About 10% of the population is left-handed, but a disproportionate amount of people with schizophrenia—20%—are lefties. Schizophrenia is just one of a number of psychiatric and developmental disorders that are more common in left-handed people, including dyslexia, ADHD, language difficulties, and mood disorders. Researchers think this could have something to do with the fact that each half of the brain performs distinct functions. Typically, a person's brain has a dominant side; in righties, that side is usually the left hemisphere. But 30% of lefties exhibit right-hemisphere dominance or a distributed pattern of dominance, which could lead to the greater risk of brain disorders.
In addition, those who have no dominant hand—just 1% of the population—have more symmetrical brains, which have also been linked to a greater risk of neural disorders, the Wall Street Journal reports. (Adding insult to injury: Lefties also make less money than righties, with salaries an average of 10% lower.) Researchers say environmental factors are more important than genetics when it comes to determining handedness. Babies born at a low birth weight, or babies born to older mothers or mothers who experience stress during their pregnancies, are more likely to be left-handed. That could be because the stress hormone cortisol can interfere with brain development. But all hope is not lost: The Journal notes that of the last 12 presidents, half were lefties, a group that includes Barack Obama and George H. W. Bush. (Read more left handed stories.)