Dynamics of Population

How population is measured and changes: the census, birth and death rates, fertility and the replacement level, sex ratio, ageing and the demographic dividend.

Key ideas

Demography is the study of population and how it changes; India counts its people in a census every 10 years.
The birth rate is live births per 1,000 people in a year.
A fertility rate of 2.1 children per woman is the replacement level; long below it, a population stops growing and may shrink.
Sex ratio is the number of females per 1,000 males.
Life expectancy is the average number of years a newborn is expected to live.
A demographic dividend is the gain a country can make when many people are young and of working age.
Japan’s population is ageing because few children are born and people live long.
People migrate to cities for jobs, education and health care.

Remember

Replacement level	2.1 children per woman
Sex ratio of 943	943 females per 1,000 males
India’s population	about 361 million (1951) to about 1,210 million (2011)
Most populous country	India, since 2023
India’s median age	about 28 (Japan about 50)
Life expectancy	about 30 at Independence, about 70 in 2024
China’s One-Child Policy	1979 to 2015

Watch out

Reading sex ratio per 100 or per 1,000 people: it is females per 1,000 males.
Thinking a fertility rate of 2 keeps a population steady: replacement level is 2.1.
Mixing up the birth rate (per 1,000 people) and the fertility rate (per woman).

Worked example

India became the most populous country in 2023, and China’s One-Child Policy ended in 2015. How many years before 2052 was each?

Working: 2052 − 2023; 2052 − 2015

Answer: 29 years; 37 years