If you know an older left-handed person,
如果你认识一个比你大的左撇子，
chances are they had to learn to write or eat with their right hand.
他们都有可能曾经被迫学习过用右手写字和吃饭。
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，依旧有惯例强迫孩子使用“合适”的手。
Even the word for right also means correct or good, not just in English, but many other languages, too.
就连“右”字，在英语里也代表着正确和好，不仅在英语中，其他很多语言也是。
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对的，那为什么会有左撇子出现呢？
Today, about 1/10 of the world's population are left-handed.
现今，大约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,
考古证据显示，这种比例已经存在有50万年了，
with about 10% of human remains showing the associated differences in arm length and bone density,
约有10%的人在手臂长度和骨密度上显现出关联差异，
and some ancient tools and artifacts showing evidence of left-hand use.
古代一些工具和手工艺品也证明有左撇子的存在。
And despite what many may think, handedness is not a choice.
与大多数人所认为的相反的是，要用哪只手不是我们选择的。
It can be predicted even before birth based on the fetus' position in the womb.
它甚至可由胎儿出生前在母体的胎位预测出来。
So, if handedness is inborn, does that mean it's genetic? Well, yes and no.
所以，如果用那只手是与生俱来，意思是那是遗传的吗？
Identical twins, who have the same genes, can have different dominant hands.
即使是有相同基因的同卵双胞胎，也会有不同的主导手。
In fact, this happens as often as it does with any other sibling pair.
事实上，这种情况在随便一对兄弟姐妹身上都可见到。
But the chances of being right or left-handed are determined by the handedness of your parents in surprisingly consistent ratios.
你用左手或右手的机率由双亲的惯用手决定，而且比率出奇的一致。
If your father was left-handed but your mother was right-handed,
如果你的父亲是左撇子，而母亲是用右手的，
you have a 17% chance of being born left-handed, while two righties will have a left-handed child only 10% of the time.
你有17%的机会是天生的左撇子，而都用右手的父母生出左撇子子女机会只有10%。
Handedness seems to be determined by a roll of the dice, but the odds are set by your genes.
用哪一只手看似如掷骰子随意，但是机率其实由基因决定。
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.
这意味着进化之所以产生这么少数的左撇子，并在过去数千年都维持如此，是有理由的。
And while there have been several theories attempting to explain why handedness exists in the first place,
过去也有一些理论尝试解释为何从一开始就有不同惯用手，
or why most people are right-handed,
或者为什么多数人是用右手，
a recent mathematical model suggests that the actual ratio
然而最近的一种数学模式却认为使用不同手的实际比率，
reflects a balance between competitive and cooperative pressures on human evolution.
反应出人类进化过程中竞争和合作的压力的平衡。
The benefits of being left-handed are clearest in activities involving an opponent, like combat or competitive sports.
左撇子的优点在有对手的活动中最为明显，如打斗或竞争性的运动。
For example, about 50% of top hitters in baseball have been left-handed. Why?
例如，棒球的顶尖击球手有50%是左撇子。为什么呢？
Think of it as a surprise advantage.
你就设想这是一个意外的优势。
Because lefties are a minority to begin with,
因为左撇子本来就比较少，
both right-handed and left-handed competitors will spend most of their time encountering and practicing against righties.
无论选手是右撇子或左撇子，大家在练习或竞赛中碰到右撇子的时间一定比较多。
So when the two face each other, the left-hander will be better prepared against this right-handed opponent,
所以当他们面对彼此时，左撇子会对右手对手比较有准备，
while the righty will be thrown off.
所以右撇子就被打败了。

This fighting hypothesis, where an imbalance in the population results in an advantage for left-handed fighters or athletes,
这个打斗的假设，基于（右撇子）人数上不平衡造成左撇子战斗者和运动员占优势，
is an example of negative frequency-dependent selection.
是一个负频率依存天择的例子。
But according to the principles of evolution, groups that have a relative advantage tend to grow until that advantage disappears.
但是根据进化的原则，有相对优势的团体会成长到优势消失为止。
If people were only fighting and competing throughout human evolution,
如果人类在进化中只打斗和竞争，
natural selection would lead to more lefties being the ones that made it until there were so many of them,
天择会让左撇子一直繁衍到他们成为多数，
that it was no longer a rare asset.
然后他们就不再是稀罕资产了。
So in a purely competitive world, 50% of the population would be left-handed.
所以在纯竞争的世界里，50%的人会是左撇子。
But human evolution has been shaped by cooperation, as well as competition.
但是人类进化是由合作和竞争两者塑型的。
And cooperative pressure pushes handedness distribution in the opposite direction.
合作的压力将惯用手的分配推向反方向。
In golf, where performance doesn't depend on the opponent, only 4% of top players are left-handed,
在打高尔夫球时，个人表现与对手无关，只有4%的高手是左撇子，
an example of the wider phenomenon of tool sharing.
这是分享工具这种更普遍的现象的例子。
Just as young potential golfers can more easily find a set of right-handed clubs,
就像年轻有潜力的高尔夫球手更容易能够找到右撇子用的球杆，
many of the important instruments that have shaped society were designed for the right-handed majority.
社会上大多数的工具是为占大多数的右手人所设计的。
Because lefties are worse at using these tools, and suffer from higher accident rates,
因为左撇子在用这些工具时比较不顺手，出意外的比率就比较高，
they would be less successful in a purely cooperative world, eventually disappearing from the population.
所以他们在纯合作的世界里就会比较不成功，最后就会从人群中消失。
So by correctly predicting the distribution of left-handed people in the general population,
通过正确预测左撇子在一般人口中的分配率，
as well as matching data from various sports,
以及在不同的运动找到的匹配数据，
the model indicates that the persistence of lefties as a small but stable minority
这个模式显示左撇子的比率维持稳定少数，
reflects an equilibrium that comes from competitive and cooperative effects playing out simultaneously over time.
反映出并存的竞争和合作在时间推移下达到某种平衡。
And the most intriguing thing is what the numbers can tell us about various populations.
然而，最奇妙的是不同族群的数据显示出什么。
From the skewed distribution of pawedness in cooperative animals,
从合作型的动物使用的手（脚）掌的偏态分布，
to the slightly larger percentage of lefties in competitive hunter-gatherer societies,
到竞争型的授猎采集社会中有较多左撇子的比率，
we may even find that the answers to some puzzles of early human evolution are already in our hands.
我们甚至会发现早期人类进化的一些谜题的答案就在我们手中。