The Left Handed

I'm writing with my left hand so am left handed , as i heard this method about how do the person writes with the left hand and what are the reason for that to happen 

"If the left side of your brain controls the right side of your body, and the right side of your brain controls the left side of your body, then left-handed people must be the only ones in their right minds."   -

In current society, "handedness" is simply defined as the hand you use to write with, but scientists measure handedness in many ways.