Demographic Crises
Introduction
Most everyone has heard of the Black Death. They may even be able to date it to the middle of the 14th century. But the most important thing to know about the Black Death is that it was part (a large part, certainly) of a larger crisis in the population that included frequent famines and outbreaks of other diseases. The demographic crisis began in the early 14th century, well before the arrival of the plague, and it persisted long afterward.
Importance of Demography
"Demography" is the study of populations, and it's an important element in understanding how any society works, whether past or present. In a pre-industrial society, there's a close relationship between land and population, so that studying the one sheds light on the other. Changes in population directly affect not only the economy but also the society. Studying demography leads one directly into such topics as child-bearing and patterns of marriage, as well as mortality.
Awareness of demographic patterns and change is unusually important for understanding the later Middle Ages because there were such dramatic shifts in population. Here I mean not only the famous Black Death, but also changes between western and eastern Europe and changes between towns and countryside.
Nature of Sources for Demographic History
Unfortunately, our sources are poor and scattered. There were no general censuses taken in the Middle Ages. We have almost no parish registers, those records of births, baptisms and deaths that give such invaluable information for early modern Europe.
The records we do have are either notoriously unreliable or are tricky to interpret. We have estimates of population written by various medieval writers. These are of little value of themselves but can help provide confirmation of other estimates. We have various tax records, most especially of hearth taxes, that at least give us an indication of how many hearths there were in a community in a particular year. The difficulty, of course, lies in estimating how many people a hearth represents. We also have widely scattered occasional sources, such as the Florentine catasto of 1427 that permit us to be more precise. The difficulty there is in generalizing from the specific town or village to regions or nations.
Beyond statistical sources we must rely on inference. For example, we can see that town walls were expanded and can infer growth (but the walls don't contract to indicate falling populations). We can project backward from later periods where the numbers are better known. We can make some high and low estimates based on area of land under cultivation.
In short, we have to guess a lot. You'll see this reflected in the books that talk about population; they will vary widely in their numbers, and their range of estimation will be quite wide. At the end of this essay are some references that will let you look for yourself.