Natality rates are those related to births. Mortality rates are those related to deaths. Migration rates are those related to movements of populations into a country (immigration), out of a country (emigration), or simply inside a country from one place to another (internal migration). All these are important for the study of populations and their growth. Let us look at the meanings of some of these terms.
Natality is an expression of the production of new individuals in the population. In human population it is equivalent to birth rate, and is usually expressed as the number of births per year per thousand persons in the population. As such, it is quite distinct from population growth rate as it may be zero or positive but never negative. The maximum number of individuals that can be theoretically produced per individual under ideal environmental conditions is called potential or physiological natality. It is constant for a given population. The natality may be expressed as specific natality that refers to population increase under specific conditions. It is not constant for a given population. The natality rate of the population is expressed by
B = (Nn)/t
Where B = birth or natality rate, Nn = number of new borns, and t = time
The specific natality over a period of time is expressed by
B = (Nn)/ (NΔt)
Where b = natality rate per unit of time per individual in the populaitn. Δt = change in time or number of individuals.
Mortality refers to death of individuals. In a population, members die due to various causes, such as malnutrition, disease and old age. Mortality is expressed as:
d = D/t
Where d = mortality or death rate, D = total number of deaths, and t = time.
Migration is the movement of people of new homes either within the boundaries of a country (internal migration) or across the boundaries to another country (international migration). It is clear that only international migration can affect the growth of population within a country. In some countries it is large enough to have a significant effect on the growth rate. For example, legal and illegal immigrants into the USA number more than 1 million a year, which is about two-thirds of the total annual growth of that country. In times of war or hardship, too, large numbers may migrate. Millions of Jews left Europe as a result of the Second World War, and large numbers of Arabs were displaced from Palestine. More than 3 million Afghanis have migrated into Pakistan in the 1980’s as a result of the Afghan war. I order to take migration into account in calculating population growth, we must therefore, add the net immigration (which is negative if emigration is greater than immigration) to the population increase by births.
Internal migration, too, is of concern to demographers. The less developed countries are witnessing large-scale migration by the rural population into the cities due to pressure on the land. This kind of migration can cause great strain on the urban facilities such as water, housing, sanitation, etc.