Happy Left-Handers Day! Why are lefties so rare?

Thursday marks International Left-Handers Day, a holiday to celebrate the 10 percent of the population that struggles with spiral notebooks, scissors, and elbow bumping every day.

Researchers have disproven a plethora of popular myths about left-handed people, such as their supposed higher levels of creativity and introversion and the belief that they die earlier than their right-handed counterparts. But there’s still a lot that scientists don’t know about lefties, including the definite origin of left-handedness. 

A common misconception is that handedness is a result of nurture rather than nature, so many left-handed children in the past were forced to “correct” themselves by learning to write and eat with their right hand. 

However, research in recent years – such as one 2013 study which found that variants in the gene PCSK6 contributes to handedness determination – shows that there is likely a genetic component to handedness as well. The fact that a couple’s chances of having a left-handed child is bumped up from 10 percent to 25 percent when both parents are lefties confirms that your handedness is determined before you’re even born.

Other animals, including polar bears and chimpanzees, have also demonstrated a preference for one hand over the other when searching for food or using tools. But these populations are generally divided 50/50 between righties and lefties, making humans the only species with such a strong imbalance. 

This 9-to-1 ratio of human handedness has remained constant since the era of cavemen, according to archaeological evidence dating back around 500,000 years. Given the disadvantage left-handed people have living in a world of tools made for right handed-people, it might seem unlikely that this trait would live on. 

However, one theory known as the “fighting hypothesis” suggests that lefties have stuck around thanks to their advantage in physical competitions or combat. Thanks to the rarity of left-handedness, lefties are used to competing physically with righties, but right-handed people are usually thrown for a loop when faced with a left-handed opponent. 

Professor Daniel M. Abrams, who studies the physics of social systems at Northwestern University, points out in a TED-Ed talk that 50 percent of the top hitters in baseball are left-handed; being in the minority proves to be a great advantage when going up against pitchers who are used to throwing to righties. 

In terms of evolution, this advantage would normally cause the number of lefties to grow until the ratio of handedness was 50/50, Abrams says. However, the advantages of the fighting hypothesis are balanced out by the difficulty left-handed people have when using tools made for righties, and the high number of accidents that result in their misuse of these tools.

Today, the pros and cons of being a lefty in Western societies are relatively insignificant, and usually don’t play a role in life or death situations. Five out of the past seven US presidents have been left-handed, including Barack Obama, and other famous lefties include Oprah Winfrey, Robert Redford, Lady Gaga, and Morgan Freeman. 

Happy International Left-Handers Day!