Originally Posted by Mölyapina

I think it's about 90%. However, the Northern Hemisphere has most of the land mass. All of the below is in the Northern Hemisphere:

• All of continental Europe excluding overseas territories in the Southern
• Hemisphere and Eastern Hemisphere
• All of North America, Central America and the Caribbean excluding overseas territories
• The vast majority of Asia, except East Timor, and part of Indonesia (which
• straddles the equator)
• About 2/3 of Africa, just above the "horn"
• About 1/10 of South America, north of the mouth of the Amazon River

Here is a picture, with the equator in red:



There is overcrowding in some areas for sure, but that doesn't neatly correspond to major wars. I think that the circumstances leading to each individual war are so complex that it is futile to identify one common cause for all of them, unless you want to go with something really broad such as the existence of evil.