The Majority's Habits Overrule those of the Individual - Batlah Daato Eitzel Kol Adam
General Rules
One cannot prove "normal behavior" from the acts of a Mentally Incompetent Individual: Shabbat 104b
Whether an entire country, although other countries differ in practice, still constitutes Popular Method: Shabbat 144b; Eruvin 28a, 29b-30a