Somewhat surprisingly, the answer seems to be "nothing". In a primitive society, population self-regulates through early deaths, caused by diseases, physical i

Somewhat surprisingly, the answer seems to be "nothing". In a primitive society, population self-regulates through early deaths, caused by diseases, physical injury, famines, etc... Then there is a transition to modern society (demographic transition) where though improvement in medicine, agriculture, and the general idea of not letting people die, population grows exponentially, but it doesn't last as people simply stop making children. It has already happened in the first world, most countries would have a negative growth rate if it wasn't for immigration. Asia are just at the end of their transition and Africa is in the middle of it. Assuming the trend still holds, it will be the end for Africa and therefore, the entire world in a few decades. It is actually reason for concern, as it will result in an aging population, and unlike today in first world countries, there will be no immigration to compound the problem. But changing the question and asking "How was it done?" instead, then I would say "birth control", the voluntary kind, the pill in particular. Safe abortion too. As someone mentioned, women in the workforce also contributed, but I would go further and say that for many women, having a job is not a right, it is an obligation! Many couples simply can't live decently on a single pay, especially with a child. Also, with laws regarding child labor, long studies, and changes in mentality, children are more of a burden than they once were, as most can't support themselves before their 20s. It used that at that age and even way before that, they supported their parents instead, especially boys (that's the idea behind dories). Besides the inability to live on a single pay, these are generally good things, but the result is less children, which is a good thing for ending a potentially catastrophic exponential growth, but it may turn out to be a problem later. Not an unsolvable one though.