One of the persistent human mysteries is handedness. Handedness is clumsy sounding term for being right-handed or left-handed. Despite over 100 years of research, we don’t know why 10 percent of humans throughout history prefer to use their left hand. If you go back and look at cave paintings, you will find about 10 percent of cave men throwing spears left handed. [1,2]
Throughout history, left-handers have been stigmatized as being odd or cursed. The word “right” has been associated with strong and proper. The word “left” has been associated with wrong and clumsy. Statistically, about 90 percent of people are right-handed and about 10 percent of people are left-handed. A very rare person may use both hands equally, called being ambidextrous. Men are slightly more likely to be left-handed than women. [1-2]
Is Handedness Genetic?
Handedness is not like having blue eyes. It is not strictly a matter of genes. About 40 genes have been linked in some way to handedness, but none is very strong. If both parents are left-handed, the chance of left-handedness only goes up by about 10 percent in their children. If one identical twin is left-handed, the other twin is often right-handed. 
Handedness Starts in the Womb
Until now, researchers thought that handedness was determined by which side of your brain you use to send movement signals to your spinal cord. A recent study published in the journal eLife, shows that may not be the case. Ultrasound studies show that babies in the womb at 13 weeks already prefer sucking their right or left thumb. 
The researchers point out that at 13 weeks, the part of the brain that would send signals for handedness, is not yet communicating with the spinal cord, so the brain-side theory may not work. In any case, this study supports other studies that suggest babies are born right-handed or left-handed.  My grandson Ethan – who is left handed – was a leftie the first time he was able to throw a ball.
Does Handedness Matter?
Many smart people are or were lefties, including Einstein, Da Vinci, Clinton, and Obama. Studies have been done to find out if left-handers are smarter, more artistic, more introverted, more likely to have mental health problems, more likely to have physical health problems, and more likely to die younger. None of these studies has found convincing evidence that handedness matters. [4,5]
It can be hard for a left-hander to live in a right-handed world. Just try using scissors or a can opener with your left hand. Maybe lefties are not born odd, it may just be the stress of living in a right-hand world.
My mother was the ninth of 10 children, so she was due to be one of the 10 percent. As a child, whenever she reached for anything left-handed, my grandfather would slap her hand. She was naturally left-handed but grew into an ambidextrous adult. She was creative and smart. She also put sugar on tomatoes and salt on watermelon.
Animals also have handedness. Studies show that bears, chimps, and dogs are either left or right- handed, but the odds on handedness for the rest of the animal kingdom is 50/50. Only humans seem to live in a 90 percent right-handed world. [1,4] Nobody knows why and nobody can tell you if your child will be a left-handed. If you baby throws lefty, don’t worry. He or she will grow up to be unique and wondrous.