Lack of clean water.
Inadequate medical care.
Rats and other rodents.
Soon stabilize at the level it is today - about 6 billion.
Begin to decrease until it reaches 5 billion.
Increase for a short time and then decrease to current levels.
Stabilize somewhere around 9 billion by 2050.
High literacy rates result in women wanting to have more children.
Population sizes will not decline until some people start having to do without food and other necessities of life.
Many people live in cities, where large families are an advantage.
Many people have low literacy and limited access to healthcare.
Limiting education for women.
Here's an interesting quiz for you.