Take a Side on the Handedness Question

Five-time golf major championship winner Phil Mickelson has a funny
nickname, ‘Lefty.’ You may guess where it originates: Mickelson plays golf
left-handed, meaning his clubs are the reverse image of right-handed clubs, and
he lines up on the opposite side of the ball compared to his right-handed

interesting about Mickelson’s moniker is that he’s otherwise right-handed—despite
being right-hand dominant, he was able to develop professional level ability
with his left, having learned to play lefty by mirroring his right-handed father’s
movements with the club.

Such is the
nature of handedness, one of the most curious of all human attributes.

typically thought of as a discrete characteristic (that is, you are either right- or left-handed), preference for hand usage lies on a spectrum. Some perform
better on certain tasks with their right and better on other tasks with their
left, known as mixed-handedness. Mickelson might fall into this category. 

Ambidextrous people are equally skilled with both hands. Everyone else has a
dominant hand or side of the body and generally uses that side to complete
most tasks. Considering
an estimated 25 percent of handedness is genetically determined, a natural line of
inquiry questions what purpose handedness serves to have been codified in the human
genome. This investigation includes two different questions.

Firstly, why do
humans exhibit asymmetrical motor skills across the two halves of their body at
all, and secondly, why does 90 percent of the world exhibit right-handedness? The second
question is particularly pertinent, given that a host of traditionally negative
traits and disorders are associated with being left-handed, including decreased
motivation, lower spatial reasoning and greater rates of depression and

To answer
the first question of why our motor skills are unequal across the body, we
would do well to look to the brain. As you may know, the brain’s functions are specialized to occur almost entirely in a single hemisphere, a division called lateralization. Aside from motor skills, the most striking
example of lateralization is language, which is produced by the left hemisphere
in around 90 percent of humans.

The going theory as to why brain
functions are lateralized is efficiency. Cross-talk between brain hemispheres
occurs through only a few structured conduits, and it takes time. It then makes
sense to restrict important functions that must happen quickly to just one side
of the brain, increasing the efficiency with which they can be completed.

To illustrate how this works,
imagine you were ambidextrous. If I told you to copy a paragraph by hand as
quickly as possible, you almost certainly would not write one sentence with
your right hand, then one with your left, then one with your right, and so on
until you finished. The time it takes to switch hands makes the process inefficient—better to
simply get it all done with one hand.

But even if we can explain why the
brain divides its labor, we still don’t know why some people become
left-handed. A simple explanation posits that in hand-to-hand combat, lefties
enjoy an advantage because their opponent has likely fought almost exclusively

Some evidence for this theory exists. A 2014 study by researchers at
the University of Montpellier concluded that more peaceful, unindustrialized
societies (like the Dioula of Burkina Faso, with a murder rate of 1.3/100,000 and lefty
percentage of 3.4 percent) have much lower rates of left-handedness than more violent
ones (like the Eipo of Indonesia, with a murder rate of 300/100,000 and lefty percentage of 27 percent).

The ‘fighting hypothesis’ may also explain why lefties are slightly
overrepresented in the Ultimate Fighting Championship.

Another hypothesis suggests that
lateralization depends on testosterone levels in the womb, with higher levels
promoting delayed left hemisphere development and thus left-handedness.

hypothesis predicts that left-handedness will be more common in men, who have
higher testosterone levels at fetuses, and in people born during winter months,
since intrauterine testosterone increases when the days are shorter. Indeed, a
2014 study of 13,000 central Europeans confirmed exactly these predictions.  

Whether lefties exist because
they’re more likely to beat a righty in unarmed combat or because their mothers
had high levels of testosterone, the right-handed world has rarely been kind to
our southpaw minority. In 18th- and 19th-century America,
being left-handed was thought to be a disability, and the use of the left hand
was strongly discouraged.

Left-side dominance is often associated with
awkwardness or clumsiness—there’s nothing worse than dancing at Pub like you’ve
got two left feet. Hell, we don’t even bother to say people are ‘correct’ with
a whole two syllables. Instead, we just tell them they’re ‘right.’

Despite these semantic digs and the
ubiquity of right-handed tools, like scissors, knives, notebooks, standardized
tests, school desks and can openers, it might not be so bad for lefties after
all. One realm where lefties dominate is the American presidency: Five of the last seven were left-handed. Like Phil Mickelson, they all played golf.

Alas, we’re all
left to wonder how they would perform in bare-knuckle combat.

Warren Szewczyk PO ‘15 is hoping to spend the next couple years researching schizophrenia and continuing to write about science. If you are reading this, you hopefully read the entire preceding column, so thanks for that.

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