.

war group scale
A society with a larger population has a larger war group. But a smaller society devotes a larger proportion of its population to its war group.

By measuring the proportion of a society involved in war, his team calculated each society’s “demographic investment” — the extent to which it puts its resources toward conflict — and found that smaller societies can afford to make larger demographic investments simply because of scale. For instance, if a small community of 1,000 subsistence farmers needs 40 percent of its citizens to mobilize and fight, it’s reasonable to think that it can devote 400 people to its war group. But if 40 percent of the citizens in the United States need to be armed for conflict — that is, some 129 million people — the cost would decimate the economy.

“It’s just economically impossible,” says Oka.

Anthropologists say that society has not gotten less violent over time. It's simply a matter of how big a society is.

Oka says he and co-author Mark Golitko, Ph.D., were inspired to study societal violence by their professor Lawrence Keeley, who wrote the book War Before Civilization, one of the first works to comprehensively counter the notion that humans were peaceful before the formation of large states. By showing that small-scale societies were violent, Keeley tore down the idea that state-level violence is a totally novel phenomenon — which Pinker uses as the basis of his argument that humans are enjoying an unprecedented period of peace.

The team’s findings in the PNAS paper are more in line with Keeley’s position, arguing that history has not necessarily decreased levels of human violence. Their analysis led them to establish a “scaling law,” which describes a consistent relationship among population size, war group size, and conflict casualties. The law explains multiple trends: Smaller societies have larger armies proportionally, and larger societies experience fewer war casualties proportionally.

“If you have a large population, you’re going to have a low proportion. But that’s not because you’re less violent,” says Oka. “That’s just because you can’t afford to have the same proportions of people involved as if you were in a small-scale society.”

These findings may come as a disappointment to anyone who think humans have achieved great measures of peace. Even Oka is among those who wish it wasn’t so.

“Let’s not pat ourselves on the back and say we’re less violent than we were before. If this is the case, then it means that we are actually no more or less violent than we ever have been and that we’re going to have to work harder if we want to move towards peace.”

Abstract: The proportions of individuals involved in intergroup coalitional conflict, measured by war group size (W), conflict casualties (C), and overall group conflict deaths (G), have declined with respect to growing populations, implying that states are less violent than small-scale societies. We argue that these trends are better explained by scaling laws shared by both past and contemporary societies regardless of social organization, where group population (P) directly determines W and indirectly determines C and G. W is shown to be a power law function of P with scaling exponent X [demographic conflict investment (DCI)]. C is shown to be a power law function of W with scaling exponent Y [conflict lethality (CL)]. G is shown to be a power law function of P with scaling exponent Z [group conflict mortality (GCM)]. Results show that, while W/P and G/P decrease as expected with increasing P, C/W increases with growing W. Small-scale societies show higher but more variance in DCI and CL than contemporary states. We find no significant differences in DCI or CL between small-scale societies and contemporary states undergoing drafts or conflict, after accounting for variance and scale. We calculate relative measures of DCI and CL applicable to all societies that can be tracked over time for one or multiple actors. In light of the recent global emergence of populist, nationalist, and sectarian violence, our comparison-focused approach to DCI and CL will enable better models and analysis of the landscapes of violence in the 21st century.

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