Handedness: Genes, Brain, Evolution & Left vs Right
This podcast details handedness (e.g., left-, right-, mixed-, ambidextrous), exploring its genetic roots (multifactorial, not single-gene) and brain lateralization (left hemisphere dominance for language, right hand control). It covers evolutionary history from the Stone Age, fetal development, prenatal hormones, cultural biases, and links to cognitive abilities, mental health (ADHD, schizophrenia), and sports advantages.
0.000000 4.440000 Welcome to everyday explained, your daily 20-minute dive into the fascinating house and
4.440000 6.320000 wise of the world around you.
6.320000 10.160000 I'm your host, Chris, and I'm excited to help you discover something new.
10.160000 11.160000 Let's get started.
11.160000 14.840000 Ever pick up your coffee mug or maybe scroll on your phone without even thinking which
14.840000 16.120000 hand you're using?
16.120000 18.440000 Most of us definitely have a dominant hand, right?
18.440000 21.480000 But have you ever actually stopped to wonder why?
21.480000 24.520000 Why are some people right-handed and others left-handed?
24.520000 28.120000 And why do, well, most people seem to favor the right.
28.120000 33.640000 Today, we're doing a really fascinating deep dive into this whole mystery of human-handedness.
33.640000 36.960000 We've gone through a bunch of your sources, you know, articles, research papers, notes,
36.960000 40.760000 trying to pull out the key nuggets of knowledge, and maybe find some surprises.
40.760000 44.320000 Our mission basically is to unpack what's going on behind the scenes, what happens during
44.320000 48.680000 development, how much is actually in our genes, and yeah, why does the right hand seem
48.680000 50.360000 to rule the world?
50.360000 54.840000 We're aiming for those aha moments, you know, without drowning you in data.
54.840000 56.640000 Maybe share a laugh or two as well.
56.640000 58.080000 So let's get into it.
58.080000 60.320000 Just how common is right-handedness, really.
60.320000 62.640000 Well, the number is paint a pretty stark picture.
62.640000 67.760000 If you look at Western countries, it's about 90% right-handed, which leaves only roughly
67.760000 70.000000 10% is left-handed.
70.000000 74.160000 And what's really wild is that this isn't like a new thing.
74.160000 76.880000 Researchers think this ratio has been pretty stable since the Stone Age.
76.880000 77.880000 Since the Stone Age?
77.880000 79.400000 Wow, how do they even know that?
79.400000 80.960000 It's pretty clever, actually.
80.960000 82.120000 They look at ancient skeletons.
82.120000 86.200000 You can see differences in the arm bones, like asymmetrical wear, that suggests one
86.200000 88.520000 arm was used more dominantly.
88.520000 91.240000 And they even study prehistoric tools.
91.240000 95.440000 Looking at the wear patterns to figure out if the maker was consistently using their right
95.440000 96.440000 or left hand.
96.440000 98.640000 That's amazing forensic work, basically.
98.640000 101.480000 Yeah, it's a subtle clue, but tells us a lot.
101.480000 105.880000 And just to add, people who are mixed-handed, you know, using different hands for different
105.880000 109.000000 things are truly ambidextrous with equal skill.
109.000000 110.000000 Mm-hmm.
110.000000 111.000000 They're actually quite rare.
111.000000 114.560000 Okay, so this preference is ancient, practically baked into us.
114.560000 115.560000 But when does it start?
115.560000 118.280000 Is it something we're born with or does it develop later?
118.280000 123.600000 The evidence really points towards it being hardwired, probably before birth.
123.600000 128.840000 We know from ultrasound studies that fetuses start moving their arms really early, maybe
128.840000 131.440000 nine or ten weeks into gestation.
131.440000 134.240000 And get this by early in the second trimester.
134.240000 138.760000 You can often see babies in the womb showing a clear preference for sucking one thumb over
138.760000 139.760000 the other.
139.760000 142.320000 Seriously, they're picking favorites in the womb already.
142.320000 143.320000 That's incredible.
143.320000 144.800000 It really is.
144.800000 149.400000 But despite that super-really sign, most developmental folks will tell you, you probably won't get a really
149.400000 153.840000 solid sense of your kid's dominant hand until they're maybe two or three years old.
153.840000 154.840000 Okay.
154.840000 155.840000 Why the delay then?
155.840000 158.920000 Well, lots of young kids keep switching hands for different tasks.
158.920000 163.640000 It's like they're exploring, testing things out, refining those motor skills before they
163.640000 165.120000 really settle on a preference.
165.120000 166.120000 Right.
166.120000 167.120000 Makes sense.
167.120000 168.120000 An explorer, Cory, face.
168.120000 169.920000 But okay, this brings us to the big question.
169.920000 170.920000 Yeah.
170.920000 171.920000 What about genes?
171.920000 175.440000 How much handedness written in our DNA?
175.440000 176.440000 The genetics.
176.440000 179.920000 That's where it gets really interesting and maybe a bit complicated.
179.920000 181.900000 So yes, genes definitely play a role.
181.900000 185.760000 But handedness is actually considered one of the least inherited traits we have.
185.760000 186.760000 Least inherited.
186.760000 187.760000 Really?
187.760000 188.760000 How much influence are we talking?
188.760000 193.520000 Genetic seems to account for only about 25% of the variation, which is, you know, surprisingly
193.520000 196.120000 low for something that feels so fundamental.
196.120000 197.120000 25%.
197.120000 198.120000 Wow.
198.120000 202.320000 All those theories about a single lefty gene or righty gene.
202.320000 203.320000 Yeah.
203.320000 204.840000 Those single gene ideas haven't really panned out.
204.840000 210.760000 The thinking now is that it's a multifactorial trait, meaning it's determined by like a whole
210.760000 214.160000 bunch of genetic factors and environmental factors working together.
214.160000 217.520000 Some researchers even think there could be at least 40 genes involved.
217.520000 218.640000 It's a complex puzzle.
218.640000 219.640000 Wait.
219.640000 220.640000 40 genes.
220.640000 224.960000 So it's not like a genetic lottery where, you know, two lefty parents means a lefty kid.
224.960000 228.800000 It's more like a biological coin flip that sometimes just lands randomly.
228.800000 230.320000 My genes are basically saying flip a coin.
230.320000 231.320000 I don't really care.
231.320000 233.480000 That's actually a pretty good way to put it.
233.480000 239.160000 One popular theory involves something like a D gene and a C gene, D for dextral, promoting
239.160000 243.160000 right handedness, which is more common, and C stands for chance.
243.160000 246.760000 It basically allows for a random 50/50 outcome.
246.760000 251.560000 The key idea is that left handedness often pops up not because of a specific left hand
251.560000 256.160000 gene, but because there isn't that strong genetic push towards the right hand.
256.160000 259.720000 So it's the lack of a bias rather than a bias towards the left?
259.720000 260.720000 Exactly.
260.720000 264.560000 And that element of chance is probably why left handedness persists, you know, why it hasn't
264.560000 268.040000 just vanished over time, despite being less common.
268.040000 272.760000 And scientists have identified some actual genetic variance linked to handedness.
272.760000 276.680000 Interestingly, some of these genes are involved in how brain cells develop, and also in
276.680000 279.240000 the brain's plasticity its ability to adapt.
279.240000 283.640000 Okay, so genes contribute maybe nudge things one way or another, or sometimes just leave
283.640000 286.240000 it a chance, but it's not the whole story.
286.240000 289.640000 What else is going on then, especially like really early on in development?
289.640000 290.640000 Right.
290.640000 295.320000 So beyond the genes, there are these fascinating environmental factors, especially in utero.
295.320000 300.160000 One idea though it's still debated involves prenatal testosterone levels.
300.160000 305.040000 The hypothesis is that higher levels might subtly disrupt how the brain develops, potentially
305.040000 309.200000 affecting the left handness fear more because it matures a bit later.
309.200000 313.880000 This could, theoretically, increase the chance of left handedness more weaker lateralization
313.880000 314.880000 overall.
314.880000 315.880000 Hmm, interesting.
315.880000 319.080000 Hormones in the womb potentially playing a role.
319.080000 320.080000 What about the birth itself?
320.080000 321.600000 Could that have an impact?
321.600000 322.600000 That's another theory.
322.600000 324.520000 Some researchers look at birth stress.
324.520000 328.560000 The idea is that maybe some kind of neurological issue affecting the left hemisphere around
328.560000 333.520000 birth like temporary, oxygen deficiency during a difficult delivery could cause a shift
333.520000 335.440000 in hand preference.
335.440000 340.320000 Now some studies do show a slight link between birth stress and more left handers, but honestly
340.320000 343.760000 the evidence isn't consistently strong on that being a direct cause.
343.760000 347.320000 It's also interesting that twins have a higher rate of left handedness, maybe due to their
347.320000 349.720000 unique conditions in the womb like being more crowded.
349.720000 351.120000 Yeah, that makes sense.
351.120000 355.480000 And even things like low birth weight show some association with a higher chance of being
355.480000 359.760000 left handed, possibly linked to subtle differences in early brain development.
359.760000 360.760000 Okay, wow.
360.760000 365.040000 And you mentioned something truly wild earlier about body symmetry.
365.040000 366.040000 Oh, right.
366.040000 370.560000 Yeah, some of the genetic material influencing handedness also seems to guide our overall body
370.560000 377.360000 plan, our symmetry disturbances can rarely lead to sedus in versus where internal organs
377.360000 380.280000 are flipped hard on the right, for instance.
380.280000 384.760000 But interestingly, that condition itself doesn't seem to make someone more likely to be
384.760000 389.280000 left handed, suggests there are other genetic factors compensating, keeping the hand preference
389.280000 390.280000 sort of separate.
390.280000 394.120000 Man, so many hidden factors pushing and pulling before we even, you know, pick up our first
394.120000 395.120000 crayon.
395.120000 399.120000 It's like this constant balancing act inside, even if your organs are on the wrong side.
399.120000 400.120000 Exactly.
400.120000 403.440000 And all this connects directly to how our brains are organized.
403.440000 407.000000 Handedness is deeply tied to brain lateralization.
407.000000 411.320000 That just means differences in structure or function between the brain's left and right
411.320000 412.640000 sides.
412.640000 416.880000 For most varieties, language is heavily based in the left hemisphere, which controls the
416.880000 418.560000 right side of the body.
418.560000 419.560000 Pretty straightforward.
419.560000 421.920000 Okay, that part I think I knew a left brain right hand.
421.920000 422.920000 Right.
422.920000 424.360000 It's a twist for left handers.
424.360000 426.400000 It's not just a mirror image.
426.400000 432.560000 Only about, say, 25-30% of lefties show strong language dominance in their right hemisphere.
432.560000 434.640000 Wait, only a quarter, so the rest...
434.640000 439.680000 The majority, maybe 60% or so, are still left brain dominant for language, just like right
439.680000 440.680000 handers.
440.680000 445.480000 And then maybe 10% show what's called by hemispheric language, using both sides more equally for language
445.480000 447.000000 processing.
447.000000 451.280000 So bottom line, left handers tend to have less overall brain lateralization, less specialization
451.280000 452.280000 on one side.
452.280000 455.960000 Definitely not as simple as righty left brain, lefty right brain, my mind is kind of blown
455.960000 456.960000 here.
456.960000 459.200000 That shows incredible brain flexibility, doesn't it?
459.200000 460.600000 It really does.
460.600000 464.160000 And understanding this variation is actually really important practically.
464.160000 468.560000 It helps researchers develop better treatments for things like stroke or epilepsy when a specific
468.560000 473.840000 brain area is damaged, knowing how functions might be distributed differently is key.
473.840000 477.640000 Plus, some studies hint that lefties might be better at tasks requiring coordination
477.640000 482.920000 between both hands. They might even have a slightly larger corpus colosum that's the bridge
482.920000 484.760000 connecting the two brain halves.
484.760000 491.360000 A bigger bridge between brain sides, could that mean advantages, like in thinking or memory?
491.360000 492.360000 Potentially.
492.360000 497.240000 Some research links it to things like better voable fluency or memory recall, suggesting
497.240000 500.360000 maybe more efficient communication between the hemispheres.
500.360000 502.000000 But again, research is ongoing there.
502.000000 506.520000 Okay, the biology is clearly super complex and still being figured out, but what about
506.520000 511.240000 society's role? I mean, historically being a lefty wasn't exactly celebrated, was it?
511.240000 515.840000 Oh, definitely not. You're absolutely right. There's been a lot of historical and cultural
515.840000 517.240000 bias.
517.240000 521.520000 Think about the Middle Ages. The devil was often depicted as being a left-handed. That
521.520000 526.240000 tells you something. And even today, in some cultures, particularly in parts of Asia like
526.240000 530.840000 Japan and China, you see fewer left-handers than in the West.
530.840000 535.480000 This is partly due to lingering social pressure to use the right hand for things like writing
535.480000 536.480000 or eating.
536.480000 538.640000 Yeah, that pressure must have been intense.
538.640000 544.680000 It was. Even here, in the US, back in the early 1900s, some teachers and doctors actually
544.680000 549.320000 thought left-handedness was linked to mental problems. They often forced kids to switch
549.320000 551.360000 to writing with their right hand.
551.360000 553.280000 Imagine that being forced to switch.
553.280000 556.920000 My own grandma told me she was a natural lefty, but learned to write right-handed. She just
556.920000 561.520000 said it was for good measure, but yeah, it sounds like it was for good behavior back then.
561.520000 562.520000 Exactly.
562.520000 568.280000 So culture can influence hand preference, especially for specific tasks like writing, but influencing
568.280000 574.360000 writing doesn't necessarily change someone's preference for, say, throwing a ball. It's tasks
574.360000 576.200000 specific sometimes.
576.200000 581.200000 The good news is things have changed a lot. As that stigma has faded over the last century,
581.200000 585.120000 the rate of left-handedness has actually been rising. And now, you see tons of products
585.120000 590.040000 specifically for lefties, scissors, knives, even measuring cups. Life's getting easier
590.040000 593.960000 for the south paws. That's definitely a positive shift. So, okay, let's buy us now. Let's
593.960000 599.360000 talk up sides. Are there actual advantages to being a lefty, like secret superpowers?
599.360000 603.360000 Uh-huh. Maybe not superpowers, but there are some interesting potential advantages. Sports
603.360000 608.400000 is a big one. An interactive sports thing, boxing, fencing, tennis, baseball lefties seem
608.400000 613.080000 to have a strategic edge. Why? Because most opponents are used to facing right-handers.
613.080000 615.480000 The lefties movements can be surprising, unexpected.
615.480000 617.280000 Ah, the element of surprise.
617.280000 621.720000 Exactly. But it's what they call frequency dependent. The advantage is bigger when lefties
621.720000 626.280000 are rarer. He doesn't really apply a non-interactive sports, like swimming or bowling, where you're
626.280000 629.360000 not directly facing off against someone's hand in this.
629.360000 633.840000 Okay. So, surprise attacks on the tennis court. What about other areas, like creativity
633.840000 635.400000 you're thinking?
635.400000 639.320000 There are some studies linking left-handedness to things like creativity, particularly
639.320000 644.280000 in men. Also, maybe higher scores on verbal reasoning, or a greater likelihood of being
644.280000 649.680000 in gifted programs, but I have to stress, other research doesn't find these links, or
649.680000 655.080000 finds different results. So, the evidence is still a bit mixed on those cognitive aspects.
655.080000 659.080000 Right. So, being a lefty picture might be more about confusing the batter than having
659.080000 664.440000 a fundamentally different brain. Although, hey, lefties do stand closer to first base, always
664.440000 669.840000 thinking strategically. But it can't be all sunshine and surprising fastballs, right?
669.840000 672.120000 Are there potential downsides or costs?
672.120000 677.200000 That's the other side of the coin, yes. There are potential trade-offs. Some research suggests
677.200000 682.720000 statistical links, and I emphasize statistical between left-handedness and a slightly higher
682.720000 690.520000 risk for certain conditions, like schizophrenia, mood swings, maybe ADHD symptoms, or even neuroticism.
690.520000 694.920000 There's also been debate about longevity. Some older studies suggested lefties live shorter
694.920000 699.200000 lives, possibly due to increased accident risk in a world designed for righties, take
699.200000 704.040000 power tools, machinery, that kind of thing. But the evidence on longevity is really conflicting
704.040000 706.040000 and controversial now.
706.040000 708.880000 And what about school? You mentioned ADHD.
708.880000 712.960000 Yeah, some studies show non-right handed students statistically might face more challenges
712.960000 716.920000 in school. What's particularly interesting is that mixed-handed kids, those who don't
716.920000 721.400000 have a consistent dominant hand, seem to be about twice as likely to experience things
721.400000 726.560000 like dyslexia, compared to consistently left-to-right-handed kids. It suggests maybe having
726.560000 731.360000 that inconsistent dominance could be more problematic than being consistently left-handed.
731.360000 735.920000 That really raises a crucial question, then. If there are these potential costs and there
735.920000 742.680000 are minority, why has left-handedness stuck around for hundreds of thousands of years? Why
742.680000 745.840000 didn't evolution just favor righties completely?
745.840000 750.040000 That's the million dollar evolutionary question, isn't it? The very fact that left-handedness
750.040000 755.240000 has persisted, always at this lower frequency, strongly points to something called balancing
755.240000 759.280000 selection. It suggests there's an evolutionary trade-off happening.
759.280000 764.200000 Balancing selection. Meaning that if left-handedness only had advantages, you'd expect it to
764.200000 769.520000 become much more common, maybe even reach 50%. The fact that it stays around 10%, suggests
769.520000 773.120000 there must be some associated costs that balance out the benefits.
773.120000 777.800000 So the advantage in fighting or sports, which remember works best when lefties are rare,
777.800000 781.560000 could be one side of the balance. And the potential health risks, or the difficulties
781.560000 787.120000 using right-handed tools, or maybe even slightly smaller average body size, some studies suggest,
787.120000 790.040000 those could be the costs on the other side, keeping things in equilibrium.
790.040000 795.280000 So it's this constant evolutionary give and take, a dynamic balance that's kept lefties
795.280000 799.800000 and all those special scissors around for ages. And it's not just us, right? You mentioned
799.800000 800.800000 chimps.
800.800000 804.000000 Exactly. Our closest relatives show hand preferences, too.
804.000000 808.800000 Jimpansese, for example, often favor their left hands for delicate tasks, like fishing termites
808.800000 814.040000 out of mounds. But they might favor their right hand for tasks, needing more brute force,
814.040000 815.040000 like cracking nuts.
815.040000 816.040000 Yeah.
816.040000 819.000000 So it's complex, even in other primates. It's a really ancient trait.
819.000000 820.000000 Fascinating.
820.000000 823.760000 Yeah. And we're still learning so much about how genes, development, and the environment
823.760000 827.680000 all weave together here. Scientists still need to figure out if there are meaningful
827.680000 832.840000 categories of left-handedness, like, is familial left-handedness different from left-handedness
832.840000 837.600000 that might arise from, say, birth stress. If those categories even really exist cleanly,
837.600000 840.560000 understanding that can tell us more about the evolutionary forces.
840.560000 845.960000 Wow. What an incredibly deep dive that was from, you know, thumb sucking in the womb
845.960000 850.240000 to surprising pitches on the baseball field. It's so clear that hand-edness is way
850.240000 856.000000 more than just which hand you write with. It's tied into our biology, our deep history,
856.000000 861.680000 the very wiring of our brains. We've touched on the genetics, that complex mix of influence
861.680000 866.840000 and chance, the developmental journey, brain lateralization, which is definitely not as
866.840000 871.000000 simple as I thought, and also how culture has played a role, plus those intriguing trade
871.000000 875.840000 offs, the costs and benefits, and how evolution seems to keep everything in this delicate balance.
875.840000 879.800000 Absolutely. And it leaves us with a really interesting thought to consider, doesn't it?
879.800000 884.240000 As our society keeps becoming more accepting, more accommodating of left-handedness, maybe
884.240000 888.800000 reducing some of those historical costs. How might that change the evolutionary balance
888.800000 889.800000 in the future?
889.800000 893.800000 Yeah. Well, the number shift, if the world gets friendlier for self-pause, definitely
893.800000 896.800000 something to chew on. Thanks so much for taking this journey with us today.
896.800000 913.200000 Until next time, keep exploring and stay curious.