The term demographic transition refers to a historical shift of birth and death rates from high to low levels in a population. The decline of mortality usually precedes the decline in fertility, thus resulting in rapid population growth during the transition period.
The key defining features of demographic transition are:
- A dramatic decline in death rates from the general ballpark of death rates seen in agricultural societies to the ballpark of death rates seen in industrial and post-industrial societies.
- A dramatic decline in birth rates from the general ballpark of birth rates seen in agricultural societies to the ballpark of birth rates seen in industrial and post-industrial societies.
- The decline in death rates begins earlier than the decline in birth rates, and the lag between the declines leads to a period of considerable population growth.
While these general features are shared by all examples of demographic transition, the specifics vary quite a bit. There is another related idea called the second demographic transition, a speculative theory about the future.
The term demographic transition is tricky to understand because it could refer both to a specific aspect of the history of a particular population's demography, and to the general theoretical model. For this reason, it should be treated as a fuzzy rather than a precise term.