Study of History
The study of history is discouraging in a number of respects. For one thing, it's pretty clear that genocide is the major player in practically every history of note.
That's a corollary, by the way, of the first principle of economics, the Law of Malthus. As soon as humans managed to become a major predator, and perhaps before, human population expanded until it was controlled by starvation, war, and disease. Once we "advanced" to agriculture, our productivity crowded us every more densely. If such a society is to be stable, it needs mechanisms for controlling population well enough to prevent starvation.
Typically civilizations develop a range of methods, including restrictions on marriage, like dowry and bride price. War is another popular alternative, and when successful, is often followed by genocidal elimination of the conquered. Another technique is establishment of a poor class - the designated victims, if we may so designate them. Their job is to die off at sufficient rate that the non-poor can remain comfortable. Of course that also requires that you need some downward social migration unless you have other means for controlling the numbers of the more economically favored. I wonder how that happened in India, for example. I suspect that there must have been some mechanism for demoting some members of higher castes, but maybe the real system was different.
The unique thing about today is that we have more and better methods for escaping the Malthusian trap than ever before. We would be really foolish not to take advantage of it.