Final answer:
By 2050, India is expected to be the world's most populous country, followed by China, with the US retaining third place and Nigeria's population surging to fourth place. Japan and Russia are projected to fall in the rankings due to declining birth rates.
Step-by-step explanation:
According to the United Nations' World Population Prospects: the 2015 Revision, the most populous countries in the world in 2050 are expected to be India, China, the United States, and Nigeria. India is projected to surpass China around 2025 to become the most populous nation. The population of India is projected to be significantly higher than China's due to the latter's earlier implementation of population control measures and India's relatively higher fertility rates. Both countries' populations are over a billion but are growing at different rates.
The United States is expected to remain the third most populous country with an estimated population of 423 million. Significant growth is also anticipated in Nigeria, which is forecasted to have a population of 402 million, skyrocketing from its 2010 population of 166 million due to its high fertility rates.
On the other hand, nations like Japan and Russia are expected to experience a decline in their population, falling from the 9th and 10th most populous nations to 16th and 17th respectively. This is due to declining birth rates in these countries. Global population growth trends indicate a high growth rate in developing countries while populations in many developed regions such as Europe and North America are either stable or in decline.