This book is concerned with commonsense questions about, for instance,
the effect of a lowered death rate on the proportion of old people or the
effect of abortions on the birth rate. The answers that it reaches are not
always commonsense, and we will meet instances in which intuition has to
be adjusted to accord with what the mathematics shows to be the case.
Even when the intuitive answer gives the right direction of an effect, technical
analysis is still needed to estimate its amount. We may see intuitively
that the drop from an increasing to a stationary population will slow the
promotion for the average person in a factory or office, but nothing short
of an integral equation can show that each drop of 1 percent in the rate of
increase will delay promotion to middle-level positions by 2.3 years.
The aim has been to find answers that will be serviceable to those working
on population and related matters, whether or not they care to go deeply
into the mathematics behind the answers. My earlier book, Introduction
to the Mathematics of Population, had the opposite purpose of developing
the theory, and mentioned applications mostly to illuminate the theory.
Because of their different objectives there is virtually no overlap between
the two books.