It’s Easter, and instead of continuing with my series on the New Caliphate, which is quite gloomy, I thought I would take a break from it and write something more appropriate for the holiday. I’ll post the next installment in the series after the holidays.
Corpus Christi procession. Oil on canvas by Carl Emil Doepler
But I also want to do something “cliodynamicky,” because that’s what this blog is about. Fortunately, I have just the thing, resulting from my early analyses of the dynamics of religious conversion, which was included in Chapter 6 in my 2003 book Historical Dynamics.
One of the cases I looked into in that chapter was conversion to Christianity. My interest was motivated by Rodney Stark’s delightful book, The Rise of Christianity: How the Obscure, Marginal Jesus Movement Became the Dominant Religious Force in the Western World in a Few Centuries. Stark developed a quantitative model for the growth of the numbers of early Christians. The model is based on the idea that it is people who convert people. This simple idea (which is supported by sociological studies of religious conversion) is similar to the “autocatalytic” model in chemical kinetics. Basically, the more converts there are the more likely an unconverted unbeliever to encounter them and become converted. So the predicted dynamics is initially exponential — an accelerating curve.
To get at the shape of the curve, Stark made a guess that there could be around a thousand converts in the Roman Empire in 40 CE, shortly after the crucifixion of Jesus. He then proposed that their numbers grew at the rate of 40% per decade. Several years after he made these estimates, a colleague attracted his attention to the reconstruction by the American Classicist Roger Bagnall of the growth of Christianity in Egypt. Bagnall came up with an estimate of how many Christians there were in Egypt based on the number of people with identifiable Christian names mentioned in Egyptian papyri.
Since Stark was unaware of Bagnall’s data at the time when he constructed his prediction, we have here a true prediction here (in technical terms, test of theory with out-of-sample data).
When I wrote my 2003 book, I fitted the autocatalytic model to Bagnall’s data and found that it was very close to the curve guesstimated by Stark. Here’s what it looks like:
Source: Peter Turchin
As you can see, the proportion of Christians grows very slowly for the first 200 years following the Easter of 30 AD (the most probable year of the Crucifixion). Then, it suddenly explodes during the fourth century. Actually, nothing changes in the generating mechanism — slow initial growth followed by blow-up is what exponential models do. Eventually, the curve bends down and approaches 1, because the proportion of converted cannot exceed that upper limit.
So what we see here is an interesting example of how predictions in Cliodynamics can be tested with data. But the story gets better.
Two years after I wrote the chapter on conversion in Historical Dynamics, I happened on a reference to a German dissertation that gave a list of Pagan and Christian office-holders between 324 and 455. The dissertation was by R. von Haehling, entitled “Die Religionszugehörigkeit der hohen Amtsträger des Römischen Reiches seit Constantins I. Alleinherrschaft bis zum Ende der Theodosianischen Dynastie,” published in Bonn in 1978.
I immediately realized that von Haehling’s data enable us to make another test of the autocatalytic model.What I then did was to plot von Haehling’s numbers on top of the previously published curve:
Source: Peter Turchin
The results are quite amazing. We see that the curve fitted to the Bagnall data (showing the proportions converted before 300 CE, filled circles) does a very good job predicting the course of Christianization in the von Haehling data (after 330 CE, hollow circles). The two data sets, each collected using different methods and from different parts of the Roman Empire, merge seamlessly — and line up very closely to the theoretical curve.
It’s possible to do history as an analytical, predictive science!
Happy Easter!
Fascinating.
What I’m curious about, though, is why some movements/religions sustain high growth rates while others do not?
Or maybe all our descendants will be Mormons 5 centuries hence.
That’s a very good question, and the answer is that nobody yet knows. Eventually we will get around to testing theories with a Seshat approach, but for now I can recommend David Sloan WIlson’s Darwin’s Cathedral, and Rodney Stark’s books.
The numbers of adherents to the Latter Day Saints (Mormon) church has been increasing exponentially ever since the church was founded. No signs of slowing down yet.
What happens when you repeat the exercise for Islam in Egypt?
I don’t have data for Egypt, but for Iran and Spain it’s the same — logistic curve with a similar rate of growth, so it takes ~3 centuries to go from small numbers to near saturation level.
Carl Coon comments:
Peter, if you factor in the new technology what happens? The concept involves personal persuasion, doesn’t that also work for Facebook, or does it? This has implications for the future of humanism and for the triumph of reason and science over the old religions. In looking toward the future of humanism what can one expect?
Hi Carl,
I am not sure that new technology has changed dramatically the process of religious conversion. We don’t get religion by reading about it in a newspaper, seeing it on TV, or in a blog. Sociological research shows that conversion travels along thick connections in social networks. This is the work of Stark and coworkers, and more recently Scott Atran an coworkers on youth becoming radicalized. So I may be wrong, but so far new tech has not changed the basic dynamics.
I wonder if decline also moves in an exponential manner.
For instance, the mainline American Protestant churches (the Seven Sisters + a few others) have been in decline in recent decades. Same with the various established denominations in Europe. What can they expect in a few centuries?
Also of interest are the different paths of Buddhism in China&Japan vs. in various SE Asian countries (vs. it’s birthplace of India). In China&Japan, Buddhism seemed to enjoy phenomenal growth for a period of time, but never got close to 100% of the population, while in various SE Asian countries, it did (and in India, it was almost completely wiped out even though India featured several kingdoms that were almost completely Buddhist at various times).
The decline almost has to move exponentially. For example, Iran prior to the Islamic conquest was pretty solidly Zoroastrian. So, as Islam spread exponentially, Zoroastrianism had to fade away also exponentially. Except there was a pocket of Zoroastrianism that never went away, and still persists today. Complex dynamics!
It would be interesting to see similar data for Buddhism in China, non-Anglican Protestantism in England, and other initially successful movements that did not make it to 100%. How far along the curve do you have to go before you can see evidence that it is not headed to saturation? Or does it follow the above curve faithfully until some disruptive event alters the trajectory?
Yes, it would be very interesting, and the data, or proxies at the very least, must be available.
As I said in my response to Richard above, the actual trajectories are more complex than a simple logistic curve. Check my 2003 book for some more intricate details of the dynamics of religious conversion.
I wonder if state backing makes the difference.
In China (outside of maybe 1-2 individual emperors)* and Japan (and Korea, which I forgot originally), Buddhism never enjoyed official state backing while it did in various SE Asian kingdoms.
Christianity also became the official religion of the Roman Empire after Constantine (likewise Islam under various Muslim potentates).
*You can argue that Tibetan Buddhism was the religion of the Qing court, but it’s so different from the main strain of Buddhism in China–Chinese actually differentiate it, calling it “Lamaism” instead of “Buddhism”–that it had little chance of gaining a large percentage of Han Chinese converts.
So many people call themselves Christians yet they lack a personal relationship with Jesus Christ. The reality is Constantine created Christianity aka Catholicism to control the people and hold his throne. Catholicism was a mix of paganism and the Bible to pull Born Again Believers into paganism and Baal worship. The reality of Biblical Salvation is anyone who believes on what Jesus Christ did on the cross is a saved saint. He/She is born again and has eternal salvation. There are no good works required to be saved and that certainly includes repentance. Repentance does not even mean turning from sin, as so many religious heretics try and push. 99% of the churches in America preach a false gospel and have perverted it to the point of no return. The problem is, the people themselves are not reading the Bible because they are too busy worshiping at the throne of statism, atheism or some other ridiculous cult.