The world is heading for demographic catastrophe. Fertility rates have been falling across the globe for 40 years, to the point where, today, Israel is the only First World country where women have enough babies to sustain their population. The developing world is heading in the same direction, fast. Only

3 percent of the world’s population live in a country where the fertility rate is not dropping.

As fertility falls, populations shrink. As populations shrink, economies will sputter. Western countries will struggle to support too many retirees without enough workers, and the rest of the world (particularly places such as China and Russia) will be challenged just to maintain order as societies change in unprecedented ways: Most people will have neither brothers, sisters, aunts, nor uncles, and there will be no such thing as an extended family.

This forecast may sound apocalyptic, but it’s nearly conventional wisdom among the demographers and economists who study such things. However, the conventional wisdom also sees a silver lining to the world’s demographic decline: a “geriatric peace.” As fertility rates decline, and babies become relatively scarce, the average age of societies increases. In many countries the median age is already over 40, with geezers outnumbering children. And once the entire world looks like Florida, the thinking goes, we’ll all be more peaceable, because countries full of old men don’t go to war.

Unfortunately, Susan Yoshihara and Douglas A. Sylva suggest that geriatric peace may be elusive, and in Population Decline and the Remaking of Great Power Politics they have collected essays from an all-star squad of demographers, historians, and military strategists—Phillip Longman, Nicholas Eberstadt, Toshi Yoshihara, and Murray Feshbach are among their Murderers’ Row—who argue that a shrinking world may be more dangerous than we might expect.

In 1950, Japan was the fifth-most populous nation on earth, Germany was the seventh, and the United Kingdom the ninth. By 2050 these countries will rank twentieth, twenty-first, and twenty-second, respectively. Population is the wellspring of power, both economic and military, and the reordering of global power is, Yoshihara and Sylva argue,

inherently destabilizing.

Consider Japan. Faced with some of the lowest fertility rates on earth, Japan’s population has already begun aging and shrinking. The cresting wave of elderly Japanese is stressing the government’s budget with their demands for health care and pensions. At the same time, the decreasing labor pool has made it difficult for Japan’s Self-Defense Forces to recruit new soldiers. All of which would suggest that Japan should be retreating from international affairs. Yet precisely the opposite has happened: Japan has, instead, attempted to take on more global security responsibilities, in part as a response to growing Chinese assertiveness in the region.

Japan’s strategy-resource mismatch creates the potential for all sorts of geostrategic problems. Without manpower the Japanese defense forces will necessarily rely on advanced technologies. But tech doesn’t come cheap. And the very expense of high-tech military systems makes it more difficult to justify placing them in harm’s way.

In a sense, Japan could fall into the same trap that Western Europe already faces: the inability to formulate proportional military responses. Developed countries in demographic decline may have nuclear weapons, but for the most part, they lack basic military manpower. So when faced with a security threat or provocation, they have only two options: passivity and overreaction. Neither is conducive to peace.

There are other, tertiary effects of fertility collapse that contribute to instability. For instance, in some parts of the Third World, fertility decline has been so rapid that age structures are being scrambled by generational “echo booms.” In a chapter on the geopolitical implications of aging, Phillip Longman notes that in Iran between 2005 and 2020, the number of people aged 15-24 will shrink by 34 percent. That’s a startling shift. But then, because of echo effects, the 15-24 cohort will grow by 34 percent between 2020 and 2035. Even a stable, well-ordered society would have trouble coping with such wild demographic dislocations.

Yet for all the forward-looking concerns, the most bracing essay here is James R. Holmes’s examination of the demographic crises in classical Greece. In 464 b.c., a terrible earthquake shook Sparta. Much of the city was destroyed, along with some 20,000 Spartan warriors. The devastation was particularly bad from a demographic point of view because the gymnasium where young Spartiates were trained collapsed, wiping out an entire rising generation of warriors—and their potential progeny.

As a result of this cataclysm, Sparta’s leaders decided to retrench and abandon their traditional role as the principal counterweight to Athenian might. They became so docile that, when faced with a slave rebellion, the city begged Athens for help. (After some deliberation, Athens sent 4,000 hoplites to their aid.) Forty years later, Athens and Sparta were antagonists again, but Sparta was still chastened by its demographic decline: In 425 b.c. the Athenian Army captured 292 Spartan warriors—and the mighty Spartans sued for peace rather than lose the precious manpower. This frailty prompted the nearby subjugated region of Messenia to challenge Spartan rule. Demographic weakness, too, is a provocation.

Athens’ demographic troubles came not from a single shock but from a long-running plague that began in 431 b.c. and lasted five full years. The sickness killed between a quarter and a third of all Athenians, moving like a scythe through the city-state. Slaves and warriors alike were claimed—even Pericles, the First Citizen, succumbed. But unlike Sparta, which responded to demographic decline by turning dangerously cautious, the Athenians became reckless and impetuous. They made the rash decision to invade Sicily—a disaster that cost them nearly the whole of their navy. Terrified of losing allies (because they were so short of manpower themselves), the Athenians resorted to brutality. When the island state of Melos suggested it might leave the Athenian sphere of influence, Athens put to death the entire male population of the island and enslaved its women and children. Such brutality marked the beginning of the end of Athens’ empire.

The divergent reactions of Athens and Sparta show the strategic uncertainty inherent in demographic change. Over the next 40 years we will witness the most drastic demographic upheaval the world has seen since (at least) the Black Death. And one way, or another, the world will be remade. As Yoshihara and Sylva make clear, you may not be interested in demography, but demography is interested in you.

Jonathan V. Last is a senior writer at The Weekly Standard.

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