In 1971, Henri Tajfel split teenage boys into two groups, supposedly by their taste for the painters Klee or Kandinsky, actually at random. They didn't know each other and would never meet. Yet when sharing out money, they favored their own group, even preferring to widen the gap over maximizing everyone's gain. “Us” forms in an instant.
Remembering is not replaying a recording — it is rebuilding. Each time you recall a memory, it becomes temporarily malleable and can be altered before being stored again. That is why two people can sincerely remember the same event… differently.
In 2003, Johnson and Goldstein compared European countries. Where you must tick a box to become a donor, consent rates ranged from 4% to 28%. Where you're a donor by default unless you opt out, they topped 85%. People aren't more generous on one side; most simply stick with whatever option was chosen for them.