south park episode

In 1968, a teacher named Jane Elliott performed a seminal experiment with her class of third-graders.

Wishing to teach them about prejudice, she told them that kids with blue eyes were superior to those with brown eyes, gave the blue-eyed kids all sorts of privileges, and made the brown-eyed kids second-class citizens.

Soon enough, the kids had internalized the differences, with the blue-eyed kids picking on the brown-eyed ones.