The Fall of Rome: What was it? Why did it happen?

Peter Turchin


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There are many theories of why Rome fell. A German historian, Alexander Demandt, counted more than 220 explanations, and a few dozen more have been proposed since he published his book in 1984.

Recently, this question again entered public consciousness, when Arron Banks, insurance multi-millionaire and a big donor to the anti-immigration UK Independence Party, opined on Twitter that “the Roman Empire was effectively destroyed by immigration.”

The classicist Mary Beard thought that this was “not just bunkum, but dangerous bunkum”, and fired back: “i think you all need to do a bit more reading in Roman history before telling us what caused the fall of Rome. Facts guys!” This lead to a “Twitter storm”, and you can read about the aftermath, from Beard’s point of view on her blog.

But it should be noted that Beard never even hinted what she thought was the cause of the decline and fall of the Roman Empire. When I twitted this observation, I was immediately challenged to point to the explanation that I favor. Fair enough. I can’t fit it in 140 characters, but I believe an 800-word blog post should suffice.

First, what is “the Fall of Roman Empire”? What are we trying to explain? Let’s rephrase this question in cliodynamic terms. In other words, we need to quantify and pay attention to dynamics (how quantities change with time).

There is no question that Roman Empire reached its peak under the “five good emperors”. There are literally dozens of quantitative measures for imperial might that all agree with each other: territorial extent, overall population, internal peace and political stability, economic activity proxied by shipwrecks and the amount of industrial pollution, monument building, production of literature and art … After the death of the last “good emperor” in 180 all these indicators headed south. Together they tell us a much more quantitative and nuanced history than an artificial binary construct of “the Fall of Rome”. As a single example, here’s the trajectory of the volume of imports of particularly fine ceramics from Africa to Italy:

If we follow these trajectories, we will learn that there were peaks and valleys. For example, a key indicator, social and political instability, went up after 180 and stayed high to the end of the third century. However, there were several peaks on top of this elevated level, recurring at roughly 50-year intervals. Such dynamical richness doesn’t fit the narrative of a “collapse.”

Most of the fourth century was relatively peaceful, but then the western half really disintegrated. The center of gravity moved east, to Byzantium, which experienced its own decline in the seventh century. Which was followed up by more cycles.

Thus, a much better question is not why Rome collapsed, but why the Roman Empire experienced those massive waves of social and political instability, accompanied by political fragmentation, population decline, and (later) dramatic loss of literacy, disappearance of monumental buildings, decrease of economic activity etc.

Let’s focus on the first of these instability waves, which followed after the last “good emperor” died. Why did it happen? Any particular event, especially one of such complexity and scale, has multiple causes. There were human errors and outright villainy. Factors peculiar to the “here and now” (such as the geography of the Italian peninsula, the arrival of the Antonine plague, climate change) also shaped the trajectory. Finally, the outbreak of massive political violence was driven by more fundamental social forces that are at play in all complex, state-level societies. Yes, the infamous “Laws of History.”

Think of astronomers studying the Solar system. A particular planet has many peculiarities – its size, color, presence or not of atmosphere, how far it is from the Sun, whether its orbit is nearly circular or more elliptic, and so on. But in addition to all that wonderful diversity, all planets also obey Newton’s Laws. So what’s the equivalent of Newton’s Laws for the dynamics of political instability in large-scale complex societies?

The short answer is the structural-demographic theory.

A somewhat longer answer, suitable for a blog: Growing political instability is first and foremost a result of elite overproduction leading to excessive intra-elite competition and conflict. This main driver is supplemented by mass mobilization of non-elites resulting from popular immiseration and by failing fiscal health of the state. A really long answer is in Chapter 7 of Secular Cycles (for the crisis of the third century in Rome) and in an article by David Baker (for what happened after that).

Once again, all kinds of additional factors played a role, but if you are interested in general principles, understanding why empires decline and fall, then the structural-demographic theory is your best friend.


Here’s the reference to Demandt’s book if anybody cares to read about the 220 theories (and reads German):

Demandt, Alexander. 1984. Der Fall Roms: die Auflösung des römischen Reiches im Urteil der Nachwelt. Munich: Beck.





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Gene Anderson

I’m on board–the longer study in SECULAR CYCLES is wonderful. I still enjoy the informal summary of Edward Gibbon: “Christians and barbarians.” Of course Gibbon was actually much more nuanced. Marx’ point that you can go only so far with a slave economy is not bad–certainly works for the Byzantine Empire, which doomed itself to a thousand years of stagnation and final collapse partly by being so dependent on the slave trade–not a great way to innovate or develop.


Regarding the “additional factors” involved in the Fall of the Roman Empire, I would like to share a further approach to the problem:

al loomis

it ceased to be roman when the latifundia drove the warrior-farmers into the city. it ceased to be a republic when the professionalized warriors became the clients of their officers. when the officers contended for the purple, the economy was further degraded.
so, when the goths showed up, there were not many soldiers to meet them.
rome did not fall so much as dissolve, corroded by military success in the later years of the republic.
there is some resemblance to modern usa.

Roger Cooper

The elite overproduction argument reverses cause and effect. With the security breakdown in the Western Roman Empire, large landowners could better protect themselves from raiders than medium size holders by forming private armies. In the more secure East, this did not happen.

Loren Petrich has a nice discussion of the fall of the Western Roman Empire. From it:

Yerxa: Is there evidence that a civilization collapsed when Rome fell?

Ward-Perkins: This is an area where historians seem to be decidedly myopic. In looking closely at their texts, they have failed to notice that in every single area of the empire (except perhaps the Levantine provinces conquered by the Arabs) there was an extraordinary fall in what archaeologists term “material culture.”

Buildings shrank, even churches, and their construction materials became much more crude. Like thatched roofs instead of tiled ones. Graffiti became rare, an indicator of lowered literacy. Etc.

al loomis

listing material factors is important, but perhaps the most important factor is the social glue, a sense of community that is shaped by those material factors. the passage of huns, vandals and goths through roman lands could never have happened in the later republic simply because there was effectively endless supplies of tough farm boys to man the legions. and no one doubted that victory was inevitable, whatever setbacks eventuated. how to measure morale?

Roger Cooper

You can really talk about the Fall of Rome without comparing it to other mega-states. Prior to the “Gunpowder Empires” of the early modern period, all mega-states were unstable. The all share the problems of succession crises, suppressing regional rebellions, and dealing with barbarian (non-state) invaders.

Perhaps a better question is why did Rome last so long. Alexander the Great’s empire fell apart soon after his death, Charlemagne’s empire also quickly split. (Note that for both Alexander & Charlemagne, the more managable sucessor states lasted longer).

al loomis

yes, a very good point. the obvious answer is that rome found a way to incorporate conquered states within itself, ultimately as equals. this provided an ever renewing source of soldiers and trade.

Loren Petrich

I think that an interesting question is whether any sizable polity has ever been conquered while in an integrative phase. Could such a conquest reasonably be ascribed to brute military force or something similar? Like the conquerors having bigger armies, superior weapons and/or tactics, and the like. Or else bringing in diseases that their targets had no experience of. I’m thinking of the likes of the Aztec and Inca Empires.

Let’s see about the Roman Empire. From its origins as a city-state, it had a few centuries of integrative phase until it started suffering from civil wars around 100 BCE. Even during that disintegrative phase, Roman generals continued their conquests, like Julius Caesar conquering Gaul. The Republic became the Empire, and its early centuries were an integrative phase. However, it did not conquer very much. Then the Crisis of the Third Century, a disintegrative phase. The Empire started to fall apart, with the breakway Empire of the Gauls and with Zenobia’s Middle Eastern conquests. They were both defeated, and the Empire had an integrative phase under Diocletian and Constantine. But in the next disintegrative phase, the Western Roman Empire was destroyed, and civilization there had a big setback that it took centuries to recover from.

But the Eastern Roman Empire, a.k.a. the Byzantine Empire, recovered from this phase in the 6th century, under Justinian and his successors. It then lost out to the Arabs in the 7th century, with the Arabs even once besieging Constantinople. It also lost much of the Balkans to Slavs and the like. Then from the middle of the 9th cy. to the middle of the 11th cy., the Empire had another recovery, conquering some territories from the Arabs and the Bulgars. Then fragmentation and loss of territory, and then the 12th-cy. Renaissance. This was followed by its final decline and fall.

So some polities have survived several disintegrative phases. But could this be a case of survivor bias? Meaning that these ones are the ones that we notice more than others.


How do you definite integrative and disintegrative?

The Qin united China but they were overthrown within decades. The Han who succeeded them lasted for centuries.

The Sui united China but they were overthrown within decades. The Tang who succeeded them lasted for centuries.

The Yuan (Mongols) conquered China but they were overthrown within decades. The Ming who succeeded them lasted for centuries.

Ross Hartshorn

I think this is one of those “why” questions that can be answered on several different levels, each valid. Somewhat like, “why are rabbits in the arctic circle white?” Valid answers include:
1) they are white because their hair has no pigment
2) they are white because of certain genes in their DNA for controlling pigmentation
3) they are white because snow is (usually) white
4) they are white because they are not large enough, or well-armored enough, to fight off predators
5) they are white because the arctic circle has a lot of snow and ice, which is white
6) they are white because their chief predators use vision, at least in part, to find them

Every one of these answers could be considered correct. I am sure that the fall of the Roman civilization has several different levels of answer. In fact, if you stretch the definition of “immigration” enough, you might even be able to make an inability to control immigration into the “cause”.

I think it is more useful to turn things around. Not “why did Rome fall”, but “is your theory of how societies work consistent with the fall of Rome”? I would say SDT passes that test.

I am curious, did Ibn Khaldun ever express an opinion on the fall of Rome?

John Lilburne

In moving to thr structuralist theory you have moved somwhat away from your earlier emphaisi on abbasiya…solidaity/ trust. The advantage of the structuralist theory is that it is more easliy measurable , particularly for ancient societis than measuring something more nebulous but important assabiya/solidarity/trust. However immigration has a considerable effect both on the elite and on the masses. On the elite level, the elites begin to see themselves seperate from the masses. And the masses under malthusian pressure lose confidence for the elite (why must i die for those fat corrupt slobs). Most measurement of mass immigration in present societies show that trust declines very rapidly. When one group becomes more powerful compared to the historical power of another group, then they start to clainm their “rights”. Thus the germans were first invited into the roman empire to act as mercenaries and then later to revolt to increase their power.


Generally only when the older group doesn’t realize that their power has diminished due to a relative decline in numbers and thus abuse/take advantage of the newer group. That is what happened with the Romans and Goths:
“So many people in so small an area caused a food shortage and eventually the Thervings began to starve.Roman logistics could not cope with the vast numbers and officials under the command of Lupicinus, simply sold off much of the food before it reached the hands of the Goths. Desperate, Gothic families sold many of their children into slavery to Romans for dog meat at the price of one child per one dog.
This treatment caused the Therving Goths to grow rebellious. . .”


I am new here. I have a question. Has anyone used Adaptive Complex Systems approach using a node and edge analysis to evaluate:
1) The quality of the nodes (agents)
2) The number and quality of the edges (relationships)
3) During various times in the history of Rome
4) Between Rome and other republic/empires?
The potential value would be to integrate Nassim Taleb’s work with “Black Swan” events and how they relate to things like having “skin in the game” and how that impacts the strength of the system.

I can, a priori, intuit that the Roman republic would be inherently stronger because the institutions were flat and the agents individually stronger with shorter yet stronger edges. The Empire would necessarily have a more hierarchy creating longer weaker edges and the number of individually strong agents acting in the system would reduce. (How would immigration impact the quality of the nodes?)

If you take Carl Jung’s quote seriously,

“Whenever justice is uncertain and police spying and terror are at work, human beings fall into isolation, which, of course, is the aim and purpose of the dictator state, since it is based on the greatest possible accumulation of depotentiated social units.”

than one might believe it is in the best interest of an emperor to depotentiate his own subjects to protect power and control. The consequence is a weakened society (i.e. weak nodes, few and weak edges). This is a very unstable situation in an adaptive complex system.


Population is an indicator that curiously is not mentioned and surely has something to do with “fundamental laws of motion that affect all planets (or empires)”. The city of Rome experienced a massive population decline in the early centuries of the Christian era (see the population chart for the city of Rome).


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There’s a physical analogy that helps to understand what happened. During convection in Benard cells, liquid from the periphery has to drawn to the center (immigration of talented youth), then risen up (social lifts) and distributed back to periphery (dissemination if culture). If some part of this cycle gets stuck, the systems gets disorganized, – talents are unrecognized or wasted, there’s little chance to climb up the ladder, and those who do climb it don’t disseminate anything worthwhile.

This analogy also allows insight into rise and fall of empires, – Benard cells that encompass other cells due to better organization, and then shrink when disorganized.

I can write more about it if you are interested.

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