Why do we think bad things really happen in threes?

Humans are very adept at spotting patterns in the world around us, and reading significance in them – even when they’re meaningless. When one bad event occurs, we may dismiss it as just a coincidence, but after two misfortunes, we start to suspect a pattern. When bad luck strikes a third time, we’re then tempted to dismiss mere coincidence as too unlikely, and give the ‘run’ of misfortune some credence – however misguidedly.