In Japan, until 1868, a social superior had the right to kill a social inferior if he failed for an instant to keep smiling while in the great man's presence. In the family, the wife had to smile in the presence of her husband and the children in the presence of their parents. This was what made the Japanese appear to Western travellers such a cheerful people; it was a case of survival of the fittest.
Source: "On smiling" [From: Mortals and Others: Bertrand Russell's American Essays, 1931-1935, v.1 (1975)]
More info.: https://russell-j.com/SMILING.HTM"