  In the same way as tree rings give us useful information about the climate
many decades ago (or even centuries ago in the case of big trees), population
pyramids allow us to know birth or death rates several decades earlier.
Naturally, they can fulfill such promises only if they have been recorded
accurately. That is why we start this study by listing a number of pitfalls
that may occur in the process of taking censuses. In this paper our main goal
is to show how it is possible to "read" population pyramids. Sudden changes in
birth rate give rise to the clearest signatures. This indicator reveals that
major tragedies like famines, wars, or epidemics are generally accompanied by a
fall in birth rates. Thanks to this observation, population pyramids can be
used to identify and gauge the blows suffered by populations and nations. As an
illustration of how this works, we compare the population pyramids of North and
South Korea. This allows us, among other things, to gauge the scale of the food
shortage that North Korea experienced at the end of the 1990s.
