Many of the greatest tragedies of the 20th century came from overestimating the importance of biological and cultural differences between groups of people, especially biological differences.
Human beings haven’t learned that much, or rather human beings tend to learn all the wrong lessons and then err in the opposite direction.
Many of the greatest tragedies of the 21st century will come from underestimating the importance of biological and cultural differences between groups of people, especially cultural differences.
Many people, especially progressives have a tendency to believe that cultural and value differences don’t exist between groups of people and when they do exist they can easily be negotiated by the appropriate social policy.
That is not true and lots of people are going to pay a high price for that belief.