The world's population has increased rapidly over the last 200 years not because people have begun breeding like rabbits, but because they have stopped dying like flies. Population density is a measurement of the number of people per unit area or volume, and is often used to describe human populations. Settlements can range in size from a small number of dwellings to large cities, and factors that encourage their growth include coastal locations, fertile soil, mineral resources, and places where transportation routes intersect or rivers can be crossed. Large settlements face problems in both more and less economically developed countries.