出典： ラッセル『教育と社会秩序（or 社会体制）』
All the higher animals have methods of expressing pleasure, but human beings alone express pleasure when they do not feel it. This is called politeness and is reckoned among the virtues. One of the most disconcerting things about infants is that they only smile when they are pleased. They stare at visitors with round grave eyes, and when the visitors try to amuse them, they display astonishment at the foolish antics of adults. But as soon as possible, their parents teach them to seem pleased by the company of people to whom they are utterly indifferent.
Source： On smiling, Aug. 17, 1932, in Mortals and Others；Bertrand Russell's American Essays 1931-1935, v.1, 1975.