Births, deaths and migration
Demographers are concerned with the structure of populations and how population structures change. In a closed population the only way to enter the population is by being born, and the only way to leave the population is through dying. However, in an open population (i.e. all national and local populations) an individual can also enter a population by immigrating into it, and may leave a population by emigrating out.
It is these three processes – births, deaths and migration – that determine the growth (or decline) of a population.