What percentage of the world's population lives in the northern hemisphere?

Just under 90 percent of the world's population lives in the northern hemisphere. In fact, more than half of all humans can be found north of 27 degrees latitude above the equator. For reference, 27 degrees north passes through South Padre Island in Texas.
This all raises the question of why most people live in the northern hemisphere. The clearest answer is that most of the world's habitable landmass is north of the equator. However, this still does not account for the full discrepancy. As the other educator notes, life expectancy in southern regions tends to be less than in the north. This could account for some of the imbalance.
Another factor that demographers have considered has to do with human migration. The largest landmass in the northern hemisphere, Eurasia, can be traversed on foot. There are no major bodies of water to split up the continent. This is not true for the southern hemisphere, with its large oceans and many islands. Southern Africa is about 2,000 miles across at its widest point. Depending on where you measure, the Eurasian continent is well over 6,000 miles from east to west. This ease of travel allowed early human populations to move and settle in these lands much earlier and more thoroughly than in the southern hemisphere.
Another consideration is that, discounting Antarctica, most of the land in the southern hemisphere is in the tropics, where agriculture is more difficult and diseases are more rampant. Conversely, most land in the north is in regions with temperate climates, where humans have historically fared best.
https://bigthink.com/strange-maps/563-pop-by-lat-and-pop-by-long?page=all

The world’s population is constantly changing; however, in 2018, it was estimated that 90% of the world’s population lived in the Northern Hemisphere.
When looking at the earth, it is important to realize that there is more land in the Northern Hemisphere, making it more inhabitable. In fact, close to 67% of the land on the earth is found in the Northern Hemisphere. There is still, however, a proportionately larger population on the north than in the Southern Hemisphere, which holds 33% of the earth’s land.
To account for this difference, we must consider the location of cities and climates. The vast majority of the world’s largest cities are located in the Northern Hemisphere. Though there are a few large cities in the Southern Hemisphere, the populations in those few cities does not compare to the populations in the many cities located north of the Equator. In addition, the largest mass of land in the Southern Hemisphere is Antarctica, which is uninhabitable due to its harsh climate. Antarctica is the world’s largest cold desert. There are other parts of the Southern Hemisphere that are difficult to inhabit due to their climates. For example, there is not a large human population in the Amazon Rainforest.

Demographers estimate the world population to be 7.41 billion people (estimate as of July 2017.)  88% of the world's population lives in the Northern Hemisphere.  Given that the Northern Hemisphere has only 67.3% of Earth's total landmass, this implies that the population density in the Northern Hemisphere is much greater than in the Southern Hemisphere. 
Why does the Southern Hemisphere have a lower population density?  One third of the Southern Hemisphere's land mass is Antarctica.  Even if we factor out Antarctica, the remaining areas of the Southern Hemisphere are still less dense than the population density in the Northern Hemisphere.  While birth rates are high in the southern portion of Africa, average life expectancy is much lower than the rest of the world (averaging 50 years versus 70 years worldwide average).  Other parts of the Southern Hemisphere have stable but low growth rates.  A second reason for a lower population density is that the two countries projected to have the most growth in the near future are in the Northern Hemisphere (India and China), and their average life expectancy averages 70 years, meaning the net growth is greater than in areas with both high birth and mortality rates.
https://brilliantmaps.com/human-hemisphere/

https://www.cia.gov/library/publications/the-world-factbook/geos/xx.html