population change, which means the rate at which the population is increasing or decreasing at a given time. In this lesson, we'll go over the three main factors that cause population change:birth,migration, anddeath. Let's talk about each of these in detail.

Birth Rate

Now, this might seem very obvious to you, but when babies are born, it results in a population change! Social scientists who study this trend have some key terms, though. First, thefertility ratein a country refers simply to the number of children born. This is different from thefecundity rate, which is the number of children whocouldbe born in a given society. In other words, fecundity is the biological capacity of humans to produce children.

For a woman, this is generally the time frame between her early teens and her mid-forties. For men, this window is longer, but factors like sperm count diminish as a man ages. The fecundity rate is greater than the fertility rate in most cases. Birth rates vary by society for a number of reasons, including personal choice and social and political factors. Factors such as poverty level, access to contraception, maternal and infant care, and availability of nutrition all impact the birth rate in a society.

Death Rate

Just as when babies are born, when members of a society die it changes the population. This might be a bit morbid to think about, but scientists refer to this as themortality rate, which is simply the number of deaths that occur in a society. This is usually calculated by looking at the number of deaths per 1,000 individuals. In the United States, for example, the death rate is 8/1000 people. Just like birth rate, the death rate in a country depends on things such as availability of quality medical care, disease, war, or famine.

Migration

One important factor that accounts for a change in population ismigration. At the broadest level, migration refers to movement; it's the flow or circulation of people in societies. There are two kinds of migration: emigration and immigration.Emigrationmeans to leave one's own country for another.Immigrationmeans to enter into a country other than one's own. Both of these process impact population change.

Emigration can decrease a country's population size. For example, if you live in New York City and emigrate to Miami, you are decreasing the population of New York City but increasing the population of Miami, because you immigrated there. However, it's important to note that immigration and emigration don't change the earth'soverall population, or the number of people alive at a given time. Immigration and emigration simply change the number of people in a given place.

Today, most migration occurs when people leave rural areas and head for urban areas. Think of the word 'overpopulation.' What comes to mind? For many of us, it's likely a crowded, noisy, and polluted city of Africa or Asia. An influx of people from rural areas, who often can't sustain their families via traditional ways of life, head to cities to find work.