From the outside, it’s hard to see how the annexation of Crimea was a rational decision for Russia. As yesterday’s vote in the U.N. General Assembly showed, Moscow is now more politically isolated than ever. Its economy is paying a steep price thanks to harsh sanctions and tumbling markets. Moscow now has responsibility for yet another autonomous unrecognized region of questionable strategic importance—one that just happens to depend on Ukraine for its power and water.

Then again, when it comes to territory, states often behave in ways that seem illogical to outsiders. Why is public opinion in South Korea, China, and Japan so inflamed by the question of who owns a series of uninhabited rocks? Why does Israel continually subject itself to international opprobrium by constructing new settlements in the West Bank?

In the most recent issue of the journal International Security, Monica Duffy-Toft and Dominic Johnson, political scientists at Oxford, argue that a new theoretical framework is needed to analyze such behavior, one rooted in evolutionary biology. They write:

Territorial behavior facilitates effective competition for resources such as food, mates, shelter, breeding sites, and security from predators. Territory per se—a particular patch of ground—is not necessarily intrinsically valuable. For example, you cannot eat land, but you can eat food that grows there. Territory is therefore a proxy through which organisms secure access to key resources and protect them from competitors. Across the animal kingdom, as well as in preindustrial human societies, access to and control over resources have been essential for survival and reproduction, and adaptations to acquire these via territorial behavior have been subject to strong selection pressure throughout evolutionary history.

As Duffy-Toft told me an interview today, “It comes back to survival and reproduction. There’s an instinct that we need land in order to exist. We need to have the capacity to get resources to live our lives.”

But as she points out, territorial behavior need not always lead to violence. “It’s to our advantage to have borders, to delineate them and understand the basis of who lives where,” she said. “[The model] may explain why it is that states go after territories that have no material basis, or that the costs are higher than what they will get out of them.”

Generally speaking, when opponents enter into conflicts—whether birds fighting over a nesting site or states going to war over a border dispute—the actor that previously occupied the territory has an advantage.

“If somebody’s a resident, they seem to fight harder. They’re much more apt to be aggressive,” Duffy-Toft says. “As residents, they know the feel of it and the smell and where to find food, but if they come to the conclusion that they don’t have the capacity to defend that territory, they will abandon it.”

A version of this dynamic played out in Ukraine. Even though Crimea was part of Ukraine, local sentiment and ground conditions were favorable to Russia, and it was clear that Kiev didn’t have the means to challenge Russia’s territorial aggression. So Putin’s move went unchallenged, and the territory—in terms of de facto control if not international recognition—changed hands without much bloodshed.

Duffy-Toft acknowledges that the thesis is controversial. While their piece is currently the lead article in International Security, one of the more prestigious journals in the field, it took almost 10 years to get it published.

“We’re pushing up against real biases in our field,” she says. “Scholars don’t want to admit that our behavior can be constrained by the fact that we’re animals. They say we’ve developed norms in place to mitigate against that. That’s true, but let’s look at where those institutions came from.”

Fighting over territory may be a fundamental biological impulse, but it’s become much rarer, particularly since World War II. This is part of the argument the United States has used to make the case that Russia’s actions are beyond the pale for a modern state. “You just don’t in the 21st century behave in 19th-century fashion,” said John Kerry. “The United States does not view Europe as a battleground between East and West,” said President Obama, “nor do we see the situation in Ukraine as a zero-sum game. That’s the kind of thinking that should have ended with the Cold War.” (This is also key to the controversial distinction Obama drew this week between the Iraq war and the invasion of Crimea. Whatever actions the United States may have taken in Iraq, in Obama’s view, the country’s sovereignty was not violated as Iraq’s borders never changed.)

Part of the reason why territorial wars are now rare may be that the killing capacity of states has increased so much due to improvements in weaponry, particularly nuclear weapons. The costs of challenging a state’s residency in a given territory is now so high that few countries are willing to try it, and international norms have been developed to preserve the current status quo.

This has left us with a situation in which basically every piece of land on the Earth is recognized as part of one state or another and borders are rarely challenged—or at least rarely challenged by force. “There’s a recognition that warfare is a lot more costly today,” Duffy-Toft said. “Preserving [the existing] boundaries is recognition that this equilibrium is the best all-around.”

The small exceptions to that rule—contested territories like the West Bank or Kashmir, breakaway states like Abkhazia or Somaliland—are where fights over territory are most likely to occur.

Ukraine’s residency was weak enough in Crimea that Putin was able to take it without a fight, but that’s not the case in eastern Ukraine, where Kiev’s claim to the territory is much stronger. An evolutionary model suggests the Ukrainians would feel compelled to fight back if Russia challenged their hold over this territory.

Duffy-Toft, currently attending a conference of the International Studies Association in Toronto, noted that in addition to her paper finally being published, “territoriality has made a comeback” in the world of international relations scholarship. Though the conference was planned before the invasion of Crimea, “there’s a lot of interest this year in questions about human attachment to land, territory, and identity.”

This could be a sign, as some are now arguing, that an older model of geopolitics is making a comeback. As Duffy-Toft put it, “people are going back to fundamentals.”