The demographic transition is like an antibiotic

And fertility is like antibiotic resistance

This article was inspired by ‘Age of the Bacteriophage’, an excellent article by Léa Zinsli in the latest issue of Works in Progress.

We have become increasingly familiar with the story of antibiotic resistance.

Antibiotics work, but bacteria can evolve resistance to them. By ‘evolve’, we mean within extremely large groups of bacteria that we try and wipe out with our drugs, there are sometimes bacteria who, due to random mutation, have adaptations that make them immune to the antibiotic’s mechanism of action.

After their bacterial siblings and competitors have been wiped out, the immune bacteria reproduce and build the colony/infection back up to its previous size. Like chopping off a hydra’s head – another one regrows – but even worse as your sword now doesn’t cut through it.

Since we are not developing new antibiotics as fast as bacteria are developing immunity to our old ones, we potentially have a growing problem, hence the revival of interest in alternative therapies like bacteriophages – see more in Léa’s article.

My interest here is using this as a makeshift analogy that might help us think about another societal problem we have. Before modernity, people by and large aimed to have large families. If they could afford to get married, and feed, clothe and house them, and they were lucky with infant mortality and the harsh vagaries of premodern life, they ended up having a large number of children. Before the demographic transition, completed fertility rates were usually around 7.5 per woman, as they are in countries like Niger today.

Modernity, and especially the industrial revolution, changed all this. Starting with France, followed by Britain, Germany, the USA, and eventually the rest of the developed world, parents began to have something more like 2.5 children. Most of this happened before the pill or the cheap, widely-available condom, and before legal and available abortion, not to mention before the major waves of feminism culturally, legally, and economically. In some countries it happened before the decline of child mortality, in others it happened after.

There is still a debate about what the deeper causes are – some ascribe it to wealth itself; others to a modern system of production where ‘quality’ of children is worth more than quantity; others still to cultural changes and secularisation; and others to exogenous changes in the cost of having children like increased pressure to school children and norms and laws proscribing child labour.

Whatever the causes, post-demographic transition countries have generally produced only just-above-replacement fertility, with one exception, in their post-Second World War baby booms. In recent years, this just-above-replacement fertility has steadily slid to somewhat below replacement, sparking widespread concern and the beginnings of some policy responses.

But what I want to argue here is, whatever the ultimate cause, humans are like bacteria. Within human groups, and within humanity between groups, the future belongs to those who show up. Those who reproduce steadily increase their fraction in the group and those who don’t steadily decline. Quakers were a vital part of the American founding population, but since they had very few children, and Puritans had as many as ten per family, Puritans ended up as a much more important part of what made later America.

The demographic transition is not quite like an antibiotic: though it does cause childlessness and the complete and permanent cessation of some lineages, it mostly works through reducing the number of children per family, while most families still have at least one. Eighty percent of British women born in 1975 (the youngest cohort to have finished having children) had at least one. But like an antibiotic, some groups seem more resistant than others.

First let’s consider groups who have never gone through the demographic transitions, like Nigeriens, Hasidic Jews, and the Old Order Amish. It’s entirely possible that they will come up against whatever it is that has caused fertility to decline amongst everyone else. But if they don’t, then naive projections of population suggest they will become strikingly large fractions of the overall human population in coming decades and centuries. Humans as a whole will have evolved around the problem, like, say, homo sapiens hypothetically beat out Neanderthals due to their greater cooperativeness, even if most of us leave very few genetic descendants.

But I think that even if these groups do go through a demographic transition, we are already evolving resistance. Recent data suggests that wealth is now correlated with fertility – the rich are having more babies than the middle class. Women in countries with higher female labour force participation now have more children than those who don’t.

If everyone has lots of kids, then wanting to have kids is not selected for. But if nearly everyone has very few children, then intrinsic desire to procreate is one of the most important things we select for.

To put this another way, before the demographic transition, yours and your parents’, and your children’s fertilities were all only correlated because of your wealth and other similar things that drove fertility. Since wealth, even in the past, varied quite a bit between generations, this is fairly low correlation overall.

However, since the demographic transition, the fertility of different generations of particular families has begun to correlate substantially. Children from families with four children tend to be more likely to go on to have four themselves.

One downside here is shared from the bacterial example. What if having resistance to the demographic transition antibiotic correlates – as it sometimes does in bacteria – with something that makes us less effective. Some people believe that being religious is evidence of serious flaws in a person’s reasoning. But in some countries it is precisely the religious that are having the most offspring. If we select only for fertility, then we relax selection on other attributes of people.

Another downside is speed. Antibiotics kill billions of bacteria really quickly, leaving a tiny fraction to build back better, which they do incredibly quickly. Here, the selection is (thankfully) much less extreme. This means that populations could get pretty low before they recovered.

A simplistic little model I built suggests that if one percent of the fertile UK population had a fertility rate 1.2 above the 1.8 norm, it would take over 200 years for the population to beat its current level, assuming zero immigration and everything staying static.

But I still think the story is true and important: humans are like bacteria, and we are evolving bacterial resistance to the antibiotic of low fertility.