If families continue to have fewer than two children and if most people survive into old age the demographic pyramid will

Turn upside down

Explanation:

In the past, families tend to have a lot of children since most of their economies revolved around agriculture , so having more children typically resulted in more workers for the field.

This make the demographic pyramid for people who grows old could be 5 times the initial amount from each generation.

If the families keep having less than 2 children,  the number of mortality rate will outpace the fertility, which will turns the pyramid upside down.

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