All Western countries have birthrates below the replacement rates, suggesting that soon all countries will experience a graying of, and a decline in, population. Jonathan Last asks why this has happened in his new book, the cleverly titled What to Expect When No One’s Expecting, putting demographic decline in a broader context.

Before we get to Last’s argument, we should revisit a debate between two great Enlightenment philosophers: Montesquieu and David Hume. In Spirit of the Laws, Montesquieu argues that ancient republics had more people than modern ones. Catastrophes aside, this lack of fecundity shows that a country is plagued with “internal vice and bad government.” The internal vices can range from a proud celibacy to a self-indulgent libertinism.

Montesquieu argued that population trajectory is partly the product of marriage laws. Ancient republics encouraged men and women to marry, and penalized bachelorhood. “Old Roman laws sought to induce the citizens to marry,” and censors were established to mind marriage mores. Roman law penalized parents without children and rewarded those with children with special honors and exemptions.

Hume accepts Montesquieu’s comparison of ancient and modern republics, but questions the reliability of the evidence from the ancient poets and historians. “In the flourishing age of the world,” he writes, “it may be expected, that the human species should possess greater vigour both of mind and body, more prosperous health, higher spirits, longer life, and a stronger inclination and power of generation.” Inventiveness and creativity and hope in a genuinely better future move human beings to live and generate. All things being equal, Hume contends, “it seems natural to expect, that, wherever there is most happiness and virtue, and the wisest institutions, there will also be the most people.”

Giving birth, much like educating students, requires some sort of faith or hope in the future, a belief that the human condition is worth experiencing, and a confidence that one can nurture a proper environment for the education of a new life. Welcoming new life reflects openness to the gifts of life, and appreciating these is itself confirmation of a life worth living.

Hume concludes that conditions in the modern world are much better than those of the ancient world. He observes that modern political communities oppose slavery, govern by the rule of law, establish commercial relations, encourage more consensual and relatively egalitarian sexual relations, afford general protection to private property, and encourage progress in the arts and sciences. All these factors make life freer and better, and all point to the steady growth of actions leading to an increase of population. With the numbers he has access to, Hume shows that modern communities are indeed much more populous than their ancient counterparts.

Interestingly, both Hume and Montesquieu argue that Christianity tends to suppress population growth. Neither blames Christianity directly for slow growth, but each equates it with a world-denying celibacy that did much to dampen sexual interest and fecundity in late Roman and post-Roman times; modern fecundity, they seem to suggest, comes in part from embracing a worldliness that the waning of Christianity, as they see it, allows.

Enter into this debate Jonathan Last. Last synthesizes scholarly research with an engaging blend of statistics, anecdotes, and judicious observations. His is very much a book for contemporary readers, as he sympathizes with many of the advances that have, in turn, fostered population decline. I imagine Last at a coffeehouse in a toney urban neighborhood, looking out at his mini-van and wondering if he and his wife should have more children. He appreciates the pull of post-family culture, but he is still a human being in the old sense of seeing himself as part of an intergenerational compact. Neither Cassandra nor fuddy-duddy, his tone is one of detachment from the conflicts in his soul and in the American soul.

Last makes two central claims. First, he argues that declining birthrates are longstanding. At one time, most places in the world had total fertility rates of well over five children born to the average woman, but that rate has been in almost inexorable decline for hundreds of years. In part this long-term trend is due to the salutary decline in infant mortality, but Last’s second claim is that the trend has now moved way beyond that.

Today’s total replacement rates are headed dangerously south, he argues, and population decline has turned into a birth dearth. The rates are below replacement in many countries: Singapore’s is 1.1; Japan and Poland’s 1.3; Germany, Austria, and Italy’s 1.4; Russia’s 1.6; France’s 2.08; the United States’ 1.9. The number of childless women has increased dramatically over the past forty years, while families with more than three children are increasingly rare. The global nature of falling birthrates means it defies easy characterization and cannot be explained with simple causal arrows. We have a dangerous fall (to below the replacement rate) within a much larger long-term decline. Are the two declines related? Is the birth dearth merely “finishing off” what the population decline has started, or is it a new phenomenon on top of population decline?

The urgency of addressing these questions becomes evident every time we consider the wealth of nations today. Declining populations are at the root of many of our policy and social problems, Last shows. The welfare states throughout the Western world are based on the idea that the young will fund the benefits for the old, but the dearth of the young means that as debts and deficits keep rising, something has to give. Either benefits will be trimmed, or the young will have to pay an even higher portion of their income to fund their older relatives.

And as taxes rise, productivity declines. The modern economy depends on innovation and increased productivity, but, as Last argues, the few people being squeezed more and more are much less likely to take daring acts to improve our lives. There you have the contemporary American crisis of government in a nutshell.

We don’t have to travel too far to see these consequences playing out. Japan’s economic “lost decade” (still underway nearly twenty years later) is partly the result of its graying population and shrinking labor force. This same shrinkage is happening in many European countries and, probably, in the United States, where a larger proportion of our workforce is entering its post-creative years. The current “fiscal crisis” will be nothing in comparison to what it will be when our country is sustaining more retirees on fewer workers.

Last makes a strong argument for seeing the population decline and birth dearth as intimately related. Since birth rates have been falling for at least two hundred years, we can’t simply say that innovations of the past fifty years (such as accessible contraception, legal abortion, or even the modern welfare state) are responsible. While China’s “One-Child” policy and Singapore’s feminist indoctrination programs have lowered birthrates more dramatically than American child safety seat laws (which make it harder for an average family to have lots of young children at the same time and still drive an affordable car) or the price of American homes, no particular explanation would seem to suffice if all nations are caught up in the same tide. There must be some general explanation if tyrannical control (China) and free choice (the United States) yield the same general population declines.

Last’s effort to explain this situation is provocative, especially in light of the Hume-Montesquieu debate. Whereas Hume believes modernity and its political customs foster population increase, Last blames modernity for population decline and the birth dearth. Recent population declines coincide with recent developments in family experience, loosely labeled the “decline of marriage.” As marriage rates and the marriage culture decline, divorce rates, cohabitation rates, and living-alone rates rise. There is a snowball effect as these alternatives become more attractive and marriage less normative. Childlessness is much more accepted than it once was, as are the alternatives to marriage. Birthrates are at their lowest in places where, generally, trends away from marriage are most pronounced.

This seems to be the modern tide. For Last, America’s fertility problem and the world’s fertility problem is “the result of an enormous, interconnected web of factors that constitute something like the entire framework of modern life.” Marriage and fertility are “pulled along by massive, invisible cultural undercurrents” and “disparate parts of modernity evolving independently.” Precisely what Last means by “modernity” is not clear, but while summarizing an argument from the late James Q. Wilson, he suggests that “when reason replaced religion and custom as the lodestars for human thinking, it became natural for the sacramental view of marriage to be replaced by the contractual view.”

Think of population decline and the birth dearth as aspects of this modern project. The “First Demographic Revolution,” Last shows, was one of modern science’s great accomplishments: Beginning in the 1750s, the mortality rate, and especially the infant mortality rate, started declining, a trajectory that accelerated through the 1800s. As more infants survived and life expectancy surpassed forty years, more people had longer “fertility” periods. This, as Hume observed, created what Last calls “demographic momentum,” as larger generations begat larger generations, even as each generation had, on the whole, fewer children. This, according to Last, is the population decline that most of today’s developing world is currently experiencing, as improvements in nutrition, sanitation, and medicine span the globe.

Is there a point when such birth rates level off? Is there a natural limit to the declines characteristic of this “First Demographic Transition”? Last’s answer is that the First Demographic Transition started a second, one directed at controlling birth instead of limiting death and reconstructing marriage to favor individual autonomy. Here, Last intimates, the modern project turned more than a little on itself and caused a genuine demographic problem; the First Transition was mostly salutary; the second caused a much more dangerous and destabilizing plunge.

Yet those demographic transitions are united in principle, as Last’s general comments suggest. His suggestive comments force us to understand with greater depth the meaning of modernity. The heart of the modern project lies in conquering nature, in making “nature” obey the creative wills of human beings. First in defying death. Second in controlling life and in redesigning marriage, the institution capable of generating life. Contraception (which asserts a power to control birth), abortion (which asserts a power to define life), the constructivist revolution afoot in marriage (which asserts a human power to define marriage or “relationships” as whatever we wish them to be) are thinkable only in a time when we are preoccupied with making ourselves, in Descartes’s phrase, “Lords and Masters of Nature.” The Transitions are united in their aim and understanding of the world and human power.

The most respectable reason for undertaking this project is, as Last thinks, to allow individuals to actualize themselves or experience the conscious making of their own lives and life plans, which Last sees as the hallmark of the “Second Demographic Transition.” Yet the demographic collapse suggests that our mastery of our situation is not as complete as we think, which should suggest to Last the limits of modernity. Our search for autonomy and mastery over nature has led to a species of revenge from nature. Or, perhaps, our attempt to design and “control” birth can only be successful if we also must rely on nature (i.e., the natural desire for procreation), and nature in this respect seems to be letting us down, as Pierre Manent has argued. This is a problem of technology and its way of thinking; its prominence in our way of thinking is the most compelling reason to see the birth dearth as a consequence of modernity’s general population decline.

Last’s provocative book raises questions about what can be done about population decline and the birth dearth. Initial findings on efforts across the Western world to raise birthrates suggest that there is little we can do to move the needle. The most promising developments in this respect are connecting fecundity with the revitalization of faithful religious practice, as seen in Last’s fascinating discussion of the former Soviet land of Georgia. Swimming against the modern tide seems to require embracing institutions appreciative of such limits.