When seeking to understand national security issues, demographics is commonly the missing dimension. The fertility of a particular population not only determines its overall numbers, but also contributes mightily to determining the balance of ethnic and linguistic groups, and a country’s chances of achieving any kind of lasting stability. Wise governments count their children. And without knowing something about demographic factors, we are going to be baffled by the behavior of some key players in the current Middle Eastern imbroglio.

A society’s population is shaped by both birth and death rates, but at present, I will focus on births, and especially on fertility. One key measure used by demographers is the total fertility rate, TFR, the total number of children that an average woman will bear during her lifetime. If that rate is around 2.1, then the population is stable, and that figure is known as the replacement rate. If it is significantly higher than replacement, say at 4.0 or 5.0, then we have a fast expanding population, a lot of young people, and probably a lot of instability. A rate below replacement points to an aging and shrinking population, and also a crying need for immigrants and new blood. From the 1960s, European countries moved to sub-replacement rates, and that situation is now spreading rapidly—though far from uniformly—around the world.

Those rates also tell us a lot about religious behavior, and there is a close if poorly understood linkage between fertility and faith. Populations with very high fertility rates tend to be highly religious in very traditional ways, while “modern” and educated populations have far fewer children, and those societies are usually very secular. Over time, high fertility gives way to low, and religion declines accordingly. The poster child for that story is resolutely secular Denmark, with a TFR around 1.7, but a similar process has swept over most of modern Catholic Europe.

We can argue at length about whether the religious change follows fertility, or vice versa. Perhaps a decline in religious ideologies weakens commitment to family as a primary means of defining identity; or else declining numbers of children reduce the community ties that bind families to religious institutions. Either way, growing numbers of people define their interests against those of traditional religious values, spawning numerous conflicts over issues of morality, sexuality, and sexual identity. Ireland’s TFR, for instance, has halved since 1971, and the decline is much steeper if we just consider old stock Irish families, rather than immigrants. The same years have witnessed repeated brushfire wars over such issues as contraception, divorce, and same sex marriage.

But changes in fertility do not affect all parts of a nation equally or simultaneously, especially when different regions show very different patterns of wealth and economic development. Over time, the higher birth rates of poorer and more religious populations will gain in relative numbers, and over two or three generations, that pattern of differential growth can have far-reaching consequences. In Europe, for instance, even without the migrant boom of the past couple of years, the proportion of Muslims in Europe was certain to grow significantly.

Many Westerners still think of Global South countries in terms of classic Third World population profiles, with very high fertility and teeming masses of small children. That perception is correct for some areas, particularly in Africa, but it is radically wrong for others. In fact, many Asian and Latin American countries now look thoroughly “European” in their demographics.

India offers a startling example of this change, and its explosive political consequences. Half of that country’s component states now have sub-replacement fertility rates comparable to Denmark, or even lower. Meanwhile, some very populous states (like vast Uttar Pradesh or Bihar) retain the old Third World model. That stark schism is the essential basis for any understanding of modern Indian politics. As we might have predicted, the high fertility states are firmly and traditionally religious, and provide the base for reactionary and even fascist Hindu supremacist movements. Those currents are quite alien to the “European” low fertility states, located chiefly in the south, which tend to be secular-minded, progressive, and tolerant. Balancing those different regions would pose a nightmarish choice for any government, but the current Hindu nationalist BJP regime aligns decisively with the high fertility regions that provide its electoral bastions. The lesson is grim, but obvious: when you have to choose between two such distinct demographic regions, it is overwhelmingly tempting to turn to the one with all the voters, and all the young party militants. Invest in growth!

With some variations, that situation is closely echoed in Turkey, and understanding that parallel helps us explain the otherwise puzzling behavior of the nation’s current Islamist AKP government. Why, for Heaven’s sake, is Turkey not more concerned about the ISIS threat? Why, when its air forces go into action, do they strike at Kurdish forces, rather than ISIS? Is the government deranged?

Here is the essential demographic background: Overall, Turkey’s fertility rate is a little below replacement, but that simple fact obscures enormous regional variations. The country can be divided into four zones, stretching from west to east. The Western quarter is thoroughly European in demographic terms, with stunningly low sub-Danish fertility rates of around 1.5. The rates rise steadily as we turn east, until the upland east has very high rates resembling those of neighboring Iraq or Syria. “Europe” and the Third World thus jostle each other within one nation.

High-fertility eastern Turkey is of course much more religious than the secular west, and this is where we find the Qur’an Belt that so regularly supports Islamic and even fundamentalist causes. It simply makes electoral sense for the government to respond to the interests of that populous growing area, and to drift ever more steadily in Islamist directions.

But there is a complicating fact. Those fast-breeding eastern regions are also home to what the Turkish government euphemistically calls the “Mountain Turks,” but which everyone else on the planet calls “Kurds.” Turkey’s Kurdish minority, usually estimated at around 15-20 percent of the population, is expanding very rapidly—to the point that, within a generation or two, it will actually be a majority within the Turkish state. This nightmare prospect is front and center in the mind of Turkish president Recep Tayyip Erdogan, who a couple of years ago issued an apocalyptic warning of a national Kurdish majority no later than 2038. That date is a little implausibly soon, but the principle stands.

In the face of seemingly imminent demographic catastrophe, what can Turkey do? One solution is for the government to plead with citizens to start breeding again—even those western secularists—and to get the national fertility rate closer to 3 than 2. But since that outcome is highly unlikely, the government must resort to short term solutions, and to extol religious, Islamic identities over ethnicity. Ideally, a return to Islam might even provide an incentive for families to reassert traditional values, and to have more children. Alongside that policy, the government has an absolute need to suppress stirrings of Kurdish nationhood or separatism on Turkish soil.

From a demographic perspective, the Turkish government is going to find any manifestations of Kurdish identity terrifying, far more than even the hardest-edged Islamism. ISIS is an irritant; the Kurds pose an existential demographic threat.

And in large measure, that explains why Turkish jets are targeting the Kurdish PKK militias, rather than ISIS.

It’s the fertility, stupid.

Philip Jenkins is the author of The Many Faces of Christ: The Thousand Year Story of the Survival and Influence of the Lost Gospels. He is distinguished professor of history at Baylor University and serves as co-director for the Program on Historical Studies of Religion in the Institute for Studies of Religion.