system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What factors caused India and China to have such large populations?

I know they are relatively large countries that have been inhabited for a very long time, but so is Europe if taken as a whole. Yet Europe only has about half as many people as just one of these two countries. What historical factors lead to their massive populations?
gpt
It's more geographical than historical.

You need to feed a large population. India and China have lots of suitable farmland. Farming takes a great deal of labor so the more people in your family, the better you can farm, encouraging a large population. Nowadays with modern farming methods and fertilizer, you can produce even more food to support a larger population into the billions.

The other regions that have comparable farmland are the fertile crescent (Middle East) and the Nile flood plains, and the United States. The fertile crescent spanning the Mediterranean, Black, and Red Seas is where farming originally began. However, the history of conflicts there have not allowed stable farming in that region. Despite supporting the Ancient Egyptians, the Nile's flood plains have been in decline. Lastly, the US is recently settled and most of the farming goes towards exports and animal feed.