Population Change from Aging, Death, and Migration

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  • 0:05 Fertility
  • 1:39 Mortality Rate
  • 2:24 Migration
  • 4:02 Population Change
  • 5:54 Lesson Summary
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Lesson Transcript
Instructor: Valerie Houghton, Ph.D.

Valerie holds a Ph.D. in Health Psychology.

Fertility, mortality and migration are principal determinants of how a population can change, and they are at the very core of demographic studies. In this lesson, we will discuss how a population can change from fertility (birth), mortality (death), and migration.


Fertility is the number of children a woman bears during her reproductive years and is related to social behaviors and personal decisions. A typical measurement used for fertility is the crude birth rate, which is the number of live births in a given year for every 1,000 people in a population. Demographers calculate the crude birth rate by dividing the number of live births in a year by the total population and then multiplying the result by 1,000.

Fertility should not be confused with fecundity, which refers to the biological capability and capacity of bearing children. The human female is generally fertile from early teens to about mid-forties. The human male, on the other hand, generally remains fertile throughout adulthood, although sperm count and quality diminish from middle-age onward.

Worldwide, there are significant differences between birth rates. For example, differences in family size range from 8.6 children in Jordan to 5.2 children in Indonesia. In developed countries, such as the United States and the European Union, there is a tendency for the family sizes, on average, to be smaller than the replacement level.

Replacement level is the level of fertility required to replace themselves and their partner in a population. By definition, 'replacement' is considered only to have occurred when the offspring reach 15 years of age. In the United States and other industrialized countries, the replacement level is approximately 2.

Mortality Rate

The death rate is known as the mortality rate, which is a measure of the number of deaths in a population. Mortality rate is typically expressed in units of deaths per 1,000 individuals per year. For example, if there is a mortality rate of 9.5 in a population of 1,000, this would mean 9.5 deaths per year in that entire population or 0.95% out of the total.

The mortality rate can increase due to epidemics, childhood diseases and war. However, it can also decrease. For example, antibiotics and improvements in medical care have resulted in decreases in the mortality rate.


The movement by humans from one area to another is known as migration. The humans who undergo migration are called migrants. However, according to the International Organization for Migration, there isn't a universally-accepted definition for a migrant. Nevertheless, the United Nations defines migrant as an individual who has resided in a foreign country for more than a year irrespective of the causes, voluntary or involuntary.

An important distinction is that people who migrate into a territory are called immigrants, while people who leave a territory are called emigrants. Under such a definition, those traveling for shorter periods of time as tourists or business persons would not be considered migrants, immigrants or emigrants.

Migration can affect the population in many ways. For example, in areas where natural disasters or military conflicts occur, this often leads entire populations to be displaced. For example, the large wave of Cubans who legally and illegally immigrated to the United States following the 1959 Cuban Revolution increased the United States' overall population. As of today, over a million Cubans reside in the United States, with the majority living in Florida and New Jersey.

Another example of emigration is the internal relocation of many Louisiana residents following Hurricane Katrina. With their homes destroyed, the state's economy in ruin, and sea levels continuing to rise, many people reluctantly left.

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