Population Explosion: What the Phrase Means

In one line

A population explosion is a period of very rapid growth caused by death rates falling while birth rates stay high, and India's happened between about 1951 and 1981.

In the wording a student can write in an exam: a population explosion is a sudden and sustained acceleration in the rate of population growth. It occurs when mortality falls sharply while fertility remains high, so that the gap between births and deaths widens. It is a transitional phenomenon rather than a permanent condition, because fertility eventually falls too, and its effects persist for a generation after fertility has fallen because of population momentum.

The mechanism

Growth is births minus deaths, plus net migration. For a country the size of India migration is small, so growth is essentially the gap between the birth rate and the death rate.

Mortality can fall very fast. It falls because of things that can be delivered from outside a household: vaccination, antibiotics, control of vector-borne disease, clean water, sanitation, famine relief and food distribution. A public health programme can halve an infant mortality rate within a generation.

Fertility falls slowly. It falls because of things that change inside a household and inside a culture: the education of girls, the age at marriage, the survival of children so that fewer births are needed to be sure of survivors, the cost of children in a wage economy, the availability of contraception, and the position of women.

The gap between the two speeds is the explosion. It is not caused by people having more children. Fertility in India was roughly constant while the explosion was happening; what changed was that the children stopped dying.

That sentence is the single most useful thing in this chapter, because most answers assume the opposite.

India's explosion

Before 1921. Population grew slowly and erratically, with famine and epidemic producing actual declines. The census of 1921 is conventionally called the great divide, because it is the last census before sustained growth begins.

1921 to 1951. Steady growth as mortality began to fall.

1951 to 1981. The explosion. Death rates fell sharply with the expansion of public health after independence, while birth rates remained high, and the decadal growth rate rose to its peak.

1981 to 2011. The decadal growth rate declines census after census as fertility falls.

Since. Fertility has reached about replacement level nationally, and growth continues through momentum and is regionally concentrated.

A candidate should give that sequence with the census dates, because it converts a vague statement about rapid growth into a demographic history.

Why growth continues after fertility falls

Because of population momentum, and this is the second thing most answers get wrong.