If there were no limits, the growth of a population would be exponential. Starting with two people, you'd have a thousand in 10 generations, a million in 20 generations, and a billion in 30. Between 1950 and 1990, the world's population doubled from 2.5 to six billion. Population increase is not constant and uninterrupted but is governed and determined by a series of conditions and factors.
Fertility, or the birth rate, of a population is a major factor that affects its growth. When there are more births than deaths, the population of a country increases. Fertility is measured as the number of live births per 1000 in a year. As with population growth in general, fertility is not a constant process, and there are a number of conditions that affect how many healthy babies are born. These include infant mortality rates, standard of living, nutrition, reproductive health, and attitudes to and availability of contraception and abortion.
Morality, or the death rate, also affects the growth of a population. If there are fewer people dying than are being born, the population will grow. Mortality is generally measured by the number of deaths per 1000 in a year. As with fertility, the death rate of a population is influenced by certain conditions. The availability of health care and medicine, nutrition and war, for example, can all affect death rates and population growth.
Immigration, or the number of people moving into a country, also affect population growth. When there are high levels of immigration, the population increases. The immigration level is affected by pull factors -- the characteristics of a place that attract people to it. Examples of pull factors include job and educational opportunities, high wages, affordable housing and a good standard of living.
Access to resources is another factor that determines population growth. Natural resources like water, land and fossil fuels are vital in supporting the needs of a population, but are also in finite supply. When a society has plenty of natural resources and can provide food and energy to its inhabitants, fertility and immigration rates are likely to be higher and mortality rates are likely to be lower, causing the population to increase.