Stage 5 of the Demographic Transition Model: Below-Replacement Fertility

Stage 5 of the demographic transition model is the stage where the crude birth rate falls below the crude death rate, so natural increase turns negative and the population shrinks unless immigration fills the gap. Many textbooks stop at stage 4, which is why MCAT passages often add stage 5 as a "possible" extra stage and ask you to characterize it.

Stage 4 vs stage 5

Why the death rate can rise

Death rates are already low in stage 4, but an aging population has more people in the age groups where deaths are common. The crude death rate can therefore creep up even as health care improves. The age-specific death rate stays low.

Reading the pyramid

A stage 5 pyramid is narrow at the base and wide at the top. Fewer children are born than the number of older adults, so the dependency ratio shifts toward the elderly.

Consequences the MCAT likes

• Population decline without immigration.
• Higher old-age dependency ratio, with a smaller working-age group supporting retirees.
• Policy responses such as pronatalist incentives or immigration.

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Common traps

• Assuming the death rate must fall in stage 5. It is low and may rise in the crude measure.
• Confusing replacement with zero growth. Replacement fertility is about 2.1 children per woman; below that, the population eventually shrinks.
• Treating stage 5 as universal. It is not in every version of the model, so read the passage definition.

One-minute recap

Stage 5 means birth rate below death rate, negative natural increase, an inverted pyramid and an aging population. Stage 4 sits near zero growth; stage 5 sits below it. For the full set of stages see the five-stage overview.