By nature, humans are generous, kind, compassionate, aggressive, mean-spirited, cowardly, and cruel. And for the most part, we share the view that the first three qualities in this list are virtues, while the remainder are vices. But if we recognize vice as vice, why is it so difficult to eradicate? Much of the answer can be summed up in one word: tribalism. As evolutionary psychologists like Heather Heying understand, group loyalty, or tribalism, has been advantageous to our species from the very beginning. We tend to identify with groups and cooperate within them. In the post-industrial world, this has expanded beyond family and bloodline to include political loyalties, sports fandom, and more—but the rules of the game are the same: Within our tribes, we tend to practice our higher virtues. But when it comes to other tribes, all bets are off. This is why it’s possible come to the conclusion that anyone who voted for Candidate X must be a terrible person. Tribalism empowers us to...