There is one thing that can be said for certain about the size of the world population: it is never exactly right.

I spent much of my youth being warned of the dangers of overpopulation. There were books that claimed that the population had increased so far and so fast that it was now too late to avert mass famine. Whatever we did, hundreds of millions were doomed to death by starvation. 

Fifty years later, hundreds of millions are now doomed to premature death by overnutrition, through the intermediary of Type II diabetes. Obesity has overtaken starvation as the most important nutritional worry. 

I remember a pivotal point back in the early 1980s. I was on the little island of Nauru in the Central Pacific. It was then the richest place, per capita, on earth, the small population having recently gained financial control over the island’s resource of phosphate rock. 

It had little to occupy its time thereafter but eat and drink excessively, and 50 per cent of its population started to suffer from Type II diabetes. 

An Australian professor, Paul Zimmet, came to the island to study the phenomenon, which I thought at the time was a typical piece of academic time-wasting on the arcane marginalia of the world. In fact, Professor Zimmet was studying the future of the world. He was prophetic.

Now we are worrying over population decline in many countries (we need to worry about something in order to maintain our interest), not least China among them. Prime Minister Kishida of Japan has recently announced fiscal and financial measures to boost his country’s already very low, but still declining, birth-rate. 

Whether people can be persuaded to have more children by offers of free childcare and so forth remains to be seen. Is it because raising children is too expensive that they don’t have any?

According to the projections of demographers, Italy’s population will have declined by nearly 20 per cent by 2050. In fact, all western countries have rates of fecundity that fall well below replacement levels. If their populations are increasing (as some of them are), it is only by immigration. But only in Africa is population increasing by means of more births.

With regard to Africa, we are Malthusians: the growth in population is outstripping the capacity of the economy in general and the land in particular to support it, and some proportion of it is bound to be driven towards Europe in search of an easier and more abundant life. 

It will not be the poorest of the poor who migrate of course, for modern migration is comparatively expensive, what with transport costs, people smugglers to pay, etc.

With regard to the rest of the world, we are mirror-image Malthusians, as it were: there will not be enough people to support that part of the population which is not willing or able to be economically active. 

Therefore, we shall have overpopulation and underpopulation at the same time. What is clear is that we shall nowhere have precisely the size of population that we consider ideal. 

I do not know what to make of all these projections (in which I have no great vested personal interest, since I, childless, will be dead by the time of the denouement, if anything in history can be said to be a denouement). 

There is always something pleasurable, gratifying and even delightful about contemplating the horrors to come, for they make the present seem more bearable. 

Yet since past projections — which, incidentally, should not have been considered as predictions, though they always were, just as correlation is always mistaken for causation — were often so inaccurate, what reason do we have for placing more faith in current projections? 

This question itself raises the problem of induction. Just because past projections have been wrong doesn’t mean that present ones will be wrong. 

As Bertrand Russell put it in 1912, in “The Problems of Philosophy,” the chicken who thinks the farmer has come to feed him just because he always has done so in the past ends up having his neck wrung by the very man who fed him. 

This, of course, was in the days before battery farms, where chickens can be prey to no such illusions. The past is thus no guide to the future; the problem is that we have no other guide either. 

When asked what he thought the world would be like after Covid, the French novelist, Michel Houellebecq, said that he thought that the world would be the same, only worse. There comes a time in a man’s life when he is almost biologically predestined to think something like that. 

Pessimism is the joy of old age, in the same way that it is said that a man dying of cold or of his wounds feels no pain. Whether pessimism or optimism is justified — who knows? Pessimism is much more fun, though.