We like tribalism for the same reason we like fatty foods: We evolved that way.
Homo sapiens didn’t survive long on the African savannas as rugged individualists. Alone, they couldn’t scare away the scarier animals, and, for the most part, they couldn’t catch and kill the tastier ones. But in groups, humans rose to the top of the food chain thousands of years ago and have been passing down their tribe-loving genes ever since.
Customs and practices that ensured the survival of the species were worked out through trial and error and passed from one generation to the next. Over time, and with many setbacks, the knowledge accumulated until we hit the critical mass required for modernity.
Indeed, the story of modernity is the story of how we moved away from traditional, non-voluntary forms of tribalism based on familial, ethnic, or even nationalistic lines and toward voluntary forms of tribalism.
The American founding was revolutionary in its embrace of the universality of human rights (even as it fell so short of its own ideals with the institution of slavery). Since then, the West has fought several civil wars to break away from various tribal ideologies, including not just monarchism and imperialism but also Nazism (racial tribalism), Communism (economic tribalism), and fascism (national tribalism).
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