From Huffington:Why Bush Smirks.

Leonard Shalin - Huffington Post:

In observing our president's expressions over the years, I became aware of a feature of George Bush's face that revealed more about his inner self than anything issuing forth from his mouth. President Bush has a disconnect between the right side and the left side of his face. While the right side of his mouth and the corner of his right eyes portray a smile, the left side of his mouth and the corners of his left eye convey a scowl.

The result is a twisted smirk that has become his trademark expression.

[...]

Psychologists have studied the phenomenon of the split face for many years and have accumulated a reservoir of studies that conclusively indicate that the expression of the left side of an individual's face is far more revealing concerning their emotional state than is their whole face.

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