Why are more people right-handed?

In the 160 years in which "handedness" has been studied we have learned quite a lot, but we still cannot precisely describe what causes humans preferentially to use one hand over the other, or why human populations are biased toward right-hand use rather than left-hand use.

Scientists disagree over what percentage of human populations are "right-handed" or "left-handed" because there is no standard, empirical definition for measuring "handedness"; our criteria vary, and are based on various theoretical explanations because we are still trying to understand the mechanisms involved. But I can describe in general terms what we do know.

Most humans (say 70 percent to 95 percent) are right-handed, a minority (say 5 percent to 30 percent) are left-handed, and an indeterminate number of people are probably best described as ambidextrous. This appears to be universally true for all human populations anywhere in the world. There is evidence for genetic influence for handedness; however, it is non-Mendelian and geneticists cannot agree on the exact process. There is evidence that handedness can be influenced (and changed) by social and cultural mechanisms. For instance, teachers have been known to force children to switch from using their left hand to using their right hand for writing. Also, some more restrictive societies show less left-handedness in their populations than other more permissive societies.

Some researchers argue there is evidence for cases of "pathological" left-handedness related to brain trauma during birth. And many researchers trace the cause of handedness back to prenatal, interuterine developmental processes, back to the time when the fetal brain is first developing distinct cerebral hemispheres. In the 1860s the French surgeon Paul Broca noted a relationship between right-handedness and left-hemispheric brain specialization for language abilities. But the hand-brain association is neither a simple, nor reliable, correlation. Studies conducted in the 1970s showed that most left-handers have the same left-hemispheric brain specialization for language typical of all humans--only a portion of left-handers have different patterns of language specialization.

So the bottom line is, we have a good general idea of the causes of right-handedness in human populations, but we have yet to work out the precise details, including why the direction is right instead of left.

The second question (do non-human primates show handedness？) is currently a controversial one. It is important to note the difference between an individual animal being left- or right-handed, and most of the animals in an entire population being either left- or right-handed. It is not unusual for individual animals to show a preferential use of one hand over the other, to develop an individual hand preference. But there is no consensus among researchers that any non-human species shows the same species-level handedness found in humans.

There are a few researchers who argue for this, but most of these work with animals in laboratory or captive settings, performing manual tasks that are very different from how animals use their hands in the wild.

In addition to studying handedness in humans, I have also studied hand usage in mountain gorillas (in Rwanda) as well as chimpanzees, red colobus monkeys, redtail monkeys and grey-cheeked mangabeys (in Uganda). My own research shows that individual monkeys and apes often develop individual preferences (both left and right) for manual tasks, but I have found no evidence for population-level handedness, as seen in humans.

為什么大多數人是右撇子？

在“利手”現象被研究的160年的時間里，我們知道了很多的相關知識，但是我們仍然不能準確地說明導致人類傾向于右手或者左右的原因，也不能說明為什么人傾向于使用右手而不是左右。

科學家們對于人群中右撇子以及左撇子所占的百分比持有不同的意見，這是因為目前對于測量“利手”沒有一個標準的、經驗的定義；我們的標準變化不一，而且這些標準基于各種各樣的理論解釋，這是因為我們仍然試圖理解其中的機理。但是對于我們確實已知的東西，我能概括性地描述出來。

大多數的人（大概在70%到95%）是右撇子，少部分人（大概在5%到30%）是左撇子，也有一些不確定數目的人可以被認為是左右手都靈活的人。似乎全世界任何一個地方的人口都是如此。有證據表明基因能影響“利手”；不過這是非孟德爾遺傳，基因學家不能對準確的遺傳過程達成一致。也有證據表明社會和文化體制能影響（以及改變）“利手”。比如，老師就曾強迫兒童們在書寫中從使用左手轉為使用右手。相比與那些更加寬容的社會，在一些限制更多的社會中，使用左手的人要少一些。

一些研究者辯論說，有證據表明一些“病理學上的”左撇子與出生時候的腦創傷有關。許多研究者追溯“利手”的原因到產前子宮內發展的過程，以及追溯到胎兒的大腦開始形成明顯的大腦半球的時候。在19世紀60年代，法國外科醫生Paul Broca注意到“右利手”和左大腦半球專門負責語言能力的關系。但是這種手--腦的聯系既不是一個簡單的相關，也不是一個可靠的相關。在20世紀70年代進行的研究表明，大多數的左撇子具有同樣的負責典型為人類所擁有的語言的左大腦半球----僅僅只有一少部分左撇子其專門負責語言的大腦模式不一樣。

所以基本點就是：對于人口中“右利手”的原因，我們有一個好的總體的觀點，但是我們還需要找出精確的細節，包括為什么“利手”是右手而不是左手。

第二個問題(非人類的靈長目動物也表現出“利手”嗎？)目前是一個有爭議的問題。個體動物是右撇子還是左撇子與整個動物種群中大多數動物是左撇子或者右撇子之間的區別需要重點注意。一些個體動物表現出使用某一只手的傾向也并非不尋常。但是其他任何非人類物種是否也在物種層面表現出和人類相同的“利手”現象，對此研究者們還沒有一致的看法。

有少數幾個研究者認為有這樣的非人類物種存在，但是在這些工作的大部分之中，動物所處的環境是在實驗室或者在籠子里，這些動物進行的手工任務與動物在野外如何使用它們的手有非常大的不同。

除了研究人類的“利手”外，我也研究過山地大猩猩（在盧旺達）使用手的情況，還研究了黑猩猩、紅疣猴、紅尾猴以及灰頰候（在烏干達）使用手的情況。我自己的研究表明，猴個體以及類人猿個體對于手工任務經常發展出各自的用手傾向（既有右手也有左手），但是我還沒有證據表明在這些動物的種群層面有“利手”的現象，就如在人類中看到的一樣。

Vocabulary:

Handedness：用右手或左手的習慣；也稱“利手”
Precisely：精確地
Preferentially：有傾向性的
Empirical：經驗的
Criteria：標準
Theoretical：理論的
Mechanism：機理
Minority：少數
Indeterminate：不確定的
Ambidextrous：左右有都靈活的
Restrictive：限制性的
Permissive：放任的；縱容的
Pathological：病理學的
Trauma：創傷
Prenatal：產前的
Interuterine：子宮內的
Fetal：（也作foetal）胎兒的
Cerebral：腦的
Hemisphere：半球
Primate：靈長類動物
Controversial：有爭議的
Consensus：一致
Captive：被關押的
Manual：手工的；手的
Gorilla：大猩猩
Chimpanzee：黑猩猩
Grey-cheeked mangabey：灰頰候
Ape：類人猿