When I became interested in what eventually became Cliodynamics, initially I thought that I would just play with some mathematical models of historical processes, because I had read many times before that there is very little quantitative data in history. In fact, the opposite is true. It turned out that history has massive amounts of data with which we can test our theories, and this empirical corpus continues to grow. The main source of new quantitative data is the clever use of “proxies” — indirect quantitative indicators of various processes of interest in historical dynamics. I see articles publishing such new data almost weekly.
The latest one is Linking European building activity with plague history by a team of dendrochronologists based in many labs across Central and Northern Europe, led by Fredrik Ljunqvist. Dendrochronology is the method of dating tree rings, and it allows us to pinpoint the date when a tree was felled down to a year. Ljungqvist’s group went around Europe taking cores from wooden beams in old houses and then dated them. So far they have collected nearly 50,000 such dates for building construction.
The inscription on this house in Lübeck claims it was built in 1535. I wonder, would a dendrochronological date agree? (Photo by the author)
Because new houses are usually built to accommodate extra population, the distribution of dated building years provides us with a great quantitative proxy for population increases (see panel A in the chart below):
From Figure 1 of Ljungkvist et al. 2018
One thing to keep in mind is that because old houses are constantly destroyed, the probability that a 13th century house would survive to the present is much smaller than such probability for a 17th century house. Thus, the felling dates data should be detrended. If one does so, then two great oscillations become even more apparent, with slowdowns in building new houses reflecting the two periods of general crisis in Europe: the Late Medieval crisis of the 14th and 15th century, and the General Crisis of the Seventeenth Century. You can read more about these cyclic crises in our book Secular Cycles.
The second chart (panel B) in the figure shows the distribution of plague outbreaks, recorded in historical sources, from a well-known compilation based on the pioneering work by the French demographer Jean Nöel Biraben in the 1970s. As the article title indicates, the main factor, potentially explaining building slowdowns, which Ljunqvist et al looked at, is mortality resulting from epidemics. Here I must temper my praise for the article with some criticism. While the data themselves are wonderful, I am not sure that relating them to plague outbreaks is the most interesting thing one could do with them.
An old house in Toulouse (photo by the author)
As the authors themselves acknowledge, European population, and thus building activity, started to decline well before the Black Death of 1347-1352. The causes of this decline are actually well known, and are due to worsening structural-demographic conditions in Western Europe. Furthermore, and even more interestingly, population in most countries did not start growing immediately after the cessation of plague outbreaks. In our book Secular Cycles we discuss the possible reasons for late medieval England and France, and come to the conclusion that the factor that held back population growth was incessant socio-political instability (the Hundred Years War in France, and the Wars of the Roses in England).
Ljungqvist and co-authors propose a similar explanation for the Seventeenth Century population decline in Germany (the Thirty Years War), but they treat it as a singular event. In fact, the Thirty Years War was part of the General Crisis of the Seventeenth Century that affected most of Eurasia, from England to Japan.
Some years ago Walter Scheidel and I proposed that we can use the frequency of coin hoards as a quantitative proxy for internal warfare (see our article, Turchin P, Scheidel W. 2009. Coin Hoards Speak of Population Declines in Ancient Rome. PNAS 106: 17276-17279.). One could use a similar approach in the analysis of the data collected by Ljungqvist et al. As an example, here’s what coin hoard data for Bohemia (modern Czech Republic) look like:
You can clearly see civil wars associated with the Late Medieval and the Seventeenth Century crises (the Hussite Wars and the Thirty Years War). Unfortunately, Czech building dates are not yet publicly available. It will be very interesting to see whether coin hoard data provide a better predictor for the cessation of building activity, compared to the plague data.
Fredrik Ljungqvist has graciously shared with me data on another region, Central and Northern Europe (data were collected by Hans Hubert Leuschner of University of Goettingen). The coin hoard data I have for this region has fairly crude resolution (50-year intervals), but here’s what happens if we plot these two proxies together:
The blue curve is detrended building activity, and the red bars are coin hoard per half-century. It would be better to resolve coin hoards at shorter time intervals (which is something the Seshat project will soon do), but you get the rough idea—peaks of coin hoards are associated with troughs in building activity.
More generally, there are many more variables whose effects could be investigated. We provide a lot of such proxies in Secular Cycles. For example, we show the dynamics of temple or church building activity for several secular cycles (these buildings can be well dated from historical records). It would be interesting to see whether building activity by the elites and the state (who are primarily responsible for temples and churches) is paralleled by building activity driven by more humble classes.
In any case, despite my critique, it is a very welcome development that we have a new proxy for historical dynamics. We may be at the point where traditional sources, such as historical chronicles, are being mined out, but I expect that quantitative historical data will continue to flow in, thanks to new clever ways of looking at history quantitatively.
An old house in Aarhus, Denmark
Note added 17.XII.2018. Fredrik Ljungqvist clarifies: “The description that ‘Ljungqvist’s group went around Europe taking cores from wooden beams in old houses and then dated them’ does not feel fully accurate as we compiled data already available from decades of archaeological dendro-dating work in Central Europe and reused it for our study. (All the data contributors were included as co-authors.) We did not do any new dating work for this study.”