1 00:00:06,674 --> 00:00:09,301 If you know an older left-handed person, 2 00:00:09,301 --> 00:00:14,103 chances are they had to learn to write or eat with their right hand. 3 00:00:14,103 --> 00:00:15,635 And in many parts of the world, 4 00:00:15,635 --> 00:00:20,036 it's still common practice to force children to use their "proper" hand. 5 00:00:20,615 --> 00:00:24,378 Even the word for right also means correct or good, 6 00:00:24,378 --> 00:00:27,557 not just in English, but many other languages, too. 7 00:00:28,366 --> 00:00:30,838 But if being left-handed is so wrong, 8 00:00:30,838 --> 00:00:32,962 then why does it happen in the first place? 9 00:00:33,499 --> 00:00:37,618 Today, about 1/10 of the world's population are left-handed. 10 00:00:37,618 --> 00:00:40,127 Archeological evidence shows that it's been that way 11 00:00:40,127 --> 00:00:43,400 for as long as 500,000 years, 12 00:00:43,004 --> 00:00:44,820 with about 10% of human remains 13 00:00:45,216 --> 00:00:49,925 showing the associated differences in arm length and bone density, 14 00:00:49,925 --> 00:00:54,542 and some ancient tools and artifacts showing evidence of left-hand use. 15 00:00:55,099 --> 00:00:59,496 And despite what many may think, handedness is not a choice. 16 00:00:59,496 --> 00:01:04,175 It can be predicted even before birth based on the fetus' position in the womb. 17 00:01:04,861 --> 00:01:08,349 So, if handedness is inborn, does that mean it's genetic? 18 00:01:08,823 --> 00:01:10,127 Well, yes and no. 19 00:01:10,005 --> 00:01:14,799 Identical twins, who have the same genes, can have different dominant hands. 20 00:01:15,294 --> 00:01:19,357 In fact, this happens as often as it does with any other sibling pair. 21 00:01:20,185 --> 00:01:22,689 But the chances of being right or left-handed 22 00:01:22,689 --> 00:01:25,810 are determined by the handedness of your parents 23 00:01:25,081 --> 00:01:27,910 in surprisingly consistent ratios. 24 00:01:28,639 --> 00:01:31,947 If your father was left-handed but your mother was right-handed, 25 00:01:31,947 --> 00:01:36,063 you have a 17% chance of being born left-handed, 26 00:01:36,063 --> 00:01:39,769 while two righties will have a left-handed child only 10% of the time. 27 00:01:40,522 --> 00:01:43,678 Handedness seems to be determined by a roll of the dice, 28 00:01:43,702 --> 00:01:45,811 but the odds are set by your genes. 29 00:01:46,354 --> 00:01:48,077 All of this implies there's a reason 30 00:01:48,077 --> 00:01:51,582 that evolution has produced this small proportion of lefties, 31 00:01:51,582 --> 00:01:53,821 and maintained it over the course of millennia. 32 00:01:54,236 --> 00:01:56,304 And while there have been several theories 33 00:01:56,304 --> 00:01:59,830 attempting to explain why handedness exists in the first place, 34 00:01:59,083 --> 00:02:01,181 or why most people are right-handed, 35 00:02:01,928 --> 00:02:03,434 a recent mathematical model 36 00:02:03,434 --> 00:02:06,899 suggests that the actual ratio reflects a balance 37 00:02:06,899 --> 00:02:11,181 between competitive and cooperative pressures on human evolution. 38 00:02:11,697 --> 00:02:13,489 The benefits of being left-handed 39 00:02:13,489 --> 00:02:16,581 are clearest in activities involving an opponent, 40 00:02:17,409 --> 00:02:20,205 like combat or competitive sports. 41 00:02:20,205 --> 00:02:25,213 For example, about 50% of top hitters in baseball have been left-handed. 42 00:02:25,285 --> 00:02:26,286 Why? 43 00:02:26,031 --> 00:02:27,999 Think of it as a surprise advantage. 44 00:02:28,836 --> 00:02:31,584 Because lefties are a minority to begin with, 45 00:02:31,584 --> 00:02:34,220 both right-handed and left-handed competitors 46 00:02:34,022 --> 00:02:36,648 will spend most of their time encountering 47 00:02:36,846 --> 00:02:39,679 and practicing against righties. 48 00:02:39,679 --> 00:02:41,223 So when the two face each other, 49 00:02:41,223 --> 00:02:45,430 the left-hander will be better prepared against this right-handed opponent, 50 00:02:45,043 --> 00:02:47,190 while the righty will be thrown off. 51 00:02:48,026 --> 00:02:49,663 This fighting hypothesis, 52 00:02:49,897 --> 00:02:51,699 where an imbalance in the population 53 00:02:51,699 --> 00:02:55,228 results in an advantage for left-handed fighters or athletes, 54 00:02:55,228 --> 00:02:58,787 is an example of negative frequency-dependent selection. 55 00:02:59,279 --> 00:03:01,311 But according to the principles of evolution, 56 00:03:01,599 --> 00:03:03,361 groups that have a relative advantage 57 00:03:03,385 --> 00:03:06,556 tend to grow until that advantage disappears. 58 00:03:07,219 --> 00:03:10,553 If people were only fighting and competing throughout human evolution, 59 00:03:10,553 --> 00:03:14,434 natural selection would lead to more lefties being the ones that made it 60 00:03:14,434 --> 00:03:15,492 until there were so many of them, 61 00:03:16,014 --> 00:03:18,438 that it was no longer a rare asset. 62 00:03:18,962 --> 00:03:21,094 So in a purely competitive world, 63 00:03:21,094 --> 00:03:23,731 50% of the population would be left-handed. 64 00:03:24,345 --> 00:03:29,068 But human evolution has been shaped by cooperation, as well as competition. 65 00:03:29,068 --> 00:03:30,223 And cooperative pressure 66 00:03:30,223 --> 00:03:33,627 pushes handedness distribution in the opposite direction. 67 00:03:34,786 --> 00:03:38,449 In golf, where performance doesn't depend on the opponent, 68 00:03:38,449 --> 00:03:41,562 only 4% of top players are left-handed, 69 00:03:41,562 --> 00:03:44,620 an example of the wider phenomenon of tool sharing. 70 00:03:45,062 --> 00:03:46,780 Just as young potential golfers 71 00:03:47,338 --> 00:03:50,489 can more easily find a set of right-handed clubs, 72 00:03:50,489 --> 00:03:53,796 many of the important instruments that have shaped society 73 00:03:53,796 --> 00:03:56,945 were designed for the right-handed majority. 74 00:03:56,945 --> 00:03:59,227 Because lefties are worse at using these tools, 75 00:03:59,227 --> 00:04:01,278 and suffer from higher accident rates, 76 00:04:01,737 --> 00:04:04,913 they would be less successful in a purely cooperative world, 77 00:04:04,913 --> 00:04:08,251 eventually disappearing from the population. 78 00:04:08,251 --> 00:04:10,251 So by correctly predicting the distribution 79 00:04:10,275 --> 00:04:12,846 of left-handed people in the general population, 80 00:04:12,846 --> 00:04:15,653 as well as matching data from various sports, 81 00:04:15,653 --> 00:04:16,699 the model indicates 82 00:04:16,699 --> 00:04:19,707 that the persistence of lefties as a small but stable minority 83 00:04:20,499 --> 00:04:22,458 reflects an equilibrium 84 00:04:22,458 --> 00:04:24,990 that comes from competitive and cooperative effects 85 00:04:24,099 --> 00:04:27,180 playing out simultaneously over time. 86 00:04:28,071 --> 00:04:29,738 And the most intriguing thing 87 00:04:29,738 --> 00:04:33,477 is what the numbers can tell us about various populations. 88 00:04:33,477 --> 00:04:36,575 From the skewed distribution of pawedness in cooperative animals, 89 00:04:37,457 --> 00:04:39,666 to the slightly larger percentage of lefties 90 00:04:39,666 --> 00:04:42,592 in competitive hunter-gatherer societies, 91 00:04:42,592 --> 00:04:47,636 we may even find that the answers to some puzzles of early human evolution 92 00:04:47,636 --> 00:04:49,639 are already in our hands.