Birth rates are tumbling across the world. This isn’t just a tragedy for the growing number of women who have fewer children than they want; below-replacement fertility rates mean that each generation will be smaller than the one before. This could lead to a permanent spiral of decline where the old always outnumber the young.

But can anything be done about it? A report published last month by the United Nations Population Fund (UNFPA) acknowledges the problem, but its analysis is rather confused. The authors complain that economic barriers prevent women from having ‘reproductive agency’ while simultaneously arguing that governments trying to reduce these barriers are, in some cases, ‘coercive’. After decades of international ‘family planning’ evangelism, perhaps UNFPA officials fear their efforts have been rather too successful.

This is fake news: multiple studies indicate that pro-natal strategies do increase fertility

Matthew Parris highlighted the report’s inconsistencies in The Spectator, pointing out that ‘better family leave and fertility care’ are very obviously not the reasons that African women have nearly three times as many children as their Swedish counterparts. But Parris also reflected some commonly-held misconceptions about falling birth rates, namely that population decline is not such a bad thing, and that nothing can be done about it anyway. These misconceptions need to be addressed if we are to have any chance of solving this existential problem.

Many people struggle to understand the scale of the challenge posed by the global baby bust because to do so is both counter-cultural and counter-intuitive. It is counter-cultural because, since the 1960s, predictions of over-population and scarcity have dominated the discussion. It takes considerable mental reprogramming (and a degree of humility) to see that that falling population might be a greater threat than a growing one.

Grasping the consequences of low fertility is counter-intuitive because the human brain struggles to comprehend the magnitude of compounding growth effects. Mortgages are a classic example. If asked to guess how much interest is due in total on, let’s say, a 30-year loan of £300,000 at a 4.5 per cent interest rate, most people vastly underestimate the true sum (over £400,000 if you were wondering).

We see the same tendency to underestimate the effects of falling fertility. We know that Britain’s total fertility rate (TFR) has fallen below the ‘replacement’ rate of 2.1 children per woman. But many people interpret this as a one-off population contraction, like the aftermath of a war or epidemic. In fact, below-replacement fertility rates mean that each generation will be smaller than the one before.

Let’s give a worked example. England and Wales’s TFR is now 1.44 children per woman. This is 31 per cent lower than what is required for population stability, so each generation will be one third smaller than the one before. In practice, this means that 100 people in Britain today are likely to have just 44 grandchildren and 30 great grandchildren between them. So, without immigration, the population of Britain could fall by as much as 70 per cent in three generations.

What’s the problem, some might ask? We used to survive perfectly well on a fraction of our current global population. Of course that isn’t true; in 1800, 80 per cent of the world lived in absolute poverty compared to just 8.6 per cent today. Population growth has made us richer, safer and healthier.

But the absolute size of the population is not the most important factor. What matters, socially and economically, is the ratio of young to old. In our modern context of long life-expectancy, we need enough people of working age to support those who have retired. In the UK, pension age benefits cost the state more than £125 billion a year, a cost that is shouldered by working-age taxpayers. In the early 1960s, Britain had up to 4.5 people of working age to support each pensioner; but, thanks to falling birthrates, we now have a ratio of just three to one. By 2070, that number could fall to just two working people to support each retiree.

This steady decline is a major reason why taxes are at record high, national debt has ballooned, and public services are chronically under-resourced. In these circumstances, the idea of returning to economic growth is just a pipe dream; it will be a miracle if we stave off economic collapse.

In Britain, politicians have failed to grasp the enormity of the threat headed our way. Other nations have not been so complacent; South Korea, Hungary, France and others have all pursued ‘pro-natal’ agendas, using tax breaks and cash handouts to encourage people to have children. The British media often reports that such policies have failed, presumably because none of these countries have managed to achieve replacement birth rates. But this is fake news: multiple studies indicate that pro-natal strategies do increase fertility. Natalist policies in France have arguably resulted in the births of between five and ten million babies that might not otherwise have been born. The evidence shows that there are effective actions that can be taken to improve fertility rates. To deny or dismiss this is both wrong and dangerous; we can’t afford to be defeatist when so much is at stake.

Collapsing birth rates are the most serious threat humanity faces. By the end of this century it is thought just six countries in the world will have above replacement fertility rates. There are many complex reasons for this – economic, social and cultural – all of which should be explored. But I suspect the underlying problem is that modernity has broken the link between having children and personal economic security.

We often romanticise the reasons why people choose to become parents, but the fecundity of our ancestors had nothing to do with ‘feeling ready,’ ‘finding Mr Right’ or owning a three-bed semi. Before industrialisation and social security, having children was not a luxury: it was essential for a family’s material survival. Yet in our modern world of centralised economies and socialised welfare, individuals can live comfortably and be provided for in old age even if they have no children of their own. In fact, most of us will be financially better off if we don’t have children, given that the costs of parenthood are privatised, while the economic benefits of those children – a lifetime of tax contributions – have been entirely socialised.

The reproductive urge will remain, but desiring children is not enough. No one has a baby to fund the NHS. We must find a way to restore the personal economic rewards of having children. Answers on a postcard please.