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  • Understand demographic measurements like fertility and mortality rates
  • Describe a variety of demographic theories, such as Malthusian, cornucopian, zero population growth, and demographic transition theories
  • Be familiar with current population trends and patterns
A street filled with people is shown here.
Earth’s population, which recently grew to 7 billion, is always on the move. (Photo courtesy of David Sim/flickr)

We recently hit a population milestone of seven billion humans on the earth’s surface. The rapidity with which this happened demonstrated an exponential increase from the time it took to grow from five billion to six billion people. In short, the planet is filling up. How quickly will we go from seven billion to eight billion? How will that population be distributed? Where is population the highest? Where is it slowing down? Where will people live? To explore these questions, we turn to demography    , or the study of populations. Three of the most important components affecting the issues above are fertility, mortality, and migration.

The fertility rate    of a society is a measure noting the number of children born. The fertility number is generally lower than the fecundity number, which measures the potential number of children that could be born to women of childbearing age. Sociologists measure fertility using the crude birthrate (the number of live births per 1,000 people per year). Just as fertility measures childbearing, the mortality rate    is a measure of the number of people who die. The crude death rate is a number derived from the number of deaths per 1,000 people per year. When analyzed together, fertility and mortality rates help researchers understand the overall growth occurring in a population.

Another key element in studying populations is the movement of people into and out of an area. Migration may take the form of immigration, which describes movement into an area to take up permanent residence, or emigration, which refers to movement out of an area to another place of permanent residence. Migration might be voluntary (as when college students study abroad), involuntary (as when Somalians left the drought and famine-stricken portion of their nation to stay in refugee camps), or forced (as when many Native American tribes were removed from the lands they’d lived in for generations).

Population growth

Changing fertility, mortality, and migration rates make up the total population composition    , a snapshot of the demographic profile of a population. This number can be measured for societies, nations, world regions, or other groups. The population composition includes the sex ratio    (the number of men for every hundred women) as well as the population pyramid    (a picture of population distribution by sex and age).

A pyramid graph depicting the 2011 population of the United States, grouped by age.
This population pyramid shows the breakdown of the 2010 American population according to age and sex. (Graph courtesy of Econ Proph blog and the U.S. Census Bureau)
As the table above illustrates, countries vary greatly in fertility rates and mortality rates—the components that make up a population composition. (Chart courtesy of CIA World Factbook 2011)
Varying fertility and mortality rated by country
Country Population (in millions) Fertility Rate Mortality Rate Sex Ratio Male to Female
Afghanistan 29.8 5.4% 17.4% 1.05
Sweden 9.1 1.7% 10.2% 0.98
United States of America 313.2 2.1% 8.4% 0.97

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In biology, a pathogen (Greek: πάθος pathos "suffering", "passion" and -γενής -genēs "producer of") in the oldest and broadest sense, is anything that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ. The term pathogen came into use in the 1880s.[1][2
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Source:  OpenStax, Introduction to sociology. OpenStax CNX. Jun 12, 2012 Download for free at https://legacy.cnx.org/content/col11407/1.7
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