Ignoring migration, which shape describes an age structure likely to remain stable in size?

• A. Rectangle
• B. Pyramid
• C. Inverse pyramid
• D. Circle

Answer: (A)

A stable population size without migration shows a roughly rectangular age distribution because the number of people in each age group stays about the same over time as they move into the next age group. When births roughly balance deaths, new people entering young age groups keep the widths of all age cohorts similar, so the diagram looks like a rectangle.

If there were many more births than deaths, the shape would tilt into a pyramid with a wide base—indicating rapid growth. If deaths outpace births or the population is aging, the shape would narrow toward the younger ages, resembling an inverted pyramid. A circle isn’t a standard representation of age structure.

So the rectangle best describes a stable, non-migrant population.