Many of us elite liberals like to think of ourselves as rational creatures trying to get by in a crazy world, but we know that we are prey to all sorts of cognitive and emotional malfunctions. What we don’t perhaps recognise so well is the power that demography exerts on us, just as it does on rats in a cage. I knew much of what David Willetts described in his BBC broadcast It’s the Demography, Stupid, but the excellent broadcast demonstrated well the brutal power of demography.

The seven billion people on the plant are distributed with one billion in each of Africa, the Americas, and Europe and four million in Asia. Most of us are Asians. By 2050 the global population will be around 10 billion, and that’s probably an underestimate. The programme didn’t discuss that this is probably more than our planet can support—the single biggest demographic threat.

All of the continents are facing the consequences that a python faces swallowing a monkey; all have to absorb a bulge in the population, but they are at different stages.

The bulge usually results from the “demographic transition” when development causes mortality to fall. As a result people have fewer children. But there is a lag between mortality and fertility falling, and the bulge in births results from that lag.

Africa is the continent most threatened by the bulge. In the poorest countries, like Niger, they are at the very beginning; mortality has begun to fall, but fertility is still high. Most of the world’s trouble spots—Congo, Yemen, Afghanistan—are at this stage.

Nigeria is a little further on with the transition and is set to become the third most populous country in the world with 400 million people. There is a huge bulge of young people and very high unemployment among them. Two crucial questions will determine Nigeria’s future: Will fertility drop fast enough? And can the economy create enough jobs? Rapidly falling fertility and growth in employment, as happened in the tiger economies of Asia, could mean rapid development and millions being lifted out of poverty. A slow drop in fertility and a failure to produce enough new jobs could mean massive migration (most of it “illegal”).

China is much further on with swallowing its bulge, and the bulge was restricted by the one child policy. Despite the lifting of the policy the fertility rate is around one, perhaps the lowest in the world, and China is moving extraordinarily rapidly from having a predominantly young population to having a predominantly old one. This is likely to mean lower economic growth, with a global effect, and problems of looking after the old.

Japan has the oldest population in the world, and as it has had almost no immigration is building robots to look after its old people.

Europe too has an aging population, but it has had longer than the Asian countries to adjust. Britain is in a better position than, say, Italy because it has had higher immigration over the past 60 years and so has more young people and a lower dependency ratio (people not earning and needing services/number of people who are economically active). Immigration has, however, been a driver of Brexit, which will create its own problems.

Britain also has writ large the problem of rich baby boomers (that’s me) with a lifetime of employment; index linked, final salary pensions; and houses worth a fortune with a younger generation that is poorer, has less stable jobs, can’t afford to buy houses, and which will have to work until they are 70. What’s more it was the old who voted for Brexit, which—despite the fashionable disdain for economists—will most likely make Britain poorer.

(As an aside, I’ve tended to think of my generation, perhaps the most privileged there has ever been, as being a failure in that poverty is still rampant, inequality has grown, and the planet is wrecked; but Willetts blithely said that in Britain at least we have been a success in creating an open, multicultural, progressive, liberal society. As usual, there is truth on both sides.)

The US, despite its turbulent past year, may be in the best place demographically. Because it has higher levels of immigration than in Europe it has a much lower dependency ratio and is swallowing the monkey more effectively than most. Again, however, immigration—combined perhaps with racism that is even more entrenched than in Europe—has contributed to Donald Trump being elected president, with a raft of policies that may halt the US’s contribution to the globalisation that has lifted hundreds of millions out of poverty.

Demography is ultimately more powerful than economics, yet we hear constantly from economists and hardly ever from demographers. We need to hear more from demographers.

Richard Smith was the editor of The BMJ until 2004.