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Why Am I Right-Handed?

The questions kids ask about science aren’t always easy to answer. Sometimes, their little brains can lead to big places adults forget to explore. With that in mind, we’re starting a new series called Science Questions From a Toddler, which will use kids’ curiosity as a jumping-off point to investigate the scientific wonders that surround us all. I want the toddlers in your life to be a part of it! Send me their science questions and they may serve as the inspiration for a column. And now, our first toddler …

Q: Why am I right-handed? — Josephine Kimball, age 5.

The short answer, dear Josephine, is that you are right-handed because most of humanity is right-handed. About 90 percent of us are righties — though the rate can vary by country and time period. Also, almost all the information we have on this comes from Western countries after the year 1900. As you grow up, you will learn to raise a mighty eyebrow at generalizations taken from such limited data. Sometimes, though, it’s all you’ve got.

It’s pretty cool that you asked the question this way — why are people right-handed? When we adults talk about handedness, we tend to focus on the lefties. There are fewer of them, so we think they’re the strange ones. But the reality is that we don’t know which is strange and which is not, because we don’t really know what causes handedness at all. If there is an inherent biological tendency toward being right-handed, then yeah, the lefties are weird. But if there’s not — and there might not be! — then it’s the righty majority whose existence becomes super confusing. Most of us think of handedness as simple and binary. Even scientists used to think the distinction between lefties and righties was mostly genetic. Turns out, that’s wrong.

Here’s what we do know: Humans are pretty asymmetrical creatures. That applies to both how we use the body parts we have and where things are in the first place. Our hearts tend to be on the left. Our livers tend to be on the right. Not only do these asymmetries exist, but some of them seem to be interconnected. People who are right-handed tend to process language on the left side of their brain.

There’s a way to see this connection: the Wada Test, a tool that doctors sometimes use to prepare people for brain surgery. First, you have your patient hold up both hands and talk. Then you inject a syringe full of barbiturates into her left carotid artery. (Please do not try this at home.) As soon as the drugs hit the left side of the brain and anesthetize it, 90 percent of right-handed people will lose control of their right hand, says William Hopkins, a professor of neuroscience at Georgia State University. It’ll just drop into their lap, and they’ll become unable to speak. But what’s strange is that about 70 percent of left-handed people will also lose their language ability if you paralyze the left side of the brain. The rest are about evenly split between processing language on the right and processing it with both sides. Nobody knows why most righties are asymmetrical and most lefties aren’t.

That alone should tell you there’s something a little weird about handedness. But scientists used to think it was a simple trait, easily explained. One of the most popular genetic models for handedness was proposed in 1985 by psychologist Chris McManus. Called the “dextral/chance” model, it proposed that handedness is determined by a single gene that comes in two varieties, dextral (D), meaning “righty,” and chance (C), meaning literally just chance. People who got a C variant from both parents would have a 50-50 chance of being lefties. The pairing of CD would take that chance down to 25 percent. People with DD would all be right-handed.

But in 2013, a paper published in the journal Heredity demonstrated that handedness had to involve more genes than that — a lot more. The researchers analyzed the genomes of 3,940 twins whose handedness was recorded from previous studies. First, they found that identical twins, who share all the same genetic material, weren’t significantly more likely to have the same dominant hand than fraternal twins, who are just regular siblings who shared a womb. That alone starts to rule out simple genetics.

Second, they failed to find any genes that stood out as connecting unrelated people who had the same dominant hand. If the genetics of handedness were simple, we ought to be able to look at the genomes of a bunch of unrelated righties and see a gene or genes they all shared. One of the co-authors was the same Chris McManus who first proposed the dextral/chance model. To John Armour, a professor of human genetics at the University of Nottingham in the U.K. and an author on the paper, that’s a big deal. “It’s safe to say the simple models are out of date now,” he told me.

“On the contrary. I think the dextral/chance model is still the best explanation we have,” McManus wrote in an email. The important thing is that there probably isn’t a gene for left-handedness. It’s still likely that we are dealing with genes that either make a person right-handed or leave her handedness to chance, the latter of which results in some left-handed people. All that has changed, McManus wrote, is that we now know this must involve many genes, rather than just one. He published a paper in 2013 that made this same basic point. Armour was one of his co-authors.

However Armour and McManus’s debate hashes out, their research fits right in with a growing body of evidence, drawn from other studies of twins, that suggest genetics only accounts for about 25 percent of the variation in handedness. Some of these studies have involved as many as 30,000 sets of twins. If there were simple inheritance going on, that should be a big enough sample to show it. But Armour says researchers were still surprised by how little of the variation seems to be genetic.

To put it in context, blood groups (the immune system categories that determine who can take a blood transfusion from whom) are based on simple inheritance and are almost 100 percent attributable to genetics. Height is more complex, involving 300-odd genes, the most powerful of which accounts for only about 4 millimeters of growth, Armour told me. But even that is highly heritable. The numbers vary depending on the study, but scientists think 60 percent to 80 percent of your height comes from genetics. Body mass index, skin color, hair color, eye color — most obvious physical traits have high rates of genetic heritability. “Sixty, 70 percent or above,” Armour told me. Handedness is a distinct outlier.

So, if you aren’t right-handed because of your genes, what causes it? We know handedness can be affected by social forces. In most Western cultures, for instance, generations born at the beginning of the 20th century had left-handedness drilled (or sometimes beaten) out of them, says Tulya Kavaklioglu, a graduate student at the Max Planck Institute for Psycholinguistics who is studying the connections between handedness, language and genetics. She pointed to a study from 1981 on the prevalence of left-handed Australians. Among the study population, only about 2 percent of Australians born in 1880 were left-handed. Of the generation born in 1969, 13.2 percent were lefties. As it became more acceptable to be a lefty, Kavaklioglu says, more people were.

But culture doesn’t totally explain it, either, Hopkins says. Human fetuses suck their thumbs in the womb, and almost all of them favor the right-hand digit, he told me. And there’s a big debate over the hand our closest relatives prefer. Some researchers who have studied chimpanzees and other apes in the wild think that the chances of an individual ape being right-handed are about 50-50. Hopkins disagrees. That research counts a broad range of behaviors, from nose-picking to the hand an ape uses to touch a friend on the shoulder. His research narrows in on specific behaviors such as tool use, and it shows that apes, like humans, are mostly righties. The ratio of right-handed to left-handed chimps is lower than the ratio of right-handed to left-handed humans — 2:1 as opposed to 9:1 — but Hopkins told me he thinks the genetic determinant for apes might be stronger than it is in humans. In other words, humans are more likely to be right-handed than chimps, but chimps could be more likely to have inherited their handedness.

So is it nature or is it nurture? It’s difficult to make all this evidence fit together in a way that makes sense. But Hopkins, Kavaklioglu and Armour say it’s easier if you understand this: Heritability isn’t just about genetics. You are right-handed because of genes. You are right-handed because of culture. And you are right-handed because of other factors that affect you both before and after birth — things that can be heritable without being genetic.

It’s not just your genes or what you explicitly learn. For instance, higher testosterone exposure in the womb is associated with increased rates of left-handedness, Kavaklioglu told me. There’s also implicit learning after birth, the kind that happens when babies watch and copy their parents’ behavior. It’s possible, Hopkins says, that chimps and humans differ because human babies are born with less-developed brains. There’s more opportunity for our environments to turn a right-handed fetus into a left-handed child. Nobody knows for sure. But either way, handedness is a great example of how our perspective on genetics, as a whole, has changed. “People used to think you could just hone in on the magic gene,” Armour says. “But we’ve come to appreciate that it can be more complicated than that.”

Have your own science question from a toddler that you want answered? Send it in!

Maggie Koerth was a senior reporter for FiveThirtyEight.