Population growth is essential for creating a strong base of high-income workers. It also helps pay for social welfare programs. When birth rates decrease, the strain on younger workers can be immense.
Nations with higher populations are better able to raise money on a national level, and this can lead to more international clout. China, for example, has a relatively low per capita income, but its large population gives it tremendous influence. Its centralized government is able to accomplish feats more developed nations cannot afford.
Many developed nations are facing problems due to declining birth rates. The cost of retirement care for the large population born in the middle of the 20th century is high, and young people are expected to pay for this through taxes. As a result, young people have a higher tax burden than the older generation had while they were working.
When birth rates were high in the middle of the 20th century, many experts predicted that they would continue to accelerate and eventually reach unsustainable levels. However, studies have shown that developed nations have lower birth rates. Developed nations with low immigration, like Japan, actually have declining populations. As the rest of the world develops, experts believe that the Earth's population will plateau.