John Maynard Keynes observed in 1936 that ‘common sense’ in matters of political economy was often nothing more than a half-understood and mangled version of the academic orthodoxy of fifty or a hundred years earlier. The same could be said today of demography. As someone who spends much of his time writing and speaking about the problems of too few children being born and population ageing and decline, I continue to be amazed by how many people’s demographic world view is stuck in the late 1960s. Then, sharply falling mortality rates (particularly among infants) in the developing world meant that the planet’s population was growing apace and people like Paul R Ehrlich were forecasting that we were all doomed, with the starvation of billions impending.

The world has since moved on but general opinion has not. Annual global population growth is now just over a third of the peak level reached in the 1960s and the rate is falling fast. The exact point at which the world’s population will start to decline is uncertain but all mainstream projections suggest that it will come before the end of the current century and possibly decades earlier. On the one hand, longer life expectancy buoys total numbers. On the other, falling natality depresses them. Falling natality is fast getting the upper hand, dooming the human population to decline.

At current levels, the South Koreans, world champions at not producing babies, will see each generational cohort decline by between half and two thirds, meaning the country will disappear into a demographic vortex. The rest of the world is in not much better shape. Low fertility was once the luxury of the wealthy parts of Europe and East Asia, but today countries as relatively poor as Thailand and Jamaica are embracing the trend. In the UK, we now have more deaths than births each year, something that only happens after decades of moderate below-replacement fertility levels. We are joining countries like Germany, Russia, China and Japan, which are seeing their populations decline year on year – in the case of Germany, even with a huge inflow of immigrants.

Not only have population numbers globally failed to take off to anything like the extent once expected but the famines and calamities predicted by Ehrlich and his neo-Malthusian ilk have also failed to transpire. This is in large part due to the extraordinary rise in agricultural productivity, itself the result of human ingenuity and efficiency. Yields per acre on staples have doubled or tripled since Ehrlich raised the alarm and overall output has grown three, four or five times, depending on the crop. Human population has barely doubled over the same period. That is why famines like the one in Ethiopia in the 1980s have become so rare. When there is a real food shortage, as in Sudan today, it is the result of conflict rather than an absolute shortage in resources. We eat better and live longer at eight billion than we did at four billion.

Yet the view that we are in danger of running out of food and just about everything else persists, despite the best efforts of writers such as Matt Ridley and Bjorn Lomborg. So we should welcome Dean Spears and Michael Geruso’s After the Spike. It is a brisk and accessible outline of the basic facts of the demographic precipice on the edge of which mankind currently stands. As the authors correctly point out, persistently below-replacement fertility rates will create not just a smaller human population once numbers start to fall but an endlessly shrinking one. It would be a futile task to calculate precisely when the last human will turn off the lights; there are far too many unknowables. We can’t yet ascertain how fast fertility rates will fall in sub-Saharan Africa or whether already low fertility rates in Europe and the Americas will fall to super-low levels, as in East Asia. But the authors are correct in thinking that, without a recovery to an average of around two children per couple, the population of the globe will not stabilise. Its decline will be an altogether bad thing.

There is much to commend in this book. The authors take on the environmentalist misanthropy in both its varieties: the beliefs that refraining from reproduction will save the planet (it won’t) and that the world is too ghastly for any sane and responsible person to bring a child into (it isn’t). They make the case for a global population not far in number from the current total, which, they correctly assess, the planet will be quite capable of supporting, especially as technology advances. As they also point out, the lack of a large and vibrant population will hamper the technological progress that will enhance life further.

Spears and Geruso are on less firm ground when they dismiss the problem of ageing populations. The old-age dependency ratio (the number of retired people for every hundred working-age individuals) is rising, putting pressure on economies and governments. As the population continues to decline, the average age will rise and rise. When you go from having eight or nine workers for every retiree (as in Japan in 1960) to only two (the proportion in Japan today), it is not just that your level of innovation and creativity sinks. It’s also that you end up having to spend more money on health care, social care and pensions, in the process placing an unsustainable burden on taxpayers or racking up huge debts. Small wonder a recent Japanese prime minister talked of ‘societal collapse’. The situation in Japan is just a preview of what the rest of us will face.

The authors of this book are right to strike a progressive note and to assert that the rolling back of women’s education, participation in the workforce and reproductive rights cannot be part of the answer. But if population decline is a problem we are going to tackle individually, communally and internationally, the best way of attracting the attention and resources required is to show how the economy will become unsustainable within a few short decades.