Untruthfulness, as a practice, is almost always a product of fear. The child brought up without fear will be truthful, not in virtue of a moral effort, but because it will never occur to him to be otherwise. The child who has been treated wisely and kindly has a frank look in the eyes, and a fearless demeanour even with strangers; whereas the child that has been subject to nagging or severity is in perpetual terror of incurring reproof, and terrified of having transgressed some rule whenever he has behaved in a natural manner. It does not at first occur to a young child that it is possible to lie. The possibility of lying is a discovery, due to observation of grown-ups quickened by terror.
Source: On Education, especially in early childhood, 1926
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