*Townsend is now moving on to the American Academy of Arts and Sciences, where I'm excited to see that he'll manage the Humanities Indicators—my first real programming/data project was putting together the first version of them together with Malcolm Richardson immediately after college.
Numbers between 500 and 2000 are almost always years. You can see here that the vast bulk of historical study has been in the period since 1750: the three spikes out of the landscape correspond to the Civil War and the two world wars. Output decreases in the late 20th century in large part because the data set goes back to about 1850; but as we'll see in the next chart, not entirely.
*This exaggerates the case somewhat in favor of post-1800 years, because before the modern period historians are more likely to talk about longer periods: "Late Antiquity," "The Ming Dynasty," "Elizabethan England." I find it impossible to believe they're doing so in a way that fundamentally changes the distribution above, though. As always, you should feel free anything you find possible.
More surprising is how the spread of years changes over time. Below, each of the dots represents the use of a year in a dissertation title: the blue and red lines are the mean and median years written about, respectively, in a moving 20-year window. (I've excluded years before 1500 for legibility, and to antagonize medievalists).
For most of the 20th century, the typical dissertation became later and later in time. In both cases, the early 1980s saw a steep rise up—I suspect that has to do with changes in the schools the AHA has been tracking rather than a more interesting explanation (the rise of cultural history, say). But recently, the forward march has stalled. The median year mentioned in titles has been stuck at 1900 since the mid-1980s; the
I have zero evidence for this).
Although the line of history has stopped moving, the time periods taken in by historians have gotten longer. There are 8,000 dissertations out of the 30,000 in the set that have two years in the title: usually, those are start and end dates. (If there are three or more, I take the outer limits). We can see that the average time span covered by a dissertation has shifted from 20 to 30 years at midcentury to 75 to 100 today. (Take care--log scale on the left here).
So since about 1965, dissertations have covered longer and longer periods. (The data is sparse, but there's some reason to think there might even be a trend toward more focused dissertations until the 1970s). [Edit--with parsing of decades, this trend is less dramatic but still present. Graphs later).]
We can also look at this by the period of the dissertation, not the time it was written. The pattern you'd expect is longer dissertations about periods longer ago: 1933-1939 still seems like a useful period of time to study in a ramp-up to the war, but 1765-1775 is probably too short. That's indeed what we see, but it's not a steady decline: (I've pushed the window for the moving averages here out to a full century.)
Instead, the differences we have seem to match up against well-understood periods. Dissertations about the late middle ages typically cover a century (although particularly in that period, they tend to explicitly use phrases like "Fourteenth century" as well); the lengths are possibly a bit longer around 1000, and they drop dramatically immediately after the renaissance, where they plateau at about 60 years for most of the early modern period. Those ending around 1900 are the shortest, about 30 years (1865-1900 is a classic American periodization, and 1870-1900 makes sense for French and German historians), and then they creep up through the twentieth century.
You see a funny curved line toward the right--that's the moving frontier of "1900-1945," "1900-1989," and so forth. The bump up at the end may be an artifact of some sort, but I think it's partly real--the median length for dissertations ending in 2000 is 55 years, which means that dissertators tend to take on the full post-war period at once. (Plus, remember, dissertations ending in the 1990s were by definition written after 1990, when coverage had gotten broader).
So that's the general outline. The next interesting thing to do would be to try to recover the most interesting years and the default periodizations from the set: maybe I'll get a chance soon. I also have a much prettier, wackier visualization of this same data sitting around somewhere. But as I worry more about my own dissertation, I felt like tossing something like this up on the blog.