Paul Ehrlich’s 1968 bestseller The Population Bomb starts with an introduction “The battle to feed all of humanity is over. In the 1970s hundreds of millions of people will starve to death in spite of any crash programs embarked upon now.” Of course 45 years later food has never been cheaper or more abundant. So Ehrlich style Malthusian panic is old school. New school is panicking about the centuries long decline in human fertility, which if it continues will mean humans go extinct. So depending on your disposition you can panic about too many babies or too few. As a corrective to this let’s pull out the toolkit of population genetics, and see what it says for humans.
First let’s debunk old school Ehrlich fear of Malthusian overpopulation. The chart below from wikipedia shows a model of birth rates and death rates as a country industrializes. This population model is called the Demographic Transition.
Demographic Transition stages:
- Stage 1: Pre-industrial – Malthusian steady state with fluctuations.
- Stage 2: Developing Country – death rates fall, but faster than birth rates decline. Rapid population growth. Think England in the mid 1800’s, Japan in the early 1900’s, or much of Africa now.
- Stage 3: Birth rates decline due to education of women, urbanization, etc.
- Stage 4: Fertility is low but so are death rates. Population stable with fluctuations.
- Stage 5: [proposed not agreed to] Fertility continues to fall and overall population starts to decline.
What’s amazing is that the demographic transition happened across so many economic systems, time periods and global cultures. Highly unusual for any trend in social science. By now it’s clear even at a global level since most of the world has gone through the transition as you can see in the chart below.
Of course this chart leads us to the new school way to panic about population. What happens if fertility just continues to drop forever (stage 5), and we wind up going extinct? For example see this recent Slate article. This is a much fresher way of panicking.
Below is a world map of global fertility rates, where 2 children per woman is replacement level. Blue means fertility rate of <2, green is 2-3, yellow 3-4, orange red purple are higher (still industrializing). As you can see, essentially every continent except Africa is already through the demographic transition.
Japan has a fertility rate of 1.4, China is 1.5, and most of Europe is easily under 2. Also note that the US population would be declining as well if it weren’t for immigration.
Now is when we have to bust out the population genetics tools. From a pure Darwinian point of view, the rapid transition to an industrialized urban lifestyle amounted to a massive environmental shock. One reaction was people switched to having fewer children but investing more in ones they had. This matches r/K selection theory where organisms in stable environments spend more resources investing in fewer offspring, while organisms in unstable environments tend to produce more offspring but invest less in each. Obviously for people the cultural aspects of adaptation matter far more than the genetics, but for our purposes here we fortunately don’t have to disentangle the two.
The key point here is that the global human population is not a monoculture, genetically or culturally. In that sense the new panic around fertility decline duplicates the fallacy Ehrlich made in the 1960’s. Ehrlich ignored the demographic transition by lumping everyone into a single monoculture of ever growing population. His critics, even at the time, could see the demographic transition was progressively sweeping the globe since they were smart enough to split their data analysis out country by country.
Once you think in terms of sub-populations, it’s possible to see how declining fertility will play out. Some groups will just disappear through the Darwinian sieve. But others will adapt or are already adapted and will do just fine. Of course religion is the best predictor of fertility, with Hutterites being fertility’s poster child. But there are many ways to be reproductively successful, so we’ll see a wide variety of sub-populations make it across the divide. Short term, let’s say the next 50 years, we’ll see continued slowing of population growth. And this will be a big deal in countries like Japan or Europe or shortly China, which are seeing or will shortly see declines in population. This is a real story and will have economic consequences. But longer term, say 100 years, fertility will rebound as all that will be left will be groups adapted to the industrialized urban environment. The next 50-100 years will in fact be an era of moderation in terms of population, where the balance of upward and downward fertility won’t go out of control either way. Here’s more along these lines if you want to dig in.
An interesting observation here is that the people most worried about environment also tend to be the most worried about the population. This makes sense since it’s obviously true our environmental problems such as global warming are due to sheer number of people on the planet. Personally, I view environmentalism as a type of luxury good, where it’s hard to get people to worry about it if they don’t have enough to eat. The fact that the whole world is finally getting through the demographic transition is great news for environmentalism, because it’s finally becoming politically practical to pay for it. With that said, piling on gratuitous panic about people starving (old school) or going extinct (new school) doesn’t match the social science data. From a credibility point of view, we should drop the population apocalypse aspect of the environmental message. Ehrlich is still kicking around, and in my view did more harm than good by recently claiming there’s only a 10% chance of civilization’s survival. This just makes everyone on his side sound like a crank. Our generation’s environmental challenges are plenty big. No need to go apocalyptic.