Final answer:
The population size from 1961 to 1977 changed due to varying birth and death rates, with a significant decrease in death rates and a decline in birth rates, especially in developed countries. In developing nations, despite large reductions in death rates, birth rates remained high, contributing to rapid population growth. The demographic transition model explains these trends.
Step-by-step explanation:
The change in the size of the population from 1961 to 1977 can largely be attributed to differences in birth rates and death rates. During this period, in developed countries, the death rate had significantly decreased due to improvements in nutrition, sanitation, and health care, a trend that began during the Industrial Revolution. Concurrently, birth rates began to decline as children became more of an economic liability rather than an asset, and women delayed marriage for education, leading to smaller family sizes.
In developing nations, while the death rate saw a dramatic drop, the birth rate remained relatively unchanged, leading to rapid population growth. Considering that immigration and emigration rates were equal, the change in population was a direct result of the difference between the number of births and the number of deaths. Therefore, a higher birth rate compared to the death rate led to an increase in population size, as more individuals were born than those who died within that time frame.
The concept of demographic transition explains these changes, where countries move from high birth and death rates to low birth and death rates as they develop economically and socially, eventually leading to a stabilization or decline in population growth.