There are many demographic and historical reasons behind the fact that 36% half of world’s population is concentrated in two Asian countries. Let’s start with stating some facts that I have gathered from the Pew Research:
• India is a top destination for world’s migrants.
• 12th largest immigrant population in the world is in India.
• India has one of the world’s lowest emigration rates.
Other sources tell that half of total the world population is in seven countries. Following the topmost nations, India and China, are US, Indonesia, Brazil, Pakistan, and Nigeria.
Keeping our focus on population growth in India
and China, let’s first locate these Asian countries in the world map. Both of these countries are around the Tropic of Cancer, which is one of the best places to offer the habitable and hospitable land. Be it topography, options for agriculture, or climatic conditions, there is hardly any land which can beat India and China.
Apart from a variety of crop choices, these two countries offer good options and diversity for hunting and gathering, making these two lands better than many other for survival. More space and food are two of the foremost reasons that the earliest human migration happened from Africa two these two Asian countries.
India and China contains 20% of the whole world’s arable land. They produce 50% of world’s rice (the main staple food in any country) and 30% of the world’s wheat.
Further, India and China believe in more and more agricultural activities to produce enough food grains. For this people there tend to have big families so that they can have good harvest, which also contributes to the population growth.
Coming to the topography, most of world’s land is ruled out as not being suitable for human settlement. These lands include Alaska, Sahara Desert, Antarctica, Alaska, Greenland, Siberia, central Africa, forests of Amazon, parts of Canada, Arabian Desert, and so on. This also emphasizes on the imbalance population concentration that we are discussing.
Now coming to the second part of your question, as to why America and Europe don’t have as much population as China and India, I would like to state some historical facts. China and India has never suffered from major plagues and epidemics. At least not as bad ones as those that happened in Europe and America.
Europe’s population has always remained in check as the Plague of Justinian, Black Death, etc. kept hitting the continent which even led to emigration from the land. The World Wars also contributed in keeping the population in control.
The Europeans that migrated to the Americas took small pox with them, thus causing great death tolls there. India and China, due to already large populations and good crops, have always succeeded in controlling such crises since the past days.