Demography is literally “Writing about people.”  However, it has come to mean the quantative or statistical study of human populations. As such, most of what the U.S. Census Bureau does can be described as “demography,”  from the taking of the census every 10 years, to collecting information on race, ethnicity, employment, income, poverty, commuting, health, crime victimization… the list would be long.

A more narrow definition of demography focuses on the processes which determine the growth and composition of human populations. These processes are the most fundamental of all human activities:  birth, death and movement. Demography is the oldest of the social sciences, tracing its origin to 1662, when John Graunt analyzed the death rolls of London.

Each of these processes is greatly influenced by age, time, and what demographers refer to as cohort. A cohort is a group of people who experience the same event at the same time. The most famous cohort is the people born during the Baby Boom, from July 1946 through June 1964.

In all human populations, the chance of dying follows a predictable pattern. The probability of dying is relatively high just after birth, and then falls to a low in the early teen age years, slowly rises during the adult years, and increases dramatically with old age.  This overall pattern seems to be determined by basic biology. In studying the chance of dying at a given age, demographers typically analyze the conditional probability: when a person has reached a given birthday, what is the chance he or she will not survive to the next birthday?

Of course, while the general pattern may be the same, the level of mortality can differ greatly, as can the specific details. These are determined by things such as nutrition, public health, access to health care, and smoking. Thus, demographers quickly return to wider issues that affect humans.

Similarly, fertility tends to follow a general pattern, with few births to women under 16 or over 40, with the peak generally in the 20s. This much is driven by biology. However, the exact level and pattern is driven by customs of marriage and social expectations as well as factors such as nutrition and access to birth control. Of course, all of these factors are related to wider issues such as education, class, income, race and ethnicity. To understand fertility, the demographer must again tackle a wider set of issues.

Population movement is the least biologically driven of the three basic processes. There is a general tendency for young adults to be more mobile than young children or older adults, but population movement can be driven by economics (jobs), laws, availability of housing as well as crisis-driven movement due to natural or man-made disasters. One needs only to remember Hurricane Katrina to see how quickly population movement can take place.

How do Census Bureau demographers use these concepts?  They:

• Produce estimates of the state, county and local populations in the years between censuses that are widely used to administer Federal and state programs.
•  Project what the U.S. population might be like in 10, 20 or 50 years.
• Analyze a number of dimensions of U.S. fertility, for example, the relation between fertility and employment.
• Analyze marriage and divorce, including understanding trends in same-sex relationships.
• Study geographic mobility within the United States.
• Study the foreign-born population,  producing such reports as “Foreign-Born Households are Larger, Include More Children and Grandparents.”

What is happening demographically around the world will affect the United States in many ways. So, the demographers at the U.S. Census Bureau  do not restrict their work to only the U.S. population. They have an active program to gather and study population statistics from around the world to inform Federal agencies as well as U.S. businesses of these trends.  A fine example of this is the report “An Aging World,” describing the effects of reduced fertility and mortality around the world to produce a population that is, on average, older than ever experienced in human history.

Births, marriage, deaths, movement, aging — demographers study the processes that affect us all.