Why are some people left-handed? - Daniel M. Abrams

為什麼有些人是左撇子？ － 丹尼爾．艾布拉姆斯

譯者: Melody Tang 審譯者: Regina Chu

If you know an older left-handed person,

如果你認識年長的左撇子

chances are they had to learn to write or eat with their right hand.

很可能他們都曾被迫 要學著用右手寫字和吃飯

And in many parts of the world,

在世界各地

it's still common practice to force children to use their "proper" hand.

仍然普遍教小孩子要改用「正」手

Even the word for right also means correct or good,

甚至「右」這個字 也有「對」和「好」的意思

not just in English, but many other languages, too.

不只是英文 其他許多語言也如此

But if being left-handed is so wrong,

但是如果左撇子是錯的

then why does it happen in the first place?

那為什麼當初要有左撇子呢？

Today, about 1/10 of the world's population are left-handed.

現今，大約十分之一的 世界人口是左撇子

Archeological evidence shows that it's been that way

考古學的證據顯示

從五十萬年前到現在都如此

for as long as 500,000 years,

with about 10% of human remains

大約十分之一的人類遺骸

showing the associated differences in arm length and bone density,

在手長和骨密度顯示相關差異

and some ancient tools and artifacts showing evidence of left-hand use.

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.

與大多數人所認為的相反的是 要用那隻手不是我們選擇的

It can be predicted even before birth based on the fetus' 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?

所以，如果用那隻手是與生俱來 意思是那是遺傳的嗎？

Well, yes and no.

嗯，答案是對和不對。

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 也有用不同手的

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

你用左手或右手的機率

are determined by the handedness of your parents

由雙親的慣用手決定

in surprisingly consistent ratios.

而且比率出奇的一致

If your father was left-handed but your mother was right-handed,

如果你的父親是左撇子 而母親是用右手的

you have a 17% chance of being born left-handed,

你有 17% 的機會是天生的左撇子

while two righties will have a left-handed child only 10% of the time.

都用右手的父母生出 左撇子子女機會只有 10%

Handedness seems to be determined by a roll of the dice,

用哪一隻手看似如擲骰子隨意

but the odds are set by your genes.

但是機率其實由基因決定

All of this implies there's a reason

這意味著進化之所以 產生這麼少數的左撇子

that evolution has produced this small proportion of lefties,

並在過去數千年都維持如此 是有理由的

and maintained it over the course of millennia.

過去也有一些理論嘗試解釋

And while there have been several theories

attempting to explain why handedness exists in the first place,

為何從一開始就有不同慣用手

or why most people are right-handed,

或者為什麼多數人是用右手

a recent mathematical model

然而最近的一種數學模式

suggests that the actual ratio reflects a balance

卻認為使用不同手的實際比率

between competitive and cooperative pressures on human evolution.

反應出人類進化過程中 競爭和合作的壓力的平衡

The benefits of being left-handed

左撇子的優點

are clearest in activities involving an opponent,

在有對手的活動中最為明顯

like combat or competitive sports.

如打鬥或競爭性的運動

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

例如，棒球的最佳打擊手 有 50% 是左撇子

Why?

為什麼呢？

Think of it as a surprise advantage.

你就設想這是一個意外的優勢

Because lefties are a minority to begin with,

因為左撇子本來就比較少

both right-handed and left-handed competitors

無論選手是右撇子或左撇子

will spend most of their time encountering

大家在練習或競賽中

and practicing against righties.

碰到右撇子的時間一定比較多

So when the two face each other,

所以當他們面對彼此時

the left-hander will be better prepared against this right-handed opponent,

左撇子會對右手對手比較有準備

while the righty will be thrown off.

所以右撇子就被打敗了

This fighting hypothesis,

這個打鬥的假設

where an imbalance in the population

基於（右撇子）人數上不平衡

results in an advantage for left-handed fighters or athletes,

造成左撇子戰鬥者和運動員佔優勢

is an example of negative frequency-dependent selection.

是負「頻率依存天擇」的例子

But according to the principles of evolution,

但是根據進化的原則

groups that have a relative advantage

有相對優勢的團體

tend to grow until that advantage disappears.

會成長到優勢消失為止

If people were only fighting and competing throughout human evolution,

如果人類在進化中只打鬥和競爭

natural selection would lead to more lefties being the ones that made it

天擇會讓左撇子一直繁衍到

until there were so many of them,

他們成為多數

that it was no longer a rare asset.

然後他們就不再是稀罕資產了

So in a purely competitive world,

所以在純競爭的世界裡 50% 的人會是左撇子

50% of the population would be left-handed.

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

但是人類進化是由 合作和競爭兩者塑型的

And cooperative pressure

合作的壓力

pushes handedness distribution in the opposite direction.

將慣用手的分配推向反方向

In golf, where performance doesn't depend on the opponent,

在打高爾夫球時 個人表現與對手無關

only 4% of top players are left-handed,

只有 4% 的高手是左撇子

an example of the wider phenomenon of tool sharing.

這是「分享工具」這種 更普遍的現象的例子

Just as young potential golfers

如同年輕有潛力的高爾夫球手

can more easily find a set of right-handed clubs,

比較能夠找到右撇子用的球桿

many of the important instruments that have shaped society

社會上大多數的工具

were designed for the right-handed majority.

是為佔大多數的右手人所設計的

Because lefties are worse at using these tools,

因為左撇子在用 這些工具時比較不順手

and suffer from higher accident rates,

出意外的比率就比較高

they would be less successful in a purely cooperative world,

所以他們在純合作的世界裡 就會比較不成功

eventually disappearing from the population.

最後就會從人群中消失

So by correctly predicting the distribution

藉由正確預測左撇子

of left-handed people in the general population,

在一般人口中的分配率

as well as matching data from various sports,

以及在不同的運動找到的匹配數據

the model indicates

這個模式顯示

that the persistence of lefties as a small but stable minority

左撇子的比率維持穩定少數

reflects an equilibrium

反映出

並存的競爭和合作

that comes from competitive and cooperative effects

playing out simultaneously over time.

在時間推移下達到某種平衡

And the most intriguing thing

然而，最奇妙的是

is what the numbers can tell us about various populations.

不同族群的數據顯示出什麼

From the skewed distribution of pawedness in cooperative animals,

從合作型的動物 使用的手（腳）掌的偏態分佈

to the slightly larger percentage of lefties

到競爭型的授獵採集社會中

in competitive hunter-gatherer societies,

有較多左撇子的比率

we may even find that the answers to some puzzles of early human evolution

我們甚至會發現 早期人類進化的一些謎題

其答案就在我們手中

are already in our hands.