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  • If you know an older left-handed person, chances are they had to learn to write or eat with their right hand.


  • And in many parts of the world, it's still common practice to force children to use their "proper" hand.


  • Even the word for right also means correct or good, not just in English, but many other languages, too.


  • But if being left-handed is so "wrong," then why does it happen in the first place?


  • Today, about one tenth of the world's population are left-handed.


  • Archaeological evidence shows that it's been that way for as long as 500,000 years, with about 10 percent of human remains showing the associated differences in arm length and bone density, and some ancient tools and artifacts showing evidence of left-hand use.


  • And despite what many may think, handedness is not a choice.


  • It can be predicted even before birth based on the fetus's position in the womb.


  • So, if handedness is inborn, does that mean it's genetic?


  • Well, yes and no.


  • Identical twins, who have the same genes, can have different dominant hands.


  • In fact, this happens as often as it does with any other sibling pair.


  • But the chances of being right- or left-handed are determined by the handedness of your parents, in surprisingly consistent ratios.


  • If your father was left-handed but your mother was right-handed, you have a 17 percent chance of being born left-handed, while two righties will have a left-handed child only 10 percent of the time.

    如果你的父親是左撇子而母親是右撇子,你有 17% 的機會是天生的左撇子,父母都是右撇子生出左撇子子女機會只有 10%。

  • Handedness seems to be determined by a roll of the dice, but the odds are set by your genes.


  • All of this implies there's a reason that evolution has produced this small proportion of lefties, and maintained it over the course of millennia.


  • And while there have been several theories attempting to explain why handedness exists in the first place, or why most people are right-handed, a recent mathematical model suggests that the actual ratio reflects a balance between competitive and cooperative pressures on human evolution.


  • The benefits of being left-handed are clearest in activities involving an opponent like combat or competitive sports.


  • For example, about 50 percent of top hitters in baseball have been left-handed.

    舉例來說,棒球中大約 50% 的頂尖打者是左撇子。

  • Why?


  • Think of it as a surprise advantage.


  • Because lefties are a minority to begin with, both right-handed and left-handed competitors will spend most of their time encountering and practicing against righties.


  • So when the two face each other, the left-hander will be better prepared against this right-handed opponent, while the righty will be thrown off.


  • This fighting hypothesis, where an imbalance in the population results in an advantage for left-handed fighters or athletes, is an example of negative frequency-dependent selection.


  • But according to the principles of evolution, groups that have a relative advantage tend to grow until that advantage disappears.


  • If people were only fighting and competing throughout human evolution, natural selection would lead to more lefties being the ones that made it until there were so many of them that it was no longer a rare asset.


  • So in a purely competitive world, 50 percent of the population would be left-handed.

    因此在一個純然充滿競爭的環境下,應該要有 50% 的人口是左撇子才對。

  • But human evolution has been shaped by cooperation, as well as competition.


  • And cooperative pressure pushes handedness distribution in the opposite direction.


  • In golf, where performance doesn't depend on the opponent, only 4 percent of top players are left-handed, an example of the wider phenomenon of tool sharing.


  • Just as young potential golfers can more easily find a set of right-handed clubs, many of the important instruments that have shaped society were designed for the right-handed majority.


  • Because lefties are worse at using these tools, and suffer from higher accident rates, they would be less successful in a purely cooperative world, eventually disappearing from the population.


  • So by correctly predicting the distribution of left-handed people in the general population, as well as matching data from various sports, the model indicates that the persistence of lefties as a small but stable minority reflects an equilibrium that comes from competitive and cooperative effects playing out simultaneously over time.


  • And the most intriguing thing is what the numbers can tell us about various populations.


  • From the skewed distribution of pawedness in cooperative animals to the slightly larger percentage of lefties in competitive hunter-gatherer societies.


  • We may even find that the answers to some puzzles of early human evolution are already in our hands.


If you know an older left-handed person, chances are they had to learn to write or eat with their right hand.


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B1 中級 中文 美國腔 TED-Ed 左撇子 競爭 演化 合作 右手

【TED-Ed】你是左撇子嗎?原來左撇子的歷史這麼深遠! (Why are some people left-handed? - Daniel M. Abrams)

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    Halu Hsieh 發佈於 2020 年 05 月 19 日