I believe that 90% of the population is right-handed and 10% is left-handed. I formulated this theory myself:
First of all, Leonardo da Vinci wrote backwards, but was right-handed. A video that I watched about him said something like "Why do you push a pen rather than pull it?" and that's when it struck me. Right handed people "drag" their pen across a piece of paper, but left-handers have to push their hand. Perhaps that's why most people are dominant in their right hand.