Birth, death, and the movement people into and out of a country cause changes in population. The number of children born, called the fertility of population, increases the size of population. The number of deaths, called mortality, decreases population. Migration - the number of people entering or leaving the country - can cause either an increase or decrease. A natural increase in the population of a country comes from a greater number of births than deaths.
The three factors that cause changes in population are talked about in terms of rates. Rates show often an event happens, or how common it is.
Human beings have lived on earth for tens of thousands of years, and during most of that time their numbers were relatively small. Before agriculture was developed about 10,000 years ago, the death rate was high, partly because of limited food supply. After people began to grow the crops and raise animals for food, the death rate fell. And the population of the world slowly grew. Industrialization and scientific knowledge began to spread quickly. There was a further increase in food supplies, as well as a decrease in deaths caused by diseases. The human population began to grow more rapidly.

Many persons fear that population will grow faster than the resources - such as food, fuel, and housing - needed to support all the people. If this should happen, the first result would be lower standards of living. Then, as living standards fell below what was needed to keep the poorest people alive, the death rate would rise. If living standards were reduced to a point where the dangerous diseases could not be controlled, death rates would rise among the wealthy as well as the poor.

More governments are helping families to understand the problems that are part of excessive population. They give " Family Planning" advice to families that haven't understood what 'family planning' is. Such families can plan to have number of children they want (in Indonesia the number of two children is recommended), at times when they are best able to take care for children and give them a good start in life.