National Gender Statistics Report 2021

The world’s populations are in varying stages of a demographic transition from high to low rates of
mortality and fertility (Bongaarts, 2009). In this process, the age structure gradually shifts toward older
ages. Fertility is the principal component driving population dynamics, the other two being mortality
and migration. A youth bulge results when decreases in fertility reduce the share of children in the
population at a relatively fast rate while the proportion of elderly is still relatively small. Nations
undergoing this transition can capitalise on the resulting high labour supply and low dependency rates.