Q.Why is the year 1921 identified as the year of 'great divide'?

1921 is the 'great divide' because India's population fell/stagnated before it but rose continuously afterwards. OR: productive population = working-age earners (15-60), unproductive = dependent children (<15) and aged (>60).

Why 1921 is the 'Year of the Great Divide':

• Up to 1921, India's population grew very slowly and irregularly — in several decades it even declined, because of frequent famines, epidemics (like plague and influenza), wars and a very high death rate.
• After 1921, the population started to increase continuously and at a rising pace, as death rates fell (better medical facilities, control of famines and epidemics) while birth rates stayed high.
• Thus 1921 separates two clearly different phases of India's demographic history — an earlier phase of stagnant/fluctuating population and a later phase of steady, rapid growth. For this reason 1921 is called the year of the 'Great Divide'.

OR — Productive and unproductive population:

• Productive population: The persons in the working age group (roughly 15 to 60 years) who are able to work and actually produce goods and services and earn income. They are the real workforce of the economy and are treated as an economic asset. …