demography is the study of the change in size, distribution and character of the human population and the two most basic factor in demography are the birth rate and the death rate. the former expresses the number of children born per thousand people per year.the latter indicates the number of people who die per thousand people per year. if we consider the earth as whole, we see that the population growth or decline is caused by the difference between. the number or birth and deaths over a given period. there are normally more births than deaths, and this is known as a natural increase in population.