According to a Wall Street Journal article "Other recent research suggests that mixed-handedness—using different hands for daily tasks and not having a dominant one—may be even more strongly linked than left-handedness to ADHD and possibly other conditions. About 10% of people are left-handed, according to expert estimates. Another 1% of the population is mixed-handed. What causes people not to favor their right hand is only partly due to genetics—even identical twins, who have 100% of the same genes, don't always share handedness. "
Below is a video of some famous lefties - and of course, not all lefties have ADD, Dyslexia or something similar. It's something interesting to think about.