Are you a rightie or a leftie? No, we're not talking politics here, we're talking handedness. And unless you happen to have an abundance of left-handed friends, you might have noticed how rare they are. So why are just 1 in 10 of us left-handed?
While no one's been able to definitively explain this, there are plenty of hypotheses that have been building for more than a century, and the evidence points to some kind of genetic influence. Why? Because the percentage of lefties is roughly the same, anywhere you look on the globe.
For those of you who kick a ball on a regular basis, you'll be aware that there are natural asymmetries all over the body - you'll likely opt for one foot over the other when you go to kick.
These asymmetries can be found in everything from our feet to our ears, our eyes, and the layout of our brain, Hannah Fry explains over at BBC Future.
If you hold your thumb at arm's length, then look at it using one eye and then the other, the eye that appears to show the thumb closest to you is your strongest. Similarly, you probably tend to answer the phone or listen behind closed doors with one ear rather than the other - that's your strongest ear.
But why aren't left-handed and right-handed people born to a roughly 50:50 ratio?
Some experts suggest that social cooperation, played out over thousands of years, has given righties dominant. In other words, when communities act together - in terms of sharing tools and living spaces - using the same hand as everyone else is beneficial.
Others suggest that it's to do with the way the brain is arranged in two hemispheres, with the left half controlling the right side of the body, and the right half controlling the left side of the body.
If most people's brains use the left hemisphere to control intensive language and fine motor skills, the thinking goes, that bias results in the right hand being more dominant too.
In fact, one of the more unusual hypotheses to explain the rarity of left-handedness is that a genetic mutation in our distant past caused the language centres of the human brain to shift to the left hemisphere, effectively causing right-handedness to dominate, Alasdair Wilkins explains for io9 back in 2011.
And while genetics likely play a large role in determining handedness, it's probably not the whole answer. Left-handed parents are more likely to have left-handed children than right-handed parents - a preference that can even be seen in the womb - but they still tend to have more right-handed children overall.
Researchers have struggled to identify exactly which genes are responsible for increasing the chances of being a leftie.
In 2019, an analysis of 400,000 individual records revealed the first genetic regions associated with handedness - four of them, to be exact. But other research suggests that there are probably dozens of genes that play a role in determining whether we end up writing with our left hand or our right.
On top of that, other studies have linked factors such as oestrogen levels and birth position to varying levels of left- and right-handedness.
In short, there seem to be a lot of considerations at play, and researchers are having a hard time tying them all together. That means we can't yet tell you exactly why you were born left or right-handed, but scientists clearly are working hard at finding an answer.
And when they do, they'll have to explain why some of us appear to be ambidextrous, too.
A version of this article was first published in October 2016.
#Humans | https://sciencespies.com/humans/why-are-only-10-of-people-left-handed-heres-what-scientists-know-so-far/
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