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Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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DEMOGRAPHY OF POPULATIONS

I.  Demography is the study of  populations.  Topics include the age structure of populations, mortality (death) rate and fertility (birth) rates, and the factors which influence these factors.

Populations can be studied at a number of different scales, including:

Species; which includes all individuals on Earth which can interbreed and have fertile offspring (for example, all humans)

Local populations; which are a group of individuals within the same species which live together in the same territiory but do not commonly interbreed (for example, all humans in Green Bay)

Demes; which are groups in which interbreeding occurs within a local population (for humans, socioeconomic, racial and ethnic groups. Pretty loosely constrained in America but extremely strict in other regions)

Cohorts; which are groups of individuals of similar ages (Baby Boomers, Gen X, Gen Y, Milennials, Class of 2013, etc.)

At any of these scales, a number of  variables can be calculated, including:

Net Reproductive Rate; which is the average number of offspring born per adult in a lifetime

Mean Life Expectancy; which is the average number of years left to be lived from a given age

Mean Generation Time; which is the average number of years between an adult's birth and the birth of that individual's middle child

II.  Age structure (or population pyramid) diagrams.

Some population statistics are best represented graphically.  One of the easiest to interpret are Age Structure diagrams.

To make this type of graph, you must:

(1) Count the total number of individuals in a population

(2) Divide the population up into an equal number of age classes (cohorts).  For human populations, these are
often 5-year periods.

(3) Within each cohort, count the number of females and males.

(4)  For each cohort, divide the number of males and females into the total population size.

(5)  Graph these numbers, with the vertical axis representing cohort age, and the horizontal axis representing the
proportion of the population held within each cohort.  It is common practice to put the percentage of males
on the left side of the graph, and the percentage of females on the right.

III.  Interpreting Age Structure Diagrams.

Much about the potential for future population growth, and of the history of  of a population can be determined from the shape of Age Structure diagrams.

Population growth rates:

(1) Rapidly growing populations will have a strongly triangular shape.  This means that each younger cohort is
larger than the one preceeding it.  Thus, the largest number of individuals in a population will be infants.

(2) Slowly growing populations will have approximately equal sizes among the younger cohorts, but will have a strongly pyramidal shape in the older cohorts.  This kind of diagram is created when a rapidly growing population's birth rate falls to replacement levels (on average 1 child born per adult in a lifetime)

(3) Stable populations will have all but the oldest cohorts having approximately equal sizes.

Disasters in a population's history can be identified by looking for cohorts that are much smaller than the cohorts on either side.  The size of these cohorts is smaller as individuals were killed during the disaster. For example, in Russia, among people who were young adults during World War II, women after the war outnumbered men seven to one.

Patterns of immigration can also be identified by looking for:

(1) some cohorts which are much larger than their neighbors

(2) the shape of the male side differing greatly from the female diagram.  This is happens in immigration, as both sexes usually migrate at different rates. In the 19th century U.S., for example, men immigrated in large numbers first, then women. Many women came over as "mail order brides."