This Might Be Why You're Left Handed

by Megan Grant

We've long believed that the determining factor for your dominant hand comes down to what side of the brain you use more — but science might have a new answer for why some people are left-handed. What's more, if these researchers are right, we were way off. Way, way off. So far off, we may as well have been in outer space.

Hitherto the prevailing idea, the left brain versus right brain theory says that each hemisphere of the brain controls the opposite side of the body. It's been long argued that evolution and natural selection put the left brain in charge of speech and language, explaining why most people are right-handed.

But a new study from Dr. Sebastian Ocklenburg suggests that the cause of handedness cannot be explained by the brain, but is rather the result of the spinal cord. He and his colleagues found that from the 13th week of pregnancy, activity in the baby's spinal cord is already asymmetrical. Furthermore, previous research has found that babies still in the womb seem to prefer sucking one thumb over the other.

What's happening outside the womb also plays a role. Environmental factors that have nothing to do with genes can affect symmetry, by changing the enzyme interactions around the baby and impacting how genes develop. Therefore, it can affect whether the baby prefers their right or left hand in the womb.

There have been other explanations behind hand dominance, as well. Science has typically expected to find one specific gene responsible for handedness; but despite all the research conducted, this theory remains a dead end — suggesting that this single, powerful gene doesn't really exist. Across many studies, it has proven consistent that genes are responsible for left-handedness about 25 percent of the time; however, some scientists also believe that hand dominance is largely random. Other research has found that the fetuses of women who were very depressed or stressed during their pregnancies were likelier to touch their faces with their left hands.

Yet another theory says that left-handedness made its mark in the pre-modern world because it put some people at an advantage in combat, since their opponents weren't expecting anything coming from the left side.

Genes obviously play a role but are not the end of the story — which is why identical twins (who share the same genes) can each have a different dominant hand. When it comes to handedness and nature versus nurture, both are part of the equation. In fact, one notable study turned to sports to determine just how influential competition and "survival" are when it comes to beating an opponent.

In sports like baseball, boxing, and hockey, they were able to accurately predict how many athletes would be lefties — and it was more than 50 percent for baseball players and over 10 percent in other sports. (For reference, the general population is 90 percent righties and 10 percent lefties.) Only four percent of golfers, on the other hand, were lefties. While left-handedness could put you at a major advantage when face-to-face with an opponent, more solitary games (like golf) are likely a different story.

It's not a far stretch to believe, then, that survival instincts may also play a role in handedness.

So, it would seem the the theory of the left brain versus right brain may have been slightly over-estimated. In truth, scientists haven't found much of a difference between lefties and righties in terms of personality traits like extraversion, conscientiousness, and openness to new experiences. If you need further proof that your dominant hand and its opposing brain hemisphere aren't inextricably linked, there's always this: While roughly 98 percent of righties are left-brained, so are around 70 percent of lefties.

The takeaway is that handedness isn't nearly as clearcut as maybe we'd like. While science has certainly found trends and patterns across many years and countless studies, the discussion of what causes handedness is incredibly multifaceted.