If you know an older left-handed person, chances are they had to learn to write or eat with their right hand. And in many parts of the world, it's still common practice to force children to use their "proper" hand. Even the word for right also means correct or good, not just in English, but many other languages, too. But if being left-handed is so wrong, then why does it happen in the first place? Today, about 1/10 of the world's population are left-handed. 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 bone density, and some ancient tools and artifacts showing evidence of left-hand use. And despite what many may think, handedness is not a choice. It can be predicted even before birth based on the fetus' position in the womb. So, if handedness is inborn, does that mean it's genetic? Well, yes and no. Identical twins, who have the same genes, can have different dominant hands. In fact, this happens as often as it does with any other sibling pair. But the chances of being right or left-handed are determined by the handedness of your parents in surprisingly consistent ratios. 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. Handedness seems to be determined by a roll of the dice, but the odds are set by your genes. 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. 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. The benefits of being left-handed are clearest in activities involving an opponent, like combat or competitive sports. For example, about 50% of top hitters in baseball have been left-handed. Why? Think of it as a surprise advantage. 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. 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. 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. But according to the principles of evolution, groups that have a relative advantage tend to grow until that advantage disappears. 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. So in a purely competitive world, 50% of the population would be left-handed. But human evolution has been shaped by cooperation, as well as competition. And cooperative pressure pushes handedness distribution in the opposite direction. 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. 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. 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. 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. And the most intriguing thing is what the numbers can tell us about various populations. 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.
如果你認識年長的左撇子 很可能他們都曾被迫 要學著用右手寫字和吃飯 在世界各地 仍然普遍教小孩子要改用「正」手 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 但是如果左撇子是錯的 那為什麼當初要有左撇子呢? 現今,大約十分之一的 世界人口是左撇子 考古學的證據顯示 從五十萬年前到現在都如此 大約十分之一的人類遺骸 在手長和骨密度顯示相關差異 有些古代的器具和文物 可看出是給左撇子用的 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 它甚至可由胎兒出生前 在母體的胎位預測出來 所以,如果用那隻手是與生俱來 意思是那是遺傳的嗎? 嗯,答案是對和不對。 即使是有相同基因的同卵雙胞胎 也有用不同手的 事實上,這種情況 在隨便一對兄弟姐妹身上都可見到 你用左手或右手的機率 由雙親的慣用手決定 而且比率出奇的一致 如果你的父親是左撇子 而母親是用右手的 你有 17% 的機會是天生的左撇子 都用右手的父母生出 左撇子子女機會只有 10% 用哪一隻手看似如擲骰子隨意 但是機率其實由基因決定 這意味著進化之所以 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 過去也有一些理論嘗試解釋 為何從一開始就有不同慣用手 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 競爭和合作的壓力的平衡 左撇子的優點 在有對手的活動中最為明顯 如打鬥或競爭性的運動 例如,棒球的最佳打擊手 有 50% 是左撇子 為什麼呢? 你就設想這是一個意外的優勢 因為左撇子本來就比較少 無論選手是右撇子或左撇子 大家在練習或競賽中 碰到右撇子的時間一定比較多 所以當他們面對彼此時 左撇子會對右手對手比較有準備 所以右撇子就被打敗了 這個打鬥的假設 基於(右撇子)人數上不平衡 造成左撇子戰鬥者和運動員佔優勢 是負「頻率依存天擇」的例子 但是根據進化的原則 有相對優勢的團體 會成長到優勢消失為止 如果人類在進化中只打鬥和競爭 天擇會讓左撇子一直繁衍到 他們成為多數 然後他們就不再是稀罕資產了 所以在純競爭的世界裡 50% 的人會是左撇子 但是人類進化是由 合作和競爭兩者塑型的 合作的壓力 將慣用手的分配推向反方向 在打高爾夫球時 個人表現與對手無關 只有 4% 的高手是左撇子 這是「分享工具」這種 更普遍的現象的例子 如同年輕有潛力的高爾夫球手 比較能夠找到右撇子用的球桿 社會上大多數的工具 是為佔大多數的右手人所設計的 因為左撇子在用 這些工具時比較不順手 出意外的比率就比較高 所以他們在純合作的世界裡 就會比較不成功 最後就會從人群中消失 藉由正確預測左撇子 在一般人口中的分配率 以及在不同的運動找到的匹配數據 這個模式顯示 左撇子的比率維持穩定少數 反映出 並存的競爭和合作 在時間推移下達到某種平衡 然而,最奇妙的是 不同族群的數據顯示出什麼 從合作型的動物 使用的手(腳)掌的偏態分佈 到競爭型的授獵採集社會中 有較多左撇子的比率 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