Left-handed people make up 10 percent of the human population on average, but chances are you know a southpaw. Why some people are left-handed has always been a bit of a puzzle, but researchers have found some interesting defining leftie characteristics that highlight the differences from the right-handed majority. Here are just a few of them.

1. They're over-represented in athletics. Maybe you've seen a leftie pitch a baseball game or win a tennis match and wondered why left-handers seem to be so prevalent in the sports world. It's because of the element of competition, according to a study from Northwestern University. Researchers found that left-handed people are a result of the balance between cooperation and competition in human evolution. Cooperation favors same-handedness, such as for sharing tools, whereas competition favors surprise, and a left-handed person might win in a fight, the study said.

The Northwestern scientists built a mathematical model that "can predict accurately the percentage of left-handers in a group — humans, parrots, baseball players, golfers — based on the degree of cooperation and competition in the social interaction." They claim their model accurately predicted the number of left-handed athletes in baseball, boxing, hockey, fencing and table tennis — in which southpaws numbered well above 10 percent (and up to 50 percent for baseball players).

2. They're more likely to have a slender face. According to a (somewhat odd) study, people with slender lower faces are 25 percent more likely to be left-handed. Those slender-jawed folks also are more susceptible to tuberculosis, for what it's worth.

3. They're good at math. Have you ever noticed the lefties in your life are particularly quick with arithmetic? While a link between mathematical ability and left-handedness has long been rumored, a 2016 study found a "moderate, yet significant correlation" between the two. Math scores were five to 10 percent higher among lefties in a study of 2,300 students in Italy between the ages of 6 and 17 who were asked to complete simple mathematical tasks and problem-solve.

Also, a 2006 study published in the journal Neuropsychology found that left-handed people were more likely to have faster connections between the right and left hemispheres of the brain, meaning they processed information faster. Researchers said this could be a beneficial skill in areas like video games or sports.

Lefties like this batter are over-represented in athletics. (Photo: Richard Paul Kane/Shutterstock)

4. They're less likely to be ' left-hemisphere dominant.' For 95 percent of right-handed humans, the left side of the brain is their dominant side and where language function is localized, reports Psychology Today. However, for lefties, only 70 percent are left-hemisphere dominant, with the remaining 30 percent distributing language ability across both sides evenly or moving it to the right side of the brain.

5. They're slightly more susceptible to allergies, migraines and some diseases. Left-handedness is a result of biological diversity, and while lefties may have the advantage in a fight, they may have disadvantages in other areas. For example, this study found that left-handed people may be predisposed to allergies earlier in life. And the New York Times reported on a series of studies that showed "left-handed people are more likely to suffer from learning disabilities, stuttering, migraine headaches and, according to the latest findings, autoimmune diseases, like ulcerative colitis, myasthenia gravis and celiac disease, in which the body attacks its own tissues."

6. They're more likely to have a sleep disorder. In a 2011 study, 100 people with periodic limb movement disorder (PLMD) were divided into left-handed and right-handed groups. Researchers found 69 percent of right-handed patients had bilateral limb movements compared with 94 percent of left-handed ones, regardless of age, sex and race.

"What we know of people who are left-handed is they tend to have a slightly different dominant brain hemisphere than right-handed people," study researcher Dawn Alita R. Hernandez, a professor of medicine at the University of Toledo Medical Center in Ohio, told LiveScience. "So if [PLMD] is coming primarily from the cortex, we should see a difference in handedness." Previously, PLMD was thought to originate in the spine, but researchers now believe it comes from the brain.

Studies have shown left-handed folks are more likely to be male than female. (Photo: VYarochkina/Shutterstock)

7. They feel and express emotion differently. Going back to the brain hemispheres, a 2012 study published in the journal PLoS ONE found that lefties processed motivation on the right side of the brain, whereas right-handed people had motivation activity on the left side. This finding may affect how mood disorders are treated, where the left side of the brain is stimulated.

“Given what we show here, this treatment, which helps right-handers, may be detrimental to left-handers ― the exact opposite of what they need,” one of the study’s authors, psychologist Geoffrey Brookshire, said in a statement.

8. Gay men are more likely to be left-handed. A 2003 study evaluated large numbers of heterosexual and homosexual men and women on handedness and gender-related personality traits. Here's what they found: "Homosexual men had 82 percent greater odds of being non-right-handed than heterosexual men, a statistically significant difference, whereas homosexual women had 22 percent greater odds of being non-right-handed than heterosexual women, a nonsignificant difference."

9. They tend to drink more often. If you've heard that southpaws are more likely to be alcoholics, that's a myth. A 2011 study on the rumor published in the British Journal of Health Psychology found that left-handed people do tend to drink more often, but they aren't more prone to risky drinking.

Maybe it's because they've long been forced to adapt to a right-handed world, whether it's the frustration over using scissors or a can opener designed for a rightie or the annoyance of trying to eat or write without bumping elbows with the person next to them!

10. The answer could lie in the spinal cord. There have been several theories as to what determines our preference for handedness. Over the past several decades, researchers have agreed that it is determined in the womb. Scientists previously thought that it was genetic differences between the hemispheres of the brain that decided if a person was born right-handed or left-handed, reports Business Insider. But a 2017 study found that the answer could lie in the spinal cord.

Researchers found that even though the motor cortex in the brain and the spinal cord aren't yet connected, up until about 15 weeks the baby is growing and making movements. The baby has chosen its favorite hand that early on, leading researchers to believe it's the spinal cord, not the brain, that is the key to hand preference.

Editor's note: This story was written in May 2017 and has been updated.

Angela Nelson ( @bostonangela ) is an exhausted mom of two young daughters and two old cats, and a Pulitzer Prize-winning digital editor with more than 15 years of experience delivering news and information to audiences worldwide.

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10 interesting facts about the lefties we love
Like, why are there so many left-handed athletes, and how come they're so good at math?