As such, during the late Qing reforms, mahjong became considered a social issue. Mahjong started to become outlawed indirectly as gambling and smoking opium became criminalized. The ban was not very successful, however, at least until the Chinese cultural revolution.
American Mah Jongg is considered the most difficult style because only the hands listed on a card are considered valid. This requires players to memorize the card (or at least have a good grasp of it).