为什么有人是左撇子? 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.

如果你认识一个比你大的左撇子，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“**”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, 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.

考古证据显示，这种比例已经存在有50万年了，约有10%的人显现出差异区别在手臂长度和骨密度，上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.

尽管很多人认为用手习惯不是一种选择。

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

但在出生前根据胎儿在子宫里的位置就可以预测。

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

因此，如果习惯是天生是否就意味着这是基因定的吗?

Well, yes and no.

是的，一部分也不是。

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

同卵双胞胎拥有相同的基因也会产生不同的主导手。

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

事实上，这与其他同胞对的发生概率一样。

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

你是左撇子还是右撇子取决于父母的用手习惯这有惊人的统一数据。

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，而妈妈是右撇子你有17%的机会是左撇子，而两位都是右撇子，孩子是左撇子的几率只有10%。

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

看起来用手习惯像掷骰子一样，但几率是由基因决定的。

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

这一切的原由是进化产生了这小部分的左撇子并**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.

有几种理论试图去解释为什么一开始有用手习惯存在，或者为什么大多数人习惯用右手，最近一个数学模型提出现实比例反应了人类进化中竞争和合作压力造成的人数平衡。

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

左撇子的好处很明显是在活动中处对立面，像拳击或竞技运动。

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

比如，有50%顶尖棒球击球手都是左撇子。

Why?

为什么?

Think of it as a surprise advantage.

那会是很特殊的优势。

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.

左撇子很小众，左撇子和右撇子的选手都会花大量的时间练习对抗右撇子。

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.

所以当他们面对面对抗时左撇子会更熟练地对抗右撇子对手，然而右撇子的就无力对抗了。

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.

这斗争的假设因人数上的不平均造成左撇子的战士或者运动员有优势，是一个很好的负**相关选择的例子。

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

但根据进化法则，有相关优势的组群会**至优势消失。

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.

如果在人类进化中只有战斗和竞争，自然选择会导致左撇子的数量从很少变为很多，直到他们不再是稀有资源。

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

因此在一个完全竞争**，会有50%的人口是左撇子。

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

但人类进化史.上有合作和竞争。

And cooperative pressure pushes handedness distribution in the opposite direction.

由于合作的压力造成用手习惯往相反的方向。

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.

当在高尔夫比赛中，选手的表现不用依赖对手所以只有4%的选手是左撇子这是个更广泛现象的工具共享。

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.

像年轻有潜力的高尔夫手们更容易找到右手为主的俱乐部，很多重要的装备都是为大众的右手选手设计的。

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

因为左撇子都用不惯这些装备，而且高失误率，会使他们在纯合作**里吃亏，最终造**数.上锐减。

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 **all but stable minority reflects an equilibrium that comes from competitive and cooperative effects playing out simultaneously over time.

为了能正确预测左撇子在人群中的分布，以及与各项运动中的数据配对，模型指出左撇子群体小而且数量稳定反应了人类合作和竞争的平衡影响他们同时进行着。

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

最有趣的是数据可以告诉我们很多关于人群数量变化中包含的信息。

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.

从有爪动物合作的倾斜分布，到合作狩猎社会里比重稍大的左撇子，我们会发现有关早期人类进化的某些难题的答案就在我们手中。