How do we tell left from right?

April 6, 2022 • 12:15 pm

I’m sure this topic has been covered by scientists before, but I haven’t researched it, so I’m raising it as a naive question.

First, it’s easy for you to tell up from down because down is where your feet are and up is what you see when you look away from your feet and toward the sky. Or you could drop something; the direction it falls is “down”.

It’s also easy for you to tell your front from your back. Your front is what you see when you look down, and the other side of your body is your back.

But how do you tell right from left at any given moment?

Now of course there are a number of cues that we could use to tell right from left. The side our heartbeat is most detectable by touch is on the left (unless you have situs inversus!), I wear my watch on my left wrist and my ring on my right hand, and so on. If you drive a car in the US, the steering wheel is on the left side.

But we don’t actually use these cues. When someone tells you “turn right” when you’re asking directions, you just know which way to go.  But HOW?

Presumably we learn right from left when we’re kids: a parent presumably points out your right hand and says “that’s the right hand” and vice versa. But again, what cues do we use now? Surely not the hands! (I’m sure the answer is out there somewhere, but if a reader provides it, many of us will have learned something.)

That’s my question, but it’s related to a genetics question that I pondered for years before any answer was ever given. It’s about asymmetry in animals.  There are basically two types of ways a bilaterally  symmetrical animal can be asymmetrical in some ways. I’ve posted on this three times before (here ,here, and here), so have a look at those posts. Here’s just a brief summary.

1.) Fluctuating asymmetry. Individuals are asymmetrical for some features, but the direction of asymmetry varies from individual to individual. Handedness in humans is this way, though it has a genetic component, too, making right-handed people more common. Lobsters have asymmetrical claws: one is a “cutter” and the other a “crusher”, and it’s random whether the crusher claw is on the right or left. (We know, by the way, how this comes about. Young lobsters start their lives with identical claws, but the claw that is used most often provides more neurological activity, and that activity irrevocably creates the asymmetry, which lasts for life.The most-used one becomes the grinder.) Some species of flounders are randomly flat on the left or right sides, though all start off being vertically postured fish who develop into flat fish, with the eye on the bottom migrating to the top. Many human facial features are examples of fluctuating asymmetry: the right sides of our faces are not the same as the left, but the kind of differences differ in direction from person to person. Fluctuating asymmetry is also called “anti-symmetry” since the sides are different, but not in a consistent direction.

2.) Directional asymmetry. This is what always puzzled me. There are some basically bilaterally symmetric animals, like us, in which there are some asymmetries that are directional. That is, the right side always differs from the left in a consistent way. The narwhal tusk (a hyper-developed canine tooth) is always on the left side, some owls use directionally asymmetrical ears as a way to locate prey, I’ve mentioned the human heart before, and there are many examples. (In some flounder species, individuals are always right-flat, while individuals of other species are left-flat.)

The question I always had about this rests on the observation that because every individual is directionally asymmetrical the same way, that asymmetry must somehow rest on genes for those traits that are active in development. But how does a gene know it’s on the right or left side so it can turn on or off? Given a bilaterally symmetrical individual, it’s easy to genetically specify “front” and “back”, and “up and down”, but once those are specified, then the internal features of the organism should be identical on the right and left side. So how does a gene for say, hyper-development of the canine tooth “know” that it’s on the left side to become activated? There has to be some consistent physiological or metabolic difference between the right and left sides of an animal to provide the relevant developmental cues.  But how could that occur?

We’re beginning to find out now, though we’re far from a complete understanding of the phenomenon. There are two suggestions I know of, based on either the asymmetry in the way embryonic cilia beat (causing an asymmetry in the flow of embryonic fluid) or in the “handedness” of our constituent amino acids. I describe these in the second post I wrote in the series.

Of course, once a single directional asymmetry has evolved in an animal or plant, then the evolution of further directional asymmetries can evolve using developmental cues provided by the first one.

But this is irrelevant to the question above, so I repeat it:

How do you know the difference between left and right?

111 thoughts on “How do we tell left from right?

  1. Oh, man, did I ever misunderstand the title of the post! I thought you were going to ask “how can you tell the right-wing crazies from the (ostensibly) left-wing woke loons?” After all, given that both sides are anti-science, anti-free speech, and anti-Semitic, it can be fairly difficult to tell them apart.

    But, reading the post, I see that PCC(E) is asking about the actual directions right and left. I’m not clear if it’s a general question, or rather how do specific individuals do so. I can answer the latter part. When I was very young, I actually had a lot of trouble with this. Around age 8, when I became a baseball fan (still am!), it all became very easy to me–just held out my arms at right angles to each other, and my left arm was pointing down the left-field line, while my right arm was pointing down the right-field line. I’m only a little bit embarrassed to add that mumblety-mumble years later, this is still the method I use.

    As for the genetic issue, I’d be interested in reading what the experts have to say.

      1. Yes, the field is symmetrical, but by convention (or possibly by some earlier determination for “right” and “left”), the line crossing over the third-base bag is the “left” field line, and the guy standing out there nearest to it is the “left” fielder. I would guess this comes from right- and left-handedness as you look out from home plate. In other words, I (personally) know that it’s the left-field line as I’ve absorbed standard baseball terminology, and hence I know my left hand, as it’s the one pointing to said left-field line. Or am I misunderstanding your question (again!)?

        1. I think there’s something to what you say, Mark. When I’m in a situation where I need instantaneously chose left or right — as in steering a vessel to port or starboard — I immediately think glove hand vs. throwing arm.

          That may be the reason I’ve always been wary of getting in tight spot while at the helm of a boat with a tiller rather than a wheel — since to steer to port you have to push the tiller to starboard (and vice versa), which runs contrary to one’s instincts.

          1. You might have heard this one, Ken. A famous admiral died, who had always been known for his coolness under pressure. When in a tight spot, he would take a small card out of his pocket and read it. People wondered what words of comfort or encouragement he consulted. As he was being laid out in his best uniform, his aide took the card from his pocket and read, “Port – Left. Starboard – right.” (Then there were the Civil War recruits who had to be taught hay-foot/straw-foot because they didn’t know right from left.)

            1. I distinctly remember my first captain of my U.S. Navy ship emphasizing, “Red, Right, Returning” with reference to the red buoys on the right/starboard side of the harbor channel when ENTERING a harbor, remembering that the vessel’s starboard running light is green. I paid close attention.

        2. I find it fascinating that in baseball right-handed batters are situated on the same side of home plate as the left field line, similarly for left-handed batters and the right field line.

    1. I was confused too, especially because we now have both Left and Right making excuses for Putin and blaming the US for Putin’s war on Ukraine. I wish someone would explains THIS to me! My best guess is that the Left has been outshouted in the media by Covid and cancel culture, and Class Warfare is not longer a topic of interest. So they need to hammer away at American capitalist “imperialism” and hope enough people will jump aboard to start a new revolution. Oh, I am a born lefty but do many things with my right hand only, such as throwing a ball. But I could never bowl or play tennis because I would just stand there and shift the racket or ball from hand to hand to see which was comfortable. I could never decide. I think all lefties are somewhat ambidextrous, by the way.

      1. “I was confused too, especially because we now have both Left and Right making excuses for Putin and blaming the US for Putin’s war on Ukraine. I wish someone would explains THIS to me! ”

        How about the eastward expansion of NATO during the last two decades, the Russians constantly complaining about it? Were there no reasonably predictable consequences? The dissolution of the Soviet Union apparently was not sufficiently satisfying. Regarding foreign powers attempting to establish alliances in Latin America, what is your position on the Monroe Doctrine/(Teddy) Roosevelt “Codicil”? The MSM either doesn’t want to talk about that aspect and/or perfunctorily dismisses it without explanation/justification.

    1. I was just going to write the same thing. Are you left-handed too? I wonder if it’s more difficult for lefties who grew up before special training in school was available for how to do things left-handed. Some things I had to learn with my right hand, some I learned by myself with my left, and some with my right hand and then had to transfer that to the left. I wonder if all of that caused some deep and lasting confusion. Or it’s also very likely that I’m just lacking in some directional ability that most people have.

  2. Well not everyone is able to decide on left or right without thinking about it. Perhaps the most famous person in that class was Richard Feynman. No intellectual slouch, but he always kept his loose change in one pocket so he knew right from left.

      1. I bet the cash registers were on the right – HIS right – so … yes? Easier to hand over the coins…

    1. Feynman must have been ambidextrous. I always thought everybody did it that way. I believe I’d have trouble if I woke up some morning miraculously ambidextrous and with arm muscles changed to be symmetrical. I do seem to be strongly right-handed. Maybe people where it’s less extreme don’t do the evaluation it that way. Some pro tennis males have a very pronounced muscular difference in the arms.

  3. Thanks, PCC(e), this is a very interesting question. Could it be that left/right directionality is somehow related to other axes in development? After all, even symmetric organisms have directionalities of other kinds, namely anterior/posterior and dorsal/ventral. ???

  4. I think it’s instructive that up/down and front/back are so much more instinctive than left/right — the first I suppose because of the all-pervasiveness of the gravitational field we live in and the second because of the dominance of our sense of sight (pointing only forwards) over all others. Left/right is, by comparison, arbitrary, and just has to be learnt, but I still frequently say the wrong word in a way I don’t for the other two axes. Handedness must play a part in cementing the idea of which is which, but that’s a relatively weak preference compared to gravity or the direction of sight.

  5. I’m having trouble understanding the problem. Is the question really about how an organism can develop from being, say, perfectly bilaterally symmetric to being asymmetric? Are there really any perfectly bilaterally symmetric creatures? Seems likely that all eggs start off with asymmetries. Any process that starts off with asymmetry should continue to be asymmetric. They can still be mostly symmetric but never totally.

    As far as right vs left is concerned, perhaps it is something that was decided billions of years ago and is just passed on from one creature to another. If the egg has handedness, why wouldn’t that be passed along? Although its probably an experiment that is impossible to perform, it would be interesting to rearrange the contents of a fertilized egg, swapping left with right, and see if develops into a creature with opposite handedness. And, if it did, would its offspring continue with the same?

    1. I think it’s more: how do the genes know to locate the heart slightly left of center? Why not slightly right of center, or perfectly centered? Likewise for the appendix. It’s always on the same side of the body. Why? How does it know that it should be on that side of the body?

      1. Once there’s asymmetry, which is present in all organisms, the rest is easy. There are no processes that will turn asymmetry into symmetry. Once there’s an imbalance, it is easy for processes to continue that imbalance. Even snowflakes are not actually symmetric, however much processes try to make them.

  6. “Pooh looked at his two paws. He knew that one of them was the right, and he knew that when you had decided which one of them was the right, then the other one was the left, but he never could remember how to begin.”

    A. A. Milne
    The House At Pooh Corner
    Chapter Seven in which Tigger is Unbounded
    Dutton Children’s Books, a division of Penguin Young Readers Group

  7. I can only address the question from my point of view. I’m autistic, so this may not be generally applicable, but when need to confront a left-right decision while driving etc. I mentally refer back to an injury I received when I was about a 5 year old. It was to my right hand so I feel “right” through this hand whenever I’m conscious of needing to use a right-left decision.

  8. Just speculation, animal brains seem to create a model of the world in order to be able to move through it. Visual systems and vestibular systems (and others no doubt) are part of the input and together with proprioceptors throughout the body enable our nervous systems to create a model of the environment and our body’s position and movement through that environment.

    That would seem to necessarily include left side/right side as well as up/down and front/back. It seems like this capability would be very old, in evolutionary terms, and maybe it evolved many different times too. Even jumping spiders can compose a mental map of a path to a distant prey, involving multiple changes in direction, and then execute it even when following the path means they can’t see the prey any more.

    Perhaps our brains “calculate” left side/right side from the “known” up/down and front/back data that comes directly from sensory input (visual and vestibular systems).

  9. When receiving a reference to my right, in my head I do a mental reference to my dominant (right) hand and then go from there. Oddly, a left-handed person would need to do the opposite if they were given the same task.
    Left, right, up, down … I presume we direct our attention to these things by first referencing our bodies left right up and down. Its like we are using our corporal selves as a basic reference system.

    1. People often have dominant eyes, dominant hands, and dominant feet (not always on the same side) and these are typically used first for various actions and behaviours. That gives you plenty of body signals to generate ‘handedness’ from. Perhaps you learn the cultural definition of direction and also learn to associate it with your ‘handedness’ or ‘anti-handedness’?

      1. Plus people build ‘maps’ in the brain (grid cells) which correspond to geography in the external world, so I guess ‘left’ and ‘right’ – as directions to turn – are learned.

        See: Tor Stensola, Hanne Stensola, May-Britt Moser, Edvard I. Moser. Shearing-induced asymmetry in entorhinal grid cells. Nature, 2015; 518 (7538): 207 DOI: 10.1038/nature14151

      2. As my brother and I were left-handed, growing up it was continually pointed out to us that this was “unusual,” so it stuck.

  10. For me, this has always been difficult. So when I was a child I created a “muscle memory”: When somepone says “right”, my right arm automatically contracts. Then that is the direction I go.

      1. Left & right is not easy for me:

        To consciously label left and right I have to imagine my “jab-hand” (I “did” karate as a youngster).

        In traffic I unconsciously distinguish left and right (as long as I don’t have to explicitly label them).

        In kindergarten I already could write, but my writing was a mix between normal letters & mirror-versions (I’m 50 now, but in the attic of my mom still some of my kindergarten-paintings exist)

        I started ambidextrous. I could write, box (change of “jab-hand”) and play badminton with both hands. With puberty (hormones?) my abilities with the left hand started detoriating and I am almost completely right handed now.
        My left hand is still much stronger than my right and I use cutlery in a right handed style (or was it opposite around?) interrupted only, when I use a knife.

        1. one addition:
          My son (5 years) has similar “symptoms”, Maybe there is a genetic component to it?!

        2. Interesting what you say about your left hand being stronger. My daughter and I are both left-handed. She mentioned that she uses her left hand when she needs to be precise, but uses her right hand when she needs a power grip. I realized I do the same. I don’t know if that is typical for most people or not.

  11. I don’t think we need cues, we just have the knowledge that “right” refers to one direction and “left” to the other. Isn’t it the same as how we know the difference between red and blue? We’ve just learned that when people refer to red they mean the color that looks the way that we learned red looks. We don’t need a “cue” to remind us. Or maybe I’m misunderstanding the issue?

    On the other hand (!), the mystery of how a gene “knows” it’s in the left or right side of the body seems valid. I seem to remember this question is answered, or at least addressed, in Shubin’s _Your Inner Fish_.

    1. Yes, this, and it’s so obvious that I wonder if I’ve understood the question correctly. What Aneris said in reply 5 and darrelle said in #10 are parts of the answer. More basic: there are different neurons that connect to the left versus right ear, and to the left and right sides of each visual field, in addition to the muscles as you mention. Of course an animal brain is going to learn these differences! They’re important, predictable, and highly accessible.

      Now, learning your language’s LABELS for these things might be hard, for human children, and maybe some adults. But that’s a whole ‘nother kettle of Inner Fish.

  12. How do you know the difference between left and right?

    Remembering back to the long distant days of my childhood, I seem to recall that I had a lot of difficulty working out which was left and which was right. But, by now, I seem to have gotten it.

  13. I think it was Feynman in one of his books who asked, if we were in contact with aliens many light years away, how could we tell them which was left and which was right. His answer (and I am not a physicist so I am a bit sketchy on the specifics) involved a radioactive element which emitted a particle which always spun clockwise (or counterclockwise?) as it approached you. I doubt that has anything to do with answering Jerry’s question.

  14. In 1964, Martin Gardner wrote an entire book on the symmetry problem. Its title is The Ambidextrous Universe: Mirror Asymmetry and Time-Reversed Worlds. It was updated in 1969, 1979, 1990 and 2005, with the later versions retitled The New Ambidextrous Universe: Symmetry and Asymmetry from Mirror Reflections to Superstrings. Wikipedia has a concise summary of its contents; the book itself is far too involved to describe in a mere comment, but it had a profound influence on my thinking.

    1. Yes! Good call! It was on my pile somewhere now….. might be where the letter about a paper bag on one hand was…

  15. I think some of this is on the genome.
    1] Aren’t there some studies that show that toddlers or perhaps even younger babies prefer something that occurs sequentially with the direction occurring from left to right?
    2] Test yourself with the idea of starting to explain something controversial to someone: Say to yourself, “On the 1 hand, some folks think this, & on the other hand, other folks think this.” And act it out with a bit of gesturing as you say it. OK? So — Which hand did you start with? I always start with the left hand — which suggests to me that the left-to-right motion is “natural” for me, & I expect for folks in general.
    3] When you are pondering something problematical, like some science question, & you lean back & look up & off into space, which direction do you look? I look up to my left, like that is where to start the thought process; then, as my thoughts proceed a bit, my gaze tends to move to the right.
    4] All the comic strips with multiple panels progress from left to right, in my limited experience. Do cartoons in Hebrew & Arabic read from right to left?
    5] Why does the batter in baseball run from his left to his right after he hits the ball?

    1. Baseball base running has been counterclockwise since early in the development of the game (about 1840-ish). The most generally-accepted explanation is that this accommodates right-handed throwing by infielders, most of whom have been right-handed. (There have been virtually no left-handed catchers or infielders other than first basemen in higher-level baseball for many, many years.) there’s an article about this, here:

  16. Brainstorming / thinking out here, as a Bear of Little Brain might :

    Chirality – the existence of mirror image pairs of separate objects – hands, molecules – described as “left” / sinister / levorotatory vs. “right” / rectus / dextrorotatory … I think that’s restricted to pairs of objects…

    The sidedness of a living body – the bilateral symmetry is … I suppose … fundamentally the same? …. but the two objects of opposite handedness are, in a living body, merged …? So the mirror plane is running through the middle axis, where the two objects meet …

    “left”, “right” are words of convenience … to grapple with the pair of related objects …

    akin to odd and even? Sets of pairs…

    … BTW Martin Gardner has a great way to help explain mirrors in one of his books, sent in by a reader : put a paper bag on one hand … I’ll have to find it to cite it ….


    Did anyone ever realize kitchens / kitchen layouts can have handedness? I didn’t until a few years ago. Undoubtedly this reinforces one’s perception.

    I love these questions (obviously)….

  17. My left hand just seems so uncoordinated compared to my right. It all happens in my mind instantly.

  18. The question was fundamental to a science fiction story I once read. I don’t remember the details. Basically, earthbound humans used an interdimensional radio to communicate with aliens. Describing up, down, front, and back was easy. The only question remaining was left and right. I think we were left with an open-ended question; technically, chirality matters in an electro-magnetic field. All electrical fields in our local vicinity behave the same way, with certain particles moving in a specific direction (I’m a bit rusty on my high school physics). There was no way of telling whether the radio aliens lived in the same universe with the same physical features.

  19. It appears to me that I’ve memorized which direction is left and which is right. That’s how I tell currently. How I came to memorize it is a different question.

  20. how does a gene know it’s on the right or left side so it can turn on or off?

    In some cases it may not need to. Human handedness, for example, is probably due to some neurological trait, not anything like muscle building. If you get the standard trait, right-handedness is the outcome – neither planned nor adaptational, it just consistently is.

    For the narwhal tusk I would initially hypothesize some sort of timing feature. IIRC, that’s why our fingers are different lengths – the ‘finger growing’ gene package is the same for all eight non-thumbs, it’s just turned on (and then off) for different durations depending on the finger. Something similar could be going on for the narwhal. Tooth 1: turn tooth-growing genes on for time t. Tooth 2: turn tooth-growing genes on for time t. Et cetera until…tooth 40 (or whatever number it is in development), turn tooth-growing genes on for time 1,000t. Again, the gene doesn’t know if it’s on the left or right, it’s just told to turn on and off at certain times and they happen to correspond to one specific tooth site.

    1. As far as genes knowing were they are in the body, the up/down and front/back part of the puzzle were figured out earlier, but the left/right (necessary for Directional Asymmetry traits) took a bit longer to figure out the whole story. It involved lots of research over decades putting all the pieces together, from the 60s to the 2000s, if I recall correctly. All are a matter of signaling chemical gradients being established.

      In the case of left/right, going from memory, it has to do with atypical cilia in the nodal pit of the embryo. These cilia move in a rotating motion, in a certain direction, describing a cone that is tilted in a certain direction. This creates a directional flow of fluid in the pit. Vesicles of signaling compounds are released from the floor of the pit and the directional fluid flow moves them to one side of the pit, thereby creating a gradient from one side to the other that genes can respond to.

      Jerry did a long post on this research a few years ago, one of my favorites. I didn’t take the time to look but I bet it’s one of the links he provided up above.

    2. > different durations depending on the finger.

      Does that imply a fourth fundamental axis: inside-to-outside? In my understanding, the pattern still holds for polydactylic humans.

      1. The usual physicists’ 4th here would question distinguishing past from future, i.e.’ time’, of course. Godel (1940s) invented the first cosmological solution to general relativity with paths into the future which curved around into the past. One of the 3 extra conditions on Einstein GR for (Penrose and Hawking)’s proof of the unavoidable existence of black holes (1960s) was the non-existence of such curves (where you can murder your ancestor).
        That should surely win the 2022 Premier Name-Dropping League.

  21. When I was a baby I sucked my right thumb. When I learned the words for right and left I realized I could remember which was which by remembering which thumb I sucked. My brother was left-handed; I wish I’d thought to ask him how he learned the concept. What an interesting question. Time for a poll.

    1. My kindergarten teacher messed me up by teaching all the kids “it’s easy, just remember that your strong hand is your right hand.” I am left-handed. And still occasionally get it wrong.

      1. TL;DR :

        RH guitar players have a stronger left hand than their right hand.

        Guitar players have strength in the fret/fingerboard/neck hand. Right hand is the pick hand. Most guitar players use the left hand on the neck – they are “right handed”. As such, their _left_ hand (forearm muscles, really) is effectively _stronger_ than their right hand – even though they are right-handed! (Exceptions abound, I’m sure).

  22. I can remember struggling with that when I was a kid. Eventually I a bad cut on the pinky of my left hand, which then served as a cue, which I used for quite sometime. Even now, as I think about this, I can kind of feel the presence of that scar. I wonder if at some level, I still use that as a cue.

  23. Surely most people (and other organisms) just apply an external magnetic field to a flowing electric current, determine the direction of the consequent force and apply Fleming’s Left Hand Rule. Index finger points in direction of the magnetic field and thumb is the direction of the resultant force. If your middle finger/other fingers, when at right angles to the other two (i.e. thumb and index finger), is/are pointing in the direction of the current flow, THAT’s your left hand, if the reverse, it’s the right. Dead simple. Much easier than trying to use parity violation in the weak force or trying to work through vector cross products and stuff. And, as PCC(E) points out, the possibility of situs inversus makes heart position an unreliable guide, and some people are “left handed”. Of course, in the northern hemisphere, north of the tropics, you could stick a stick in the ground, wait for the sun to rise and see which direction the shadow sweeps, recalling that it goes from left to right on the opposite side of the stick from the sun. But this takes some time and doesn’t work well on cloudy days, and if you’re in the southern hemisphere it works the other way. And in the tropics it can be confusing. So Fleming’s rule is still the most reliable.

  24. A couple of thoughts/comments:

    a) I occasionally have a fleeting problem deciding how to write a ‘b’ or a ‘d’ letter. This strikes me as a similar issue, as a b is just a left-handed (or right-handed) d.

    b) Assuming some dyslexics have problems like this to a much greater degree, I wonder if they have problems telling left from right as well?

    c) The spiral in a narwhal tusk always runs counterclockwise.

    d) Perhaps a related question is the old puzzle: why is it that when you look in mirror, that to the person looking back at you up is still up, and down is still down, but left is now right and right is now left? In other words, left to right flips but up to down doesn’t.

    1. D. Put a paper bag on one hand – see if it helps explain – see Martin Gardner’s book (I still have to find this … currently distracted by Dawkins’ Modern Science Writing book!)

  25. Right and left are purely definitional (ask any mariner which side of the boat is the “right” side) and thus the concept is a learned thing. However, I think what Dr PCC(e) is wondering is how in the hell do genes know which side of the organism is right and left during development.

    I have no idea but it strikes me that once a devloping organism has been able to

    “genetically specify ‘front’ and ‘back’, and ‘up and down'”

    it necessarily then has a three dimenisonal coordinate system to work from.

    Perhaps there are features of this kind of topography that might direct “rightness” and “leftness” to a developmental gene expression, though I can’t imagine what that might be. Perhaps there are genes which determine the frontness and topness and backness and bottomness of the developing organism and genetic controls are able to work out gradients of expression along those coordinates.

    Facinating question.

    1. No, I’m less concerned with genes than telling what side is left and right. Yes, those terms are applied arbitrarily, but everybody applies them to the same way. Right and left can be specified by the asymmetries in the human body, and I still think it’s a valid question why every human knows which was is right, and it’s always the same way for everyone. A three dimensional coordinate system that is left-right symmmetrical has no way of specifying a consistent directionality. To evolve directional asymmetry, you need an initial existing directional asymmetry, which could be either amino acids isomers (ours are all “L”) or a directional current, which could be random in terms of clockwise or counterclockwise, but once it’s evolved then there’s a directionality. Top/bottom and front/back gradients do NOT make any points on the left right side different.

  26. I’m ambidextrous. As a kid it took a while to get it right or is it left. For while I looked at the inside of my right wrist where I have a scar to get right. I throw overhand right and underhand left. I write horizontally left and vertically right. (School tried to get me to write right handed so on the blackboard I wrote right and at the desk left. Most manual tasks I do with either hand – saw, hammer, screwdriver etc. depending on the situation. Driving on the left look a while to get right.
    Does all this mean I’m mixed brain dominant ? Somewhere I read that this expression is out of favor amongst neurologists.
    Right handed musicians play the violin etc using the left hand on the finger board which is the more difficult task it seems. Why is that?

    1. A jujitsu friend told me he learned left from right by holding his hand up, and spreading index finger and thumb, it formed the letter “L.” Which is about the best way to remember that I’ve ever heard. (In jujitsu, we softened grabbed cross grip, or with both hands, and it became confusing on some techniques.)

      1. Just So Stories by Kipling tells How The Alphabet Was Made, in which “L” is formed with (oddly) a broken spear.

  27. I think it is just a matter of long habituation. When giving directions, I will sometimes have to look at my hands to help me choose the right direction to say.

  28. Car keys in the pocket on the left (with other items now and then), Phone in the pocket on the right (always alone or with business cards).

    I take the phone out with my thumb and forefinger and transfer to my left hand. I am left-handed except for baseball.

  29. I’m having a bit of difficulty understanding the question.

    As far as I’m concerned, understanding left and right as the labels for relative sides of the body/directions to a location is completely a learned behavior that becomes instinctual due to repetition.

    I mean, I remember a time before I had it ingrained in me to the level of instinct, and I remember that level of instinct developing during marching drills in high school (that is, going from sometimes messing up with a split second to respond to a command, to immediately snapping to without conscious thought).

    Is there some depth of the question that I’m missing? Is this not the same for others?

    1. I stated it differently above, but I agree with this completely, and similarly wonder what I’m missing.

  30. Honestly when I read the caption for this, I thought you were talking about literally looking at your hands and the shape of your thumb and your index finger and if it forms the shape of the Latin letter “L” that’s your left hand

  31. The vast majority of snails (my specialty as a paleontologist in an earlier life) exhibit clockwise (termed “dextral” or “right-handed”) coiling, meaning that as they grow they coil in a clockwise manner down a vertical axis. Sinistral snails exist, but they are not all that common. Almost all species are dextral; only a few are sinistral. Usually, all the individuals in a species manifest the same handedness but, occasionally, an individual of opposite handedness will turn up. I’m not aware of any species where the individuals are not biased almost entirely to the right (the vast majority) or to the left (a handful). Maybe one exists somewhere, but such species must be extraordinarily rare.

    Why left-handedness (sinistrality) is rare in snails has been a puzzle forever. Every snail specialist knows about it and most have probably wondered about it. I don’t know if anyone has figured out why snails are configured this way—with such a strong bias toward right-handedness (dextrality). I have been out of the field for 25 years now. Perhaps someone in the meantime has figured out the answer. If so, do tell!

    1. I’ve had a RWP I wanted to send in on some gastropod … conch … shells with handedness that’s been stagnant for years … basically a neat picture of the symmetry … interesting…

    2. There was a fossil snail reported from cave deposits that existed in equal numbers of sinistral and dextral individuals; turned out they were actually fossilised bat cochleae!

  32. Written English is read left to right – our left to our right.

    But to the words, if is amusing to think, from their view, they are read their right to their left.

  33. A separate but tangentially-related question:
    Cover one eye and look in the mirror. You probably see your reflection as “backwards”, i.e., with left and right reversed, but probably not as “upside-down”, i.e., with top and bottom reversed. Why?
    The solution does not require you to tilt your head.
    If you think about it carefully, you can see your reflection as “wrong” across any axis.
    It might help to think about cameras or clones.

  34. “When someone tells you “turn right” when you’re asking directions, you just know which way to go. But HOW?”

    Is it a Gestalt thing – a synthesis…

  35. I think we learn the labels left and right when we are about a year old, at the time we get dressed in things with sleeves.

    Mum to baby “right arm dear”
    baby presents left arm
    “OTHER right arm dear”
    baby presents right arm
    “aren’t you a clever boy/girl”
    “left arm dear”
    baby presents left arm (has no choice)
    “good left arm”
    little kiss on left hand

    Quite different from developmental asymmetry and from handedness.

  36. In graduate school I had a wonderful professor who had distinct problems with right and left. She was naturally left-handed, but was in elementary school in Austria in the late 1930s, where that was not accepted. Both her parents and her teachers forced her to use her right hand, but she was so dominantly left-handed that she could never write legibly. Another consequence was that, when someone told her to go to the left or right (or similar things) she experienced a sort of mental wall and rarely was able to carry out the command (that was when I knew her–when she was in her 40s).

    1. I wish I had read this before I commented above. This is exactly what I was wondering about with left-handlers who struggled as children to learn how to do tasks or were forced to do them right-handed. It is a lesson to me to read before I write. Was her hand-writing still bad in her 40s?

      1. Her hand-writing was barely legible throughout her life. The last written contact I had from her was when she was over 60, and it was still the same as it had been at 40.

  37. Notwithstanding the difficulty some people have with telling right from left, I would not be surprised if electromagnetism has something do with how genes know which side that asymmetric features develop on.

    Run an electric current through a wire, and the resulting magnetic field has a fixed direction based on the direction of current flow (given by the right hand rule). Since the body is filled with electrochemical signals, there will be small magnetic fields set up in the body. Perhaps these influence gene expression somehow.

  38. I have difficulty telling my left from my right. It’s the words themselves that give me trouble. I have an excellent sense of direction and know how to get places, but I will often say “turn left” when I mean turn right, and vice versa. I was able to drive comfortably in England when I was there. I read an article about other people who have this issue, and there was even an online test to determine how much you have it, but I can’t find the link right now. I don’t experience it as a disability, just a funny goof that happens from time to time.

  39. During my earlier life, while on a sabbatic year or numerous other shorter visits to Britain, I sometimes
    had occasion to drive. Switching to the instincts of driving on the left of course presented a problem to my own neural network. [I solved the problem by simply driving on both sides of the road. This worked so well that I have carried it over to driving in the US.] I have to confess that my brain never really assimilated the left-side driving system, with this result: whenever I used to visit the UK, at first I would
    invariably get on buses going the wrong way.

    1. The scariest times are (a) driving on the left down an empty road and then suddenly coming on to traffic and (b) walking off a curb on a busy street and looking to the left instead of the right. I’ve survived those episodes but they always leave me thinking how easy it is to die.

  40. Brainstorming here :

    Brain hemispheres : left / right
    Eyes : left / right

    … then there’s the inversion of the image on the retina…

    Those must be important for how instantaneous perception interacts with the trained brain…

  41. Occasionally when interacting with K-5 students, I will ask them to raise their right (sometimes left) hand. As they and I are facing each other in opposite directions, at least a few will raise their left (right) hands when I raise my right (left), raising the hand on the same side that I raise mine. It takes them several seconds to apprehend what is going on.

    Sometimes I will swing my arm in a (counter-) clockwise direction while turning around, my arm never changing direction, to drive home the point that whether a motion is clockwise or counter-clockwise depends on from what side the viewer is viewing the motion. Some years ago I read an article or book stating that the rotational motion of the Milky Way galaxy was counter-clockwise (or was it clockwise?). I reflected how the author possibly knew that. It’s strictly arbitrary. Sometimes I ask students to pretend that they are from a galaxy far, far away and they have never seen the Earth, and whether they could possibly know which was the North (or South) Pole and in which direction the Earth was rotating on its axis. I also remind them that they could easily rotate their spaceship a half-turn and observe the result.

    Am also reminded of Dupont chemist Wallace Carothers. The most distinct memory I have of college organic chemistry is when the professor (I remember his name) went into significant detail describing the experiment Carothers designed to conclusively demonstrate how to determine whether a given molecule was “levo” or “dextro,” both chemically identical. I wish I could more clearly remember the details. Seemed like the experiment involved a plate perforated with very tiny and numerous holes. Will try again to search the internet to pin down the particulars. (I have a vague recollection of something about the rotational characteristics of molecules in polarized light, but I feel the ice of understanding thinning under my feet. I need some refresher training on the subject.) I recall some years ago reading in the NY Times a letter by a biographer of Carothers. So much to read, so little time.

    1. Remember

      Pasteur separated tartaric acid crystals by hand to show there were L and D forms (and no others).

      I’d like to hear about this plate though – that’s how distillation efficiency can be calculated – “theoretical plates”… so I’m curious…

    1. Very interesting

      I always wondered if the origin of symmetry would lie with radiation impinging on aromatic molecules to catalyze electrocyclic reactions in the “primordial soup”, now understood to be governed by the Woodward-Hoffman rules – thus producing suprafacial or antarafacial reaction products … and also iron-containing zeolites holding organic molecules to react with sulfur. Gunther Wachterhauser argued along those lines – zeolites, Fe, S – for the origin of life.

  42. This is interesting stuff. As far as genetics goes, I am not sure we should assume that symmetry is the default, although it does seem to be mostly the case in creatures that are not plants, at least externally.
    As far as perception of left and right is concerned, It took me a long time to get it down without having to think about it. Military school sorted that out for me very quickly.
    I don’t like that left and right is perspective based. I am restoring a car, and I needed a headlight. The seller had one listed as “left”, when “driver’s side” might have been more specific.
    “Stage Left” is another one.
    I much prefer the maritime system, which was developed to avoid ambiguity. If the lookout tells me that he thinks he sees a light “two points forward of the starboard beam”, I know exactly where to look, and the direction either of us is facing is irrelevant.

  43. Fun fact :

    Car gas gauges have a triangle pointing where the gas cap is. Look for the tiny icon of the gas pump.

  44. “Individuals are asymmetrical for some features, but the direction of asymmetry varies from individual to individual. Handedness in humans is this way, though it has a genetic component, too, making right-handed people more common. ”

    Forgive me if this is … you know :

    I think the precise term for the handedness is not “asymmetry” but “chiral”, as it describes a _pair_ of objects related _by_symmetry_.

  45. Left and right are learned concepts, and if this is insufficiently done with very young children, people keep confusing the directions. Nothing innate about them.

    A counter exemple to left/ right shows shese must be learned concepts. Interestingly, some languages don’t use the concepts left and right (and front back etc), but use absolute coordinates:

    Absolute coordinates must be more usefull in a flat landscape with few distinctive marks.

  46. How do I tell left from right? I don’t always do actually.
    My girlfriend knows that when she asks for directions I often will tell her to take a left, while I mean right. I have learned to point to the direction I mean to say.

    1. I once had a job where one of my occasional duties was to train drivers on routes for newspaper dropoffs. On such occasions, I always let the other person drive, as it was a better learning experience for them than just riding along. Whenever I was asked, “Right turn?” or “Left turn?”, I quickly learned to say “yes”, “correct” or ”no”, because saying “right” – meaning “correct” – was too confusing. I became very skilled at giving and following directions as a result. I considered it a test of my eloquence and spatial sense to be able to give directions well while keeping my hands in my pockets.

  47. I know a set of identical twins who are mirror images (at least in the differences I can see). Each has a birthmark on her neck, but one has it on the right side and one on the left (otherwise the birthmarks look the same and are in the same place). And one is right-handed and the other left-handed. Genes? Position in the womb? Due to the way they initially separated when they were a handful of cells?

  48. Wow! This thread goes on & on!
    Kind of a “Welcome back, Jerry”?

    I want to add to my previous comment [#17 above].
    It seems to me that the left-right conundrum is an artifact of Civilization.
    To wit — I suspect our evolution did not prepare us for the noxious nexus of uncertainties we are discussing here.
    Something that pops up when I think about something long enuf is —
    Was it ADAPTIVE in our evolution to have concepts of right & left?
    Back in the forest as Australopithecus — climbing the trees & vines — did we have to think in such terms?
    I dont think so.
    Later, on the savanna as Homo, with language, did we need to discuss left & right?
    Again — I think it was a seldom-needed attribute.
    Hand signals — waving left or right — probably served.
    Up & down, forward & backward, just seem instinctive.
    When did we start needing to communicate about before & after?
    Like, the past & the future?

    And how about the 4 cardinal directions?
    Are they in our DNA — like the innate sense of direction some of us have?
    Surely, we had instincts, — & later words — for north, south, east & west early on.
    Maybe we solved some puzzles or problems by saying to another member of our clan — “Go west long way to big cliff, then go north little bit to hippo pool.”

    I was a platoon leader in ROTC, & a few of my guys had an issue of how to handle their feet to do an “about face” so that their 2 feet ended up side by side.
    But nobody as I recall had any issue with marching.
    And nobody had to resort to taping hay to their left shoe & straw to the right.

  49. I have a distinct memory of being taught about left-vs.-right from my grandfather when I was very young. This was in reference to either shoes or crossing the street (possibly both), and I think I basically ended up just memorizing which was which. I’m not aware of using any mnemonics or cues to identify left vs. right, but there are definitely instances where I have to think for a moment to remember which is which, so I do kind of wonder what my brain is sifting through when that happens.

  50. I think one must consider as part of the answer the sound of the English words themselves :


    I think the words themselves are paired up like this :

    [first sound]-[middle sound]-[closing sound]

    they each have a similar sound — there’s no big distinguishing feature like “zzz” vs. “ooo”, and they close on a crisp “t”.

    So it is about small details. My own impression is that “right” has a crisper snap to it. “left” has the “f”, sounding softer to me than the counterpart … what… “gh”?… or is it “eff” vs. “igh”?

    … I’m stuck now – being not a linguist.

    But the associations would form around those sounds. Perhaps other language speakers can add to that.

  51. ” “that’s the right hand” and vice versa. But again, what cues do we use now? Surely not the hands! ”

    Why not the hands? They are chiral, which means that they are symmetrical specular but not identical, and cannot be overlapped. Chirality is a concept very used in chemistry because many organic molecules are chiral. You can overlap them only if you flip them as in a mirror, otherwise you cannot.
    In order to distinguish a molecule with a given chirality from the specular molecule (namely equal but with an opposite chirality) you need the concept of right/left or counter/clockwise.

    For us, left is the the sense of the rotation needed to see the left hand. Eyes are only on the front of our body not on the back, so we can see our left hand only if we rotate your eyes toward that directions not in the other (at least when the hand is in a rest position besides the body).

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