為什麼有人是左撇子?
Why are somepeople left-handed?
If you know an older left-handed person, chances are they had to learn to write or eat with their right hand.
如果你認識一個比你大的左撇子，他們都有可能曾經被迫學習過用右手寫字和吃飯。
And in many parts of the world, it's still common practice to force children to use their"proper" hand.
在世界，上很多地方，依舊有慣例強迫孩子使用“合適”的手。
Even the word for right also means correct or good, not just in English, but many other languages, too.
就連“右”字，在英語裡也代表著正確和好不僅在英語中，其他很多語言也是。
But if being left-handed is so wrong, then why does it happen in the first place?
但是如果左撇子是不對的，那為什麼會有左撇子出現呢?
Today, about 1/10 of the world's population are left-handed.
現今，大約1/10的世界人口是左撇子。
Archeological evidence shows that it's been that way for as long as 500,000 years, with about 10% of human remains showing the associated differences in arm length and bonedensity, and some ancient tools and artifacts ;showing evidence of left -hand use.
考古證據顯示，這種比例已經存在有50萬年了，約有10%的人顯現出差異區別在手臂長度和骨密度，上古代一些工具和手工藝品也證明有左撇子的存在。
And despite what many may think, handedness is not a choice.
儘管很多人認為用手習慣不是一種選擇。
It can be predicted even before birth based on the fetus' position in the womb.
但在出生前根據胎兒在子宮裡的位置就可以預測。
So, if handedness is inborn, does that mean it's genetic?
因此，如果習慣是天生是否就意味著這是基因定的嗎?
Well, yes and no.
是的，一部分也不是。
Identical twins, who have the same genes, can have different dominant hands.
同卵雙胞胎擁有相同的基因也會產生不同的主導手。
In fact, this happens as often as it does with any other sibling pair.
事實上，這與其他同胞對的發生概率一樣。
But the chances of being right or left-handed are determined by the handedness of your parents in surprisingly consistent ratios.
你是左撇子還是右撇子取決於父母的用手習慣這有驚人的統一數據。
If your father was left-handed but your mother was right handed, you have a 17% chance of being born left- handed, while two righties will have a left-handed child only 10% of the time.
如果爸爸是左撇子，而媽媽是右撇子你有17%的機會是左撇子，而兩位都是右撇子，孩子是左撇子的幾率只有10%。
Handedness seems to be determined by a roll of the dice, but the odds are set by your genes.
看起來用手習慣像擲骰子一樣，但幾率是由基因決定的。
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.
這一切的原由是進化產生了這小部分的左撇子並保持了上千年。
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.
有幾種理論試圖去解釋為什麼一開始有用手習慣存在，或者為什麼大多數人習慣用右手，最近一個數學模型提出現實比例反應了人類進化中競爭和合作壓力造成的人數平衡。
The benefts of being left- handed are clearest in activities involving an opponent, like combat or competitive sports.
左撇子的好處很明顯是在活動中處對立面，像拳擊或競技運動。
For example, about 50% of top hitters in baseball have been left-handed.
比如，有50%頂尖棒球擊球手都是左撇子。
Why?
為什麼?
Think of it as a surprise advantage.
那會是很特殊的優勢。
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.
左撇子很小眾，左撇子和右撇子的選手都會花大量的時間練習對抗右撇子。
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.
所以當他們面對面對抗時左撇子會更熟練地對抗右撇子對手，然而右撇子的就無力對抗了。
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.
這鬥爭的假設因人數上的不平均造成左撇子的戰士或者運動員有優勢，是一個很好的負頻率相關選擇的例子。
But according to the principles of evolution,groups that have a relative advantage tend to grow until that advantage disappears.
但根據進化法則，有相關優勢的組群會發展至優勢消失。
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.
如果在人類進化中只有戰鬥和競爭，自然選擇會導致左撇子的數量從很少變為很多，直到他們不再是稀有資源。
So in a purely competitive world, 50% of the population would be left-handed.
因此在一個完全競爭環境，會有50%的人口是左撇子。
But human evolution has been shaped by cooperation, as well as competition.
但人類進化史.上有合作和競爭。
And cooperative pressure pushes handedness distribution in the opposite direction.
由於合作的壓力造成用手習慣往相反的方向。
In golf, where performance doesn't depend on the opponent, only 4% of top players are left-handed, an example of the wider phenomenon of tool sharing.
當在高爾夫比賽中，選手的表現不用依賴對手所以只有4%的選手是左撇子這是個更廣泛現象的工具共享。
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.
像年輕有潛力的高爾夫手們更容易找到右手為主的俱樂部，很多重要的裝備都是為大眾的右手選手設計的。
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.
因為左撇子都用不慣這些裝備，而且高失誤率，會使他們在純合作環境裡吃虧，最終造成人數.上銳減。
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.
為了能正確預測左撇子在人群中的分佈，以及與各項運動中的數據配對，模型指出左撇子群體小而且數量穩定反應了人類合作和競爭的平衡影響他們同時進行著。
And the most intriguing thing is what the numbers can tell us about various populations.
最有趣的是數據可以告訴我們很多關於人群數量變化中包含的信息。
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.
從有爪動物合作的傾斜分佈，到合作狩獵社會里比重稍大的左撇子，我們會發現有關早期人類進化的某些難題的答案就在我們手中。
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