Scientists generally hesitate to make predictions about the future because the passage of time cannot be manipulated in an experiment. Forecasters must rely on historical patterns and estimate what they look like projected into the future.

The past is prolog to the future. If one studies ancient complex civilizations, the prognosis is bleak. All failed, generally quite spectacularly and with remarkable speed (1). Can ours be any different?

Why Complex Societies are Unstable

When tribal groups are joined in some complex civilization, such as the Roman Empire, they do so because the combined entity is generally more successful. Either the citizens eat better, or are more secure, they are healthier, or lead happier lives.

The point of societal failure, and collapse, occurs when people do better in local tribes than they do under a large state.

Historians often point to the collapse of the Roman state as being precipitated by the sacking of Rome itself. Yet, the barbarians were not the cause of death. A more appropriate analogy is that they were like an opportunistic infection that strikes a dying person.

The sack of Rome was the result of a long string of failures stretching back for centuries. One major problem involved the cost of military defense in the far-flung territories. Another involved a decadent nobility consisting of privileged wastrels who absorbed increasing amounts of revenue and gave little in return. The Empire encountered serious financial problems that it concealed for centuries by debasing the coinage (1).

The loyalty of the military declined with the true value of their pay. At the point that they could no longer feed their families, the Roman empire was toast. Soldiers abandoned the empire and took up with warlords in their places of origin.

Joseph Tainter (1) argues that although every complex society succumbs to different weaknesses—whether it is epidemic diseases, or soil salinity, or drought, that there is a common thread in their failure. This is that the efficiency of the state declines to the point that citizens may improve their lot by abandoning the larger entity and reverting to local tribal rule.

In the modern world, this process can be observed in failed states like Afghanistan where more and more sway is held by warlords. Economic failure is illustrated by contemporary Japan where insufficient tax revenues are compensated for by ballooning public debt in one of the biggest Ponzi schemes since the Roman Empire. Ironically, Japanese public debt is perceived as having the safety of Roman currency.

Japan is the World's Canary

Japan is like any other highly developed country with one important twist. That is that they welcome fewer immigrants. This means that – unlike the U.S. – they have no rejuvenating influx of young fertile immigrants to prevent their population from aging.

Japan's low fertility produces aging of the population. This is very bad economic news because the population of retirees rises even as the proportion of working-age people shrinks.

Already Japan has seen virtually no growth in three decades amid ballooning public debt. Current attempts to revive the economy via quantitative easing are unlikely to work. Japan's major cities are projected to lose half their population before the end of the century (2). Imagine what such an erosion of the tax base will do to public services like schools, roads, and the capacity to police!

Japanese governments of the future may become more welcoming to immigrants. Yet, it will be too late as global migration cools. Indeed, as prosperity spreads around the globe, former sender countries will keep more of their best and brightest at home.

However Japan addresses her demographic and economic problems, the country offers a glimpse of the future in all developed countries.

Immigration to the U.S. seems likely to slow as the pool of migrants shrinks worldwide. Stripped of a rejuvenating influx of immigrants, the U.S. would already be Japan given that fertility in the native populations is equivalent.

The Collapse of Urban Civilization

The key to forecasting demographic change is that trends already in force are likely to continue unless the factors that propel them change (as Newton famously predicted of moving objects).

We know that fertility inevitably falls as societies develop and the underlying reasons are fairly well understood in terms of the heavy workload of working mothers and the high cost of raising children in densely-populated modern cities (2). Government subsidies tried in Russia and elsewhere have minimal impact on fertility because the subsidies are always much smaller than the actual cost of raising additional children.

Demographic winter has already struck in developed countries and is likely to spread to other nations as they develop. Sub-replacement fertility in developed countries is very bad news because no civilization with fertility this low ever recovered (1).

So if one accepts that increasing development is inevitable, then low fertility and societal collapse are in the cards as populations get progressively older and cannot maintain infrastructure and services due to a shrinking tax base.

Skeptics about the inevitability of economic development would do well to look at the trend of urbanization that marches up the graph without interruption from one decade to the next, just as fertility declines.

Moreover, anyone who doubts that the collapse of global urban civilization is imminent needs to explain how the decline of fertility can be reversed. So far, we have not seen anything that can do it.