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I am a lefty and as I recall the teachers had a hard time teaching me to hold a pencil or pen.
And, as my wife teacher tells me, and as I recall, you write with a number two pencil when learning how to write..
That would shoot down your theory, I would think...not much smudging with a pencil
My wife also believes that teachers would not slant the paper correctly for lefties
For the record, I now hold a pen correctly and my fingers remain below the writing..... I basically did that retraining myself f as an adult....a lefty can do it
I was actually just making an educated guess - this source generally confirms what I was saying...
http://handedness.org/action/leftwrite.html