Of course it doesn't help that the church DIDN'T condemn left-handedness as "of the Devil". Myth. It was seen as odd, possibly undesirable, but not condemned (and this attitude predates Christianity, mind you), and not attributed to the Devil.
The real stigmatization of left-handedness apparently has its roots in early modern psychology, or so says the author of "The Puzzle of Left-Handedness". It was seen as a sign of some kind of mental maladjustment.
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