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.

为什么大多数人是右撇子？

在“利手”现象被研究的160年的时间里，我们知道了很多的相关知识，但是我们仍然不能准确地说明导致人类倾向于右手或者左右的原因，也不能说明为什么人倾向于使用右手而不是左右。

科学家们对于人群中右撇子以及左撇子所占的百分比持有不同的意见，这是因为目前对于测量“利手”没有一个标准的、经验的定义；我们的标准变化不一，而且这些标准基于各种各样的理论解释，这是因为我们仍然试图理解其中的机理。但是对于我们确实已知的东西，我能概括性地描述出来。

大多数的人（大概在70%到95%）是右撇子，少部分人（大概在5%到30%）是左撇子，也有一些不确定数目的人可以被认为是左右手都灵活的人。似乎全世界任何一个地方的人口都是如此。有证据表明基因能影响“利手”；不过这是非孟德尔遗传，基因学家不能对准确的遗传过程达成一致。也有证据表明社会和文化体制能影响（以及改变）“利手”。比如，老师就曾强迫儿童们在书写中从使用左手转为使用右手。相比与那些更加宽容的社会，在一些限制更多的社会中，使用左手的人要少一些。

一些研究者辩论说，有证据表明一些“病理学上的”左撇子与出生时候的脑创伤有关。许多研究者追溯“利手”的原因到产前子宫内发展的过程，以及追溯到胎儿的大脑开始形成明显的大脑半球的时候。在19世纪60年代，法国外科医生Paul Broca注意到“右利手”和左大脑半球专门负责语言能力的关系。但是这种手--脑的联系既不是一个简单的相关，也不是一个可靠的相关。在20世纪70年代进行的研究表明，大多数的左撇子具有同样的负责典型为人类所拥有的语言的左大脑半球----仅仅只有一少部分左撇子其专门负责语言的大脑模式不一样。

所以基本点就是：对于人口中“右利手”的原因，我们有一个好的总体的观点，但是我们还需要找出精确的细节，包括为什么“利手”是右手而不是左手。

第二个问题(非人类的灵长目动物也表现出“利手”吗？)目前是一个有争议的问题。个体动物是右撇子还是左撇子与整个动物种群中大多数动物是左撇子或者右撇子之间的区别需要重点注意。一些个体动物表现出使用某一只手的倾向也并非不寻常。但是其他任何非人类物种是否也在物种层面表现出和人类相同的“利手”现象，对此研究者们还没有一致的看法。

有少数几个研究者认为有这样的非人类物种存在，但是在这些工作的大部分之中，动物所处的环境是在实验室或者在笼子里，这些动物进行的手工任务与动物在野外如何使用它们的手有非常大的不同。

除了研究人类的“利手”外，我也研究过山地大猩猩（在卢旺达）使用手的情况，还研究了黑猩猩、红疣猴、红尾猴以及灰颊候（在乌干达）使用手的情况。我自己的研究表明，猴个体以及类人猿个体对于手工任务经常发展出各自的用手倾向（既有右手也有左手），但是我还没有证据表明在这些动物的种群层面有“利手”的现象，就如在人类中看到的一样。

Vocabulary:

Handedness：用右手或左手的习惯；也称“利手”
Precisely：精确地
Preferentially：有倾向性的
Empirical：经验的
Criteria：标准
Theoretical：理论的
Mechanism：机理
Minority：少数
Indeterminate：不确定的
Ambidextrous：左右有都灵活的
Restrictive：限制性的
Permissive：放任的；纵容的
Pathological：病理学的
Trauma：创伤
Prenatal：产前的
Interuterine：子宫内的
Fetal：（也作foetal）胎儿的
Cerebral：脑的
Hemisphere：半球
Primate：灵长类动物
Controversial：有争议的
Consensus：一致
Captive：被关押的
Manual：手工的；手的
Gorilla：大猩猩
Chimpanzee：黑猩猩
Grey-cheeked mangabey：灰颊候
Ape：类人猿