Birth rate refers to the number of live births in a population and is closely linked to reproductive behaviour and family size. Death rate, or mortality rate, refers to the number of deaths in a population. These two demographic indicators define the trajectory of population explosion during the demographic transition.

The death rate declines much faster because it can be controlled through technological and medical interventions. As the context highlights, death rates are brought down relatively quickly through advanced methods of disease control, public health measures, and better nutrition. Additionally, increased levels of control over famines and epidemic diseases have historically caused sharp falls in mortality.

In contrast, the birth rate is relatively slow to fall because it is a sociocultural phenomenon that is deeply entrenched in societal norms. It takes longer for society to adjust its reproductive behaviour, which was evolved during periods of poverty and high mortality, to suit the new situation of relative prosperity.

Furthermore, the birth rate declines only when there is an overall increase in prosperity, a decline in infant mortality rates, and a rise in the levels of education and awareness. Since these social adjustments are gradual, the birth rate does not register a sharp fall immediately, causing a divergence between the two rates.