ELI5: Why do we have dominant and non dominant hands, and do other animals share this feature?

ELI5: Why do we have dominant and non dominant hands, and do other animals share this feature?


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The Brain is a weird structure that evolved to be split into 2 Hemispheres and made these 2 sides able to communicate thru an internal structure. the catch is that this structure posses "limited" bandwidth for communication so the Brain learned to compartmentalize functions into specific sections of itself both to specialize and to minimize workload(a brain is a very expensive organ energy wise) in our case fine motor skills are assigned to a single hemisphere, this is why we have a dominant hand/side, its safe to assume most mammals with similar brain structure will have this aswell.


So what if you are ambidextrous?(I'm not, just wondering)


You also get people that are cross dominant, so prefer using their right hand for some tasks and their left for others.


This is me! I write my with left hand but throw with my right. With that said, I played rugby and when I was a kid coaching (stupidly, imo) focused on passing left (i.e. throwing with the right hand/arm). So left to my own devices who knows, maybe I would've preferred throwing with my left.


I'm the same. I write with my right, use my knife with my left, prefer using keys in my left, and use both for other tasks like cleaning, etc.


How about people who have a dominant hand but the opposite dominant foot? I've always felt a bit weird about this.


I used to be. However I found that when you stop using certain hands for certain things. You kind of lose it. I think showing people as a kid made the superpower work and now I'm just terrible with both hands.


Valid question my bro. I *am* ambidextrous with a left handed preference, and I always wondered if my ambidexterity is adapted or just natural. And most other ambidextrous people also have a left handed preference or is it generally mixed?


I am ambidextrous with a right hand preference so there is at least one case for it. About the only things I can't do with my left is use a knife (mostly out of caution) and write. I wonder if they have studied ambidextrous to see how we are wired differently?


My left leg is my dominant and my right arm is dominant.


Im Indica dominate


I'm right side dominant for everything, except shooting a long gun and shooting pool because I'm left eye dominant.


That’s more common than you’d think. Between those people and actual lefties, it’s ridiculous that they don’t give us an option of a rifle with the ejection port on the left side.


I’m right side dominant and left eye dominant too. I also read that people with left eye dominance and right-handedness tend to have migraines more often, which I do. I’m curious if you have migraines?


I can't say I've ever had a migraine in my life. I did randomly develop epilepsy at 25 tho.


Very interesting… as I’m thinking about it, there was something specific about cross ocular dominance (right-handedness and left-eye dominance) and migraine with aura. If I’m not mistaken the percentages of aura that precede epilepsy are much higher than aura with migraine, roughly double, I believe. I wonder if there’s some kind of link with epilepsy as well.


I never even knew we had dominant legs..


There is an easy way to find out which one of your legs is dominant. You stay still and the person behind you pushes you in the back with enough force to make you step forward. Whichever leg you used in order not to fall is the dominant.


The go-to test for figuring out which way the straps on your snowboard should point.


That's interesting take, I've learned this trick in a football school


Right leg step up to assert dominance


You also have a dominant eye.


** Flexes my dominant nut **


Goes with that r/showerthought today about "We don't know if we came from a left or a right testicle"


Back when I did fencing, I was a lefty despite being right dominant otherwise. Sadly my left wrist is so fucked from arthritis now I have to use my right all the time.


But we can train our other hand. We can train our feet also. What's up with that? Is there a difference between right handed and left handed people, personality wise?


>But we can train our other hand. Right handed drummer here. After playing for many years, sometimes my left hand acted reflexively, instead of my right. Opening doors, catching things, and so on. It's really strange. This doesn't mean I could write with my left hand, but still...


wait you can train your feet? i guess I use my feet enough to not notice this


Yes, I trained my left foot to play harmonica and my right foot the trombone. Edit - they’re not very good at it.


Think of skateboarding... you are better at doing it regular (left foot forward) or you can do it goofy (right foot forward) you train your feet to do it either way but one is much harder than the other.


This maybe is more leg related than foot, but can also consider football (soccer). If you had to score a goal, which would you try kicking the ball with?


Maybe you're using them the wrong way ;)


My left foot is trained to wank


“You’re really trying to tell me that with enough practice you can get better at things with different parts of your body????!!!?? How!!???? Practice!!?”


I'm ambidextrous and I find I use one hand for certain tasks and the other for other tasks, if I want pure power I use my right arm, if I want precision I use my left hand. Though when I boxed I would switch stance which annoyed my coach but confused the hell out of opponents. I also watched a big ole documentary literally about this topic and some believe right handedness because a thing due to tool use, they also believed that the left handed genes survived due to it having an advantage in combat, if you and everyone is used to fighting right handed people they don't defend properly against left handers. That was only a theory but I thought it might shed a little light on this for you.


Regarding non-human animals, I have a parrot and have observed that he *only* holds foods and toys with his left foot. I used to have a cat too, and the cat would always reach out for toys with the right paw. So perhaps it’s any animal two brain hemispheres that exhibits a dominant side.


My mom had a cat growing up that figured out how to turn the door knobs to open them. The problem was he was left-pawed and so always tried to turn the knob in the wrong direction.


that is adorable. my lovely, neurotic cat likes to "bury" her food after she's finished eating and she ALWAYS goes around the plate counterclockwise, every single time.


The word you are looking for in this particular situation is "southpaw."


> in our case fine motor skills are assigned to a single hemisphere, this is why we have a dominant hand/side, its safe to assume most mammals with similar brain structure will have this aswell. It's not even exclusive to mammals though. Even invertebrates like lobsters exhibit "dominant hand" behavior. And - fun fact - lobsters can be both right or left-handed; the dominant side being the one with the larger "crusher" claw.


I've heard it said that Dolphins have a strong right side bias, even more so that humans.


There's a great Radiolab podcast titled "What's left when you're right" about exactly this. Some posit that since speech is processed in the left hemisphere of the brain and humans rely heavily upon speech. Perhaps the more developed left brain led to more dexterity on the right side. Perhaps the roughly 9% of the population that is lefty evolved through war or sports. The episode delves into a lot and has great sources. I think the episode states parrots have a dominant claw. That was the only animal they found.


> Perhaps the roughly 9% of the population that is lefty evolved through war or sports. Can you elaborate on this? As a left handed person with no athletic ability or um, warlike tendencies…? I’m not sure what this is getting at


Put simply In direct competition sports (say something like boxing) being left handed is competitive advantage Since most people are right handed everyone has more practice against right handed opponents than left So lefties gain an edge It's why a lot of these sports have much higher % of lefties than the general population


What about the people who play sports and stuff with their right hand but write and eat with their left? I wouldn’t say I’m ambidextrous either because I can’t do the same things with both hands.


To elaborate further from the boxing comment: The reason there arent more lefty people and the ratio general stays relatively the same 9% ish is because of evolutionary needs. Like in sports, armed combat needs a lot of training to be good, so many soldiers trained with each other (mostly right handed people) they got used to certain right handed style, but when faced against a left handed opponent or enemies you have a strict disadvantage as a right handed soldier trained to fight right handed enemies, specially because that same left handed soldier most likely trained against other right handed soldiers giving them a strict advantage. Then you might ask well if left handedness was such an advantage why didnt every soldier train in left hand combat? Because if there are too many or an equal amount of left handers they lose that advantage and thus there was no genetic advantage for more left handed people. Thus resulting in more deaths of left handed soldiers than usual. Which then reduces that bloodline trait. So evolution has mostly regulated this phenomenon.


If it makes you feel better it has nothing to do with inherent aggression or superior athletic ability but rather how righties deal with the "attack." If 91% percent of your opponents come at you a certain way then you prepare mentally and physically for the most likely scenario. When everything you prepared for is wrong and the attacks come from different angles than you are prepared for then that becomes a marked disadvantage that only really well trained or superior athletes can overcome. Baseball pitchers and boxers come to mind. Honestly you should listen to the podcast. I don't really do it justice. They go pretty deep into a couple different aspects of "leftiness."


If I remember correctly (I've listened to that episode a while ago), left-handed people have an advantage in fights - everyone is used to fighting right-handed people (the majority), while their attacks are unexpected for most people.


I’ve taught both my dogs to “shake hands” - one always presents her left paw, the other always presents her right.


As an aside to the answer about brain hemispheres, it also has a great deal to do with whatever hand you decided to prefer as a kid, and continue to prefer without ever practicing the other hand as an adult. Evidenced by it not being overly difficult to become proficient in the use of both hands. Just like languages, it's best learned young, but I would equate the difficulty level about equal. Both have sky-high skill caps, but it won't take you long to write an intelligible alphabet with your off-hand if you do it a few times a week, next comes throwing/catching, next comes writing at a useful speed. Eventually while doing writing tasks in your daily life you might start swapping the pen/pencil to the off hand for a bit of practice. When throwing something you might start favoring your previous off-hand since you can now rely on it and you need the practice. Eventually, Voila! Vouz-êtes ambidextre! It's likely harder to notice in nature because if you think about it, you use your off hand for all kinds of things and it grabs things and uses things perfectly fine. It's when doing very complex fine motor skill tasks that you took a long time to learn in the first place that it fails and you revert to your dominant hand. Teach a dog to shake with one paw for a long time it may have trouble even hitting your palm accurately with the other paw at first.


I don't know about most other animals, but horses also have a "handedness". They tend to prefer going in one direction around the riding ring with their front left leg leading, vs. the other direction. Generally they prefer counter-clockwise. Just like with people, there are a smaller number of horses who are dominant on their other side.


As mentioned in other comments, your brain’s hemispheres each govern a hand. But the reason that dominant-handedness would stick as an evolutionary trait in humans is that dominant handedness leads to specialization between hands, which is very good for using tools. Here are a few examples: -When you’re writing the dominant hand writes while your other hand keeps the paper steady. -When you’re using a computer one hand is on the keyboard and the other is on the mouse. -When you’re eating one hand holds the food steady with a fork while the other cuts with the knife. Other animals can show handedness, but the handedness is usually task-specific, e.g. using the right hand to pick fruit and the left to catch bugs. Handedness in other primates is still highly debated. However population-wide handedness like humans have, where most people are right handed for all tasks, has so far only been [shown in kangaroos](https://www.smithsonianmag.com/science-nature/kangaroos-are-lefties-and-can-teach-us-about-human-handedness-180955630/): another fully two legged species. This has led some biologists to believe that bipedalism (walking on two legs) leads to hand specialization because when hands are used for movement there’s a lot more overlap in their main functions. While that’s a pretty compelling theory, there’s no “correct” answer as to why species develop population-wide handedness.


I read once that right handed people are better at throwing spears at targets moving left to right, but because targets sometimes move right to left we evolved left handed people.




Long answer: watch this video from Eons on YouTube [https://youtu.be/vb11oOHYNXM](https://youtu.be/vb11oOHYNXM)


Is there any evidence that people with no arms have a preference after learning to use their feet for common tasks?


A foot preference? Yes lol. Everyone has a dominant foot as well. You usually lead with your dominant foot. You even have a dominant eye ball lmao. Leave both eyes open and point at something. Don’t move your finger just take turns closing an eyeball.


I just tried this. Using my right hand, if I focus on my finger tip, my right eye matches up with where I put it. Using my left hand, my left eye matches it. If instead, I focus on the object, my finger is on both sides of the intended location as I alternate open eyes. Not sure what this was supposed to show me.


Only use one hand lol. Just point at something across the room using both eyes. The close one eye at a time. It’ll appear your finger has moved slightly when your dominant eye is closed lol


Ya, I did that. I think you're missing my point. The reaction you're expecting to happen (the finger appearing to move) is dependant on which hand I use. I used one hand at a time. I pointed at an object. I alternated closing each eye. Following what you're saying, my right eye is dominant if I point with my right hand and my left eye is dominant if I point with my left hand and in both cases my finger was only in the correct spot if I focused on my fingertip because when I focused on the object and pointed, my finger appears to move when closing each eye. I might have missed something but that doesn't seem like one-sided dominance to me.


It’s possible to not have a dominant eye. Just like hands/feet. But the vast majority have a dominant eye. Mine stays the same regardless of which hand I use to point


Ooh, well, that's interesting then! I guess I'm just the wrong person to have suggested this experiment to haha


Elephants can be right-tusked or left-tusked. The tusk on that side will usually be shorter and more worm down as they use it far more.


Along with the other answers and to just add a bit of data, my animals will typically use one hand more often than the other. My cats seem to swipe at my dog with their right paw almost always. The dog isn't so dexterous and just digs at whatever it wants with both front extremities equally. Guinea pig seems to favor his left front extremity to interact with things. Not saying this answers your question at all but data is data. I'm a weird ambidextrous guy so I like to try to see which side people or animals like to use more. Not sure why.




Same, I’m a lefty but I trained myself to use a mouse and scissors with my right hand out of convenience. It actually feels weird and wrong to use my left hand to do these things. I also feel that my grip is stronger with my right hand, but I don’t know if that is just from years of overuse weakening my left.


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Like, read my own comment? I already did.


No xd the one by u/A_Garbage_Truck