Left-handedness.

From the Wall Street Journal:

•Six of the last 12 U.S. presidents, including Barack Obama and George H. W. Bush, have been lefties.

• Left-handed people earn on average 10% lower salaries than righties, according to a recent study. Findings of some earlier studies on income have been mixed.

•Despite popular misperceptions, lefties aren't more accident prone than right-handed people and don't tend to die at a younger age.

•Left-handedness has been linked to increased risk of certain neurodevelopmental disorders like schizophrenia and ADHD. Mixed-handedness is even more strongly associated with ADHD.

•Most people's brains have a dominant side. More symmetrical brains of mixed-handed people may explain the link to some neural disorders.