The 21-day figure traces back to a 1960s surgeon's informal observation. In 2009, Phillippa Lally tracked 96 volunteers adopting a new daily behavior, like eating fruit or running. It took a median of 66 days to become automatic, ranging from 18 to 254 depending on the person. Good news: missing one day doesn't reset the clock.
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.