Fertility and the Life Cycle of Civilizations
An "inverted Malthusian model" can provide a simple general theory of why civilizations rise and fall.
Introduction
The global collapse of fertility levels over the last couple of generations has been dramatic. It has made economists wonder about its implications for the future, as a growing population appears to be a necessary condition for economic growth. In this essay, I propose a model that fertility collapse is actually a natural part of the life cycle of advanced civilizations.
This model can be understood as an “inverted Malthusian model,” which implies that long-run growth for a very, very, long time is impossible: as incomes increase, fertility transition sets in, and lower fertility makes the population smaller, which decreases income.
The Civilization Lifecycle Model
Consider the following model of the rise and fall of civilization, which articulates the idea that civilizations have a natural life cycle:
Starting from an initial point of low population and low degree of economic development.
Rise of civilization: The population is growing, and more people means more ideas and scope for the division of labor, which means more prosperity and, therefore, conditions for further lowering mortality and maintaining population growth.
Fertility transition: As civilization becomes more sophisticated, the population accumulates more human capital which allows individuals to control their fertility. Individuals have disutility regarding the work needed in raising children, so the development of a capacity to control fertility results in a massive decrease in fertility. Low fertility eventually leads to population collapse.
Population collapse leads to the fall of civilization: in the short run, a decreasing population is not a big problem, as fewer people means more capital invested per person, raising living standards. In the long run, however, fewer people means fewer opportunities for division of labor and technological sophistication as less knowledge can be accumulated and utilized by a smaller population. Long-run population decline thus destroys the sophisticated civilization and leads to a return to the situation of (1), re-starting the cycle.
Empirically speaking, the situations described in (1),(2), and (4) are fairly uncontroversial. However, situation (3), that population can and will decline across a civilization if it reaches a high level of human capital, merits some discussion.
Across historical societies, it has been reported that fertility rates tend to be low in elites (as Robin Hanson told me), which have high human capital, even if these historical societies have been mostly poor and illiterate. Low elite fertility does not mean civilizational demographic collapse if this educated elite is a small enough proportion of the population; however, in more egalitarian societies where human capital accumulation occurred across a broad spectrum of the population, low fertility might have caused population collapse.
Empirical examples of civilization life cycles: Greece and Roman Italy
In Greece, archeological field surveys show that population densities increased tremendously from 800 BC onwards, peaked from 450 BC to 300 BC, and dramatically decreased over the following centuries. This coincides with the rise and fall of Classical Greek civilization.
The decrease in population density cannot be explained by economic decline: a typical Malthusian model would predict that incomes fell below the “minimum subsistence level” in Greece by 300 BC. How does that stack up to the empirical evidence? Archeologists have excavated hundreds of ancient houses and determined their roofed space; as incomes are higher, people can afford to spend more on housing, and changes in house size are a proxy indicator of changes in real incomes.
Looking at the houses at the 25% percentile in the excavated house size distribution, which are a good indicator of housing expenditures of commoners instead of elites, we have the following graph below, which shows commoner’s houses were literally ten times bigger by 300 BC compared to houses from 400-500 years earlier, brutally contradicting the Malthusian model:
Instead of poverty, Greece in 300 BC appears to have been exceptionally wealthy among pre-modern societies. It appears to be the case that low fertility was the driving force for the reduction in the population, which is consistent with the literary evidence, for example, this famous quote by ancient Greek historian Polybius (which I already cited before in my substack):
“In our own time the whole of Greece has been subject to a low birth rate and a general decrease of the population, owing to which cities have become deserted and the land has ceased to yield fruit, although there have neither been continuous wars nor epidemics...For as men had fallen into such a state of pretentiousness, avarice, and indolence that they did not wish to marry, or if they married to rear the children born to them, or at most as a rule but one or two of them, so as to leave these in affluence and bring them up to waste their substance...”
― Polybius, The Histories, Vol 6: books.XXVIII-XXXIX, dated from about 150 to 120 BC.
Regarding human capital accumulation, which might have allowed the population to control their fertility, the evidence is also consistent with this “inverse Malthusian model.” It was expected from free adult males in Greek city-states that they were literate and able to write down the names of the people they wanted to be ostracized (a practice that was not restricted to Athens but prevalent across several Greek democracies), which suggests literacy rates were very high at least among free males. Thus, human capital accumulation occurred not only at the elite level but also across the wider population (which is consistent with the increased material standard of living apparent in the archeological record).
Although the population of Greece started to collapse, thanks to a combination of Greek colonization, cultural and economic interaction, and military conquest, a process of Hellenization still occurred across the Mediterranean world. After the death of Alexander in 323 BC, Greek became the lingua franca across the ancient world (even Roman Emperors like Marcus Aurelius wrote their personal diaries in Greek). As Mary Beard explained, more Greek texts survived from the Roman period than from the Classical period). Therefore, the number of Greek speakers peaked in antiquity centuries after Greece’s population peaked.
However, after this process of Hellenization across the ancient world, it appears that the population started to collapse in other regions as well. In Roman Italy, archeological surveys show that population density appears to have peaked from around 50 BC to 100 AD, a few centuries later than in Greece:
Literary evidence also shows that Augustus, the first emperor who ruled Rome from 30 BC to 14 AD, just at the time the Italian population peaked according to archeological surveys, implemented natalist policies to encourage Italians to increase their fertility, which suggests Italy suffered from the same high development driven low fertility crisis as Greece did a few centuries earlier (as Italy at the time of Augustus was at its peak of economic prosperity in antiquity, just when fertility appears to have declined enough to prevent population growth). The decline in population density in Italy over the centuries following Augustus shows these policies apparently failed in the long run. However, I do not know if those policies continued after he died in 14 AD. Anyway, by the 5th century AD, Roman Italy was depopulated and was easily conquered by northern Germanic tribes.
Our modern world has parallels with Greco-Roman antiquity: Fertility collapse in modern times first happened in Europe and Japan. In regions that developed earlier. As economic development spread throughout the planet after WW2, fertility was also declining across all major regions. This makes an interesting parallel with the decline in fertility first in Classical Greece and then in other parts of the ancient Mediterranean world, such as Roman Italy (considering the cases of Greece and Italy, a decline in fertility was also likely among the most developed/urbanized parts of the Roman Empire, such as the territories of prosperous ancient city-states like Rhodes, Alexandria, and Massalia).
Conclusion
This evidence seems to suggest that this simple demographic life cycle theory can, alone, account for the rise and fall of Greco-Roman civilization (before I stressed institutional factors to explain the fall of Greco-Roman civilization, but these simple demographic dynamics appear to have played a major role as well). This simple theory also suggests that, as our modern world enters its low fertility regime, sustained demographic decline will follow, which suggests that the economic collapse of our civilization will occur over the following centuries.
Very interesting. For 3, Hanson was recently describing a race for status as decreasing fertility as an alternative to a disutility for child rearing.
This is utterly fascinating to contemplate. Thanks very much