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

在世界各地，教小孩子要改用「正」手仍是個普遍的習慣。

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

甚至「右」這個字也有「對」和「好」的意思，不只是英文其他許多語言也如此。

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

但是如果左撇子是錯的，那為什麼當初要有左撇子呢？

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

現今，世界上約十分之一的人是左撇子。

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.

考古學資料顯示從五十萬年前到現在都如此，大約十分之一的人類遺骸顯示手長和骨密度與這個現象的相關差異，有些古代的器具和文物可看出是給左撇子用的。

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

而且與大多數人所認為相反的是，要用哪隻手不是我們選擇的。

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

它甚至可由胎兒出生前在母體的胎位預測出來。

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

那如果用哪隻手是與生俱來的，意思是那是遺傳的嗎？

Well, yes and no.

嗯，答案是對也不對。

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

即使是有相同基因的同卵雙胞胎，慣用手也可能不同。

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

事實上，這種情況在隨便一對兄弟姐妹身上都可見到。

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

但你用左手或右手的機率是由雙親的慣用手決定，而且比率出奇的一致。

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.

用哪一隻手看似如擲骰子隨意，但是機率其實由基因決定。

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.

這都顯示演化之所以產生這麼少數的左撇子，並在過去數千年都維持如此，是有理由的。

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.

過去也有一些理論嘗試解釋從一開始就有不同慣用手的原因，或者為什麼多數人是用右手，然而最近的一種數學模式卻認為使用不同手的實際比率，反應出人類進化過程中競爭和合作的壓力平衡。

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

身為左撇子的優點在涉及競爭對手的活動，例如打鬥或競爭型的運動中最為明顯。

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?

為什麼呢？

Think of it as a surprise advantage.

就當它是個意外的收穫吧！

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.

因為左撇子一開始就是少數，不論是慣用右手或左手的競爭者，都會花大部分的時間練習如何應戰右撇子。

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.

所以當左撇子與右撇子相遇時，慣用左手的人早已做好準備，而右撇子則等著被擊敗。

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.

這個有關打鬥的假說顯示左撇子在人口分布中的不均衡，造成了使用左手的戰士或運動員會因此擁有優勢，這就是「負向頻率依賴選擇」的例子。

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

但根據演化的原則，那些擁有相對優勢的族群往往會不斷成長，直到優勢消失為止。

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.

如果人類在演化的進程中只有不斷的打鬥和競爭，天擇應該會讓左撇子成為人口中的多數，直到他們人數過多而該特質不再稀有為止。

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.

但人類的演化是同時由合作與競爭塑造而成。

And cooperative pressure pushes handedness distribution in the opposite direction.

此時，合作的壓力便把慣用手的分布現象轉往相反的結果。

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.

在高爾夫球這項不需要競爭者的活動中，只有百分之四的頂尖選手是左撇子，這是更普遍關於工具分享現象的例子。

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.

就如同年輕有潛力的高爾夫球選手會更容易找到適合右撇子的球桿，許多由社會共同形塑而成的重要工具都是專為右撇子設計的。

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.

因為左撇子在使用這些工具時表現不佳並且更容易面對高意外風險，他們在一個純然合作的環境中就不會那麼成功，最終消失在人群中。

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.

所以藉由正確預測左撇子在人口中的分布，同時參考不同運動中的數據，這樣的模組顯示左撇子做為一個數量少卻穩定的族群反映出一個平衡，而那是源自於競爭和合作的關係如何在歷史中展現。

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

而最有趣的事情是，這些數字可以告訴我們不同種族的現象。

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

從合作型動物的慣用手分布到左撇子比例較高的純粹競爭的獵人或採食者的社會。

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

我們甚至可以在我們自己的手中，找到有關古早人類演化謎題的解答。