The lost millennium
Note: This post is speculative, and you should take the claims in it with a grain of salt.
Throughout human history, we know that economic growth, for which population growth was a good proxy under Malthusian conditions, has accelerated many times. Indeed, we can see this just by extrapolating current growth rates backward: given that there are only so many people in the world and they are only at a finite multiple of subsistence income per person, it would not be possible for economic growth rates on the order of 3% per year to have lasted for more than a thousand years or so.
Though it’s difficult to do so, people have also come up with estimates for exactly how this growth happened in the past. One such estimate is from the HYDE dataset, also available at Our World In Data; and a whole collection of other estimates can be found here.
Let’s examine the HYDE dataset first. The data before 1 AD is given per millennium, and looking at the growth rates gives us the table below:
| Millennium | Mean annual population growth rate (percent) |
|---|---|
| 10000 BC − 9000 BC | 0.0236% |
| 9000 BC − 8000 BC | 0.0254% |
| 8000 BC − 7000 BC | 0.0279% |
| 7000 BC − 6000 BC | 0.0320% |
| 6000 BC − 5000 BC | 0.0368% |
| 5000 BC − 4000 BC | 0.0411% |
| 4000 BC − 3000 BC | 0.0435% |
| 3000 BC − 2000 BC | 0.0489% |
| 2000 BC − 1000 BC | 0.0415% |
| 1000 BC − 1 AD | 0.0746% |
Aside from a slight reversal in the second millennium BC[1], the estimates suggest a gradual acceleration in the growth rate of population. This would be consistent with e.g. a hyperbolic model for population growth, but here I don’t want to make any such assumption and just look at the raw data.
What is surprising, then, is the next entry that should be in this table: in the first millennium, 1 AD − 1000 AD, population growth is estimated to have averaged a mere 0.033%/year. In other words, according to this dataset, even the fifth millennium BC had faster population growth than the first millennium!
Something like this result turns out to be robust to changing the dataset we’re using. McEvedy and Jones report a mean growth rate of 0.044%/year for the first millennium, compared to 0.137%/year from 1000 BC to 200 BC and 0.062%/year from 200 BC to 1 AD. This more granular data in the first millennium BC, not available in the HYDE dataset, suggests that the slowdown in population growth likely started before the first millennium. To find a millennium with slower population growth in this dataset, we have to go back to the fourth millennium BC (5000 BC − 4000 BC).
All datasets I’ve looked at report substantial acceleration in population growth with the turn of the second millennium. The Mongol conquests and the Black Death appear to have weighed on population growth from 1200 to 1400, but despite that growth from 1000 to 1500 still averages 0.08%/year, which is faster than the first millennium BC.
The Lost Decade was a name used in Japan to refer to the ten-year period of economic stagnation following the Japanese stock market crash of 1991. Following this convention, it might be appropriate to call the first millennium “the lost millennium”. If we believe in some kind of stochastic hyperbolic growth model of economic history, a la Roodman (2020), this means that humanity got “very unlucky” in the first millennium, possibly due to persistent adverse social shocks.
This story is purely quantitative, looking at population growth, but it’s worth keeping in mind that under Malthusian conditions we expect population growth to track technological progress and capital formation. The more productive we are, the more children people should have until the population increases sufficiently for the marginal product of labor to fall back to subsistence levels. A millennium is more than enough time for Malthusian dynamics to play out, so the lack of robust population growth over the first millennium also suggests a slowdown in productivity-increasing factors such as innovation and capital accumulation.
The important question then becomes: why did this happen?, or perhaps how did this happen? We have good explanations for other slowdowns in population growth, e.g. the Mongol conquests and the Black Death for the period 1200 to 1400. However, the slowdown in the first millennium looks like a complete mystery. I can come up with some explanations but none of them fit the data that well. Even if it was “bad luck”, I’d like to be able to say more about it than that.
- ↩︎
Perhaps due to the Late Bronze Age collapse?
In the west, I think the fall of the Western Roman Empire was probably a significant hit, and caused a major setback in economic growth in Europe. China had its bloody Three Kingdom period, and later the An Lushan rebellion. There were other plagues too, like the Antonine plague. There was the Muslim conquest of the Mediterranean, Persia and Pakistan, though I don’t know if that was unusually bloody. There also were plenty of Viking raids. These might be small fluctuations in the grand scheme of things or add up to a period of enough turmoil and strife in the most populous regions of the world to slow growth down.
On this topic (and for looking at the causality direction) I think the book “In the Shadow of the Sword” by Tom Holland is quite informative. The book covers the “late antiquity” period, a lot of it involves the origins of islam and a lot of the (Eastern) romans having trouble with it all. From my memory their were either one or two devastating plagues in the Roman empire, which massively depopulated its cities. The non-urban populations (eg. the horse riding nomads) were barely effected by the plague (they were said to be immune, but it was probably just transmission opportunities). The book argues this was an important factor in the Roman Empires defeat by the new Islamic empires, a large fraction of the legionnaires the romans should have had had never been born because their parents had died of plague twenty years earlier. (The were a lot of other factors going on too, but this one stood out in this discussion.) I think that the Persians had the same problem when it was their turn to fight the same arabs, a fraction of their troops had never been born.
I’ve actually written about this subject before, and I agree that the first plague pandemic could have been significant: perhaps killing around 8% of the global population in the four years from 541 to 544. However, it’s also worth noting that our evidence for this decline is rather scant; we know that the death toll was very high in Constantinople but not much about what happened outside the capital, mostly because nobody was there to write it down. So it’s also entirely conceivable that the death toll was much lower than this. The controversy about this continues to this day in the literature, as far as I know.
The hypothesis that the bubonic plague was responsible is interesting, but by itself doesn’t explain the more granular data which suggests the slowdown starts around 200 BC and we already see close to no growth in global population from e.g. 200 AD to 500 AD. HYDE doesn’t have this, but the McEvedy and Jones dataset does.
It’s possible, and perhaps even likely, that the explanation is not monocasual. In this case, the first plague pandemic could have been one of the many factors that dragged population growth down throughout the first millennium.
Man, he has time to do such important academic work, at his age, between one Spider-Man movie and another? Talk about a polymath!
(I’m sorry)
Never underestimate the benefits of living mostly outdoors or in tents with plenty of ventilation. But yeah, I mentioned the Antonine plague but I think I remember there being more. These were pretty destructive events so obviously they’d leave a mark.
Attribution of causality is tricky with this event, but I would agree if you said the fall coincided with a major slowdown in European economic growth.
I think a problem re: China is that a lot of population decline estimates for China are based on the official census, and as far as I know China didn’t have a formal census before the Xin dynasty, and certainly not before unification in the 3rd century BC. So the fact that we don’t see comparable population declines reported may just be an artifact of that measurement issue. We certainly see plenty of them in the second millennium.
I haven’t seen estimates of this that put it anywhere near the Mongol conquests, so I would assume not particularly bloody relative to what was to come later. I would also guess that the Islamic world probably saw significant population growth around that time.
Yeah, it’s possible that this is the explanation, but if so it’s rather hard to know because there’s no principled way to compare events like these to analogs in other time periods.
Yes, I suppose the arrow could go the other way around—that economic recession caused the fall. Or really, probably just a feedback loop of stuff going to shit. Sorry for the unwarranted implication.
Yeah, just suggesting possible sources. But also, any estimates of population growth in the 1-1000 AD range must account for China, so if we can’t trust the census, are you sure your figures too aren’t affected by this fundamental problem?
Anyway, this looks like an interesting history problem—first, figuring out if the effect is real, and then, if it is, what caused it. But there’s probably enough research for a PhD, or even a whole career, in such a wide field. It’s a super complex question.
I think to answer this you need to break things down by region.
A look at our world in data shows that China was a 5th of the world population at the start of the first millennium, and it shrunk over the course of the millennium. Europe also shrunk, coinciding with the fall of Rome.
Africa, the Americas, and Australia gre at what looks like a fairly normal rate, but they are a much smaller percentage of the population.
So the explanation is likely to be local to the most populous regions, rather than some global event.
McEvedy and Jones actually discuss a regional breakdown in the final section of the book, but they speculate too much for the discussion to be useful, I think. They attribute any substantial slowdown in growth rates to population running up against technological limits, which seems like a just-so story that could explain anything.
They note that the 3rd century AD appears to have been a critical time, as it’s when population growth trends reversed in both Europe and China at around the same time: in Europe with the Crisis of the Third Century, and in China with the fall of the reconstituted Han dynasty and the beginning of the Three Kingdoms period. They attribute this to technological constraints, which seems like an unsupported assertion to me.
The other important population center is India, where we have very few records compared to Europe and China. Datasets generally report naively extrapolated smooth curves for the Indian population before the modern period, and that’s because there really isn’t much else to do due to the scarcity of useful information. This doesn’t mean that we actually expect population growth in India to have been smooth, just that in the absence of more information our best guess for each date should probably be a smoothly increasing function of the date. As McEvedy and Jones put it, “happy is the graph that has no history”.
I agree that locations isolated from Eurasia would most likely not show the same population trends, but Eurasia was ~ 75% of the world’s population in the first millennium and so events in Eurasia dominate what happens to the global population.
Well, that’s true, but at some level, what else could it possibly be? What other cause could be behind the long-run expansion in the first place, so many millennia after humans spanned every continent but Antarctica?
I’m very skeptical about explanations involving wars and plagues, except insofar as those impact technological development and infrastructure, because a handful of generations is plenty to get back to the Malthusian limit even if a majority of the population dies in some major event (especially regional events where you can then also get migration or invasion from less affected regions). I guess because they use nice round year numbers as cutoffs there could be artifacts of events right near the beginning or end of a millennium, but things happening in the 2nd or 3rd century AD aren’t candidates for that.
Technological progress being responsible for the long-run trend doesn’t mean you can attribute local reversals to humans hitting limits to technological progress. Just as a silly example, the emergence of a new strain of plague could have led to the depopulation of urban centers, which lowers R&D efficiency because you lose concentrations of people working together, and thus lowers the rate of technological progress. I’m not saying this is what actually happened, but it seems like a possible story to me.
I agree, but why would you assume wars and plagues can’t impact technological development and infrastructure?
Yes, it’s a possible story. And yes, wars and plagues impact tech development and infrastructure all the time. But I find it hard to think about how to draw a principled distinction between local growth slowdowns and hitting local limits to technological growth.
For example, in medieval Europe the Black Plague definitely depopulated the continent in ways that affected lots and lots of things, and there was no way they could reasonably have known how to do better. Resolving the plague wasn’t possible because the solutions were beyond the local limits of technological growth. If the same plague struck today, that wouldn’t happen, because we have the tools to deal with it. We understand sanitation, and disease vectors, and we can develop vaccines. It would be a blip of a decade and a few percent GDP growth, rather than half the population and centuries of recovery.
What I think would be very interesting, though highly difficult I suspect, would be a control variable related to social institutional factors. While improvements in both economic productivity and technology should support increased population growth, so would more stable and peaceful societies.
In that regard I think I would start looking at for the decline in the first millennia AD would be related to social environment and institutions rather than economic—accepting that there will be some close links between social and economic outcomes that might be hard to separate.
First of all, the population numbers are complete garbage. This is completely circular. You are just reading out the beliefs about history used to fabricate them. The numbers are generated by people caring about the fall of Rome. The fall of Rome didn’t cause of decline in China. Westerners caring about the fall of Rome caused the apparent decline in China.
Second, there was a tremendous scientific and technological regress in Rome. Not caused by the fall of Rome, but the rise of Rome. There was a continual regress in the Mediterranean from 150BC to at least 600AD. Just look at a list of scientists: it has a stark gap 150BC-50AD. It is more controversial to say that the renaissance 50AD-150AD is a pale shadow of the Hellenistic period, but it is. In 145BC Rome fomented a civil war in Egypt, destroying Alexandria, the greatest center of learning. In 133BC, the king of Pergamon tried to avoid this fate by donating the second center of learning. It was peaceful, but science did not survive.
I downvoted this comment for its overconfidence.
I will freely admit that I don’t know how population numbers are estimated in every case, but your analysis of the issue is highly simplistic. Estimates for population decline do not just depend on vague impressions of the significance of grand historical events such as the fall of Rome. Archaeological evidence, estimates of crop yields with contemporary technology on available farmland, surviving records from the time, etc. are all used in forming population estimates.
It’s far from being reliable, but what we know seems clear enough that I would give something like 80% to 90% chance that the first millennium indeed had slower population growth than the first millennium BC. You can’t be certain with such things, but I also don’t agree that the numbers are “complete garbage” and contain no useful information.
I think you’re conflating a lack of progress with regression here. I remark in the post that the slowdown in population growth seems to have begun around 200 BC, which is consistent with what you’re saying here if you take it as a statement about growth rates and not about levels. If the pace of new discoveries slows down, that would appear to us as fewer notable scientists as well as slower growth in population, sizes of urban centers, etc.
Aside from that, there are also many alternative explanations of a gap in a list of scientists, e.g. that Rome was comparatively less interested in funding fundamental research compared to the Hellenistic kingdoms. Progress in fundamental sciences doesn’t always correlate so well with economic performance; e.g. the USSR was much better at fundamental science than their economic performance would suggest.
I don’t know what you’re referring to by “Rome fomented a civil war in Egypt in 145 BC”. 145 BC is when Ptolemy VI died; but as far as I know, there was no single “civil war” following his death, Alexandria was not destroyed, and Rome was not involved directly in Egyptian politics for a long time to come. Alexandria remained one of the major urban centers of the Mediterranean until the 3rd century AD—perhaps even the largest one.