Re: are people who are both handed tend to be smarter?
One of my aunts had a brain tumour when she was about 10, the operation that removed it did something to the part of her brain which controls the left side of her body. She can still walk and pick up things etc, but lost use of fine-motor skills(eg. no more cello playing). She was originally left-handed, had to spend a few years relearning how to do everything right-handed. And she's one of the smartest people I know - dux of her highschool, topped all her subjects except one, excelled at university, high-paying govt job now yadder yadder.
Not that that's proof or anything, but still interesting.