对《左手人的产生》用英文重写了一下，由于英文逻辑性较强、资料多。这一写感觉基本明白了左撇子的产生原因。希望读者能读英文，后面有中文版。
Humans have a large complex head and a pair of dexterous hands, but almost everyone has a preference for use one of the hands. However, left-handedness is far less common than right-handedness. Today approximately 10% of the world population are left-handers. What is puzzling so far is that left-handed people seem to have nothing to do with genetic and environmental influences, or that science has so far failed to explain what causes some people to be left-handed.
Although as a minority, there have been a lot more left handed famous people than you would expect. Super geniuses left-handers include Albert Einstein, Charles Darwin, Benjamin Franklin and Sir Isaac Newton. Great business leaders like Henry Ford, Microsoft President Bill Gates, oil magnate John D. Rockefeller, Facebook founder Mark Zuckerberg, Alan Turing (founder of computer science), are examples of just a few. Five of the last eight American presidents were lefties: Gerald Ford, Ronald Reagan, George H.W. Bush, Bill Clinton and Barack Obama.
On the other hand, left-handed people have more health problems, including depression, schizophrenia, alcoholism, and so on (1), and they have shorter life.
Moreover, lefties have the following significant characteristics:
1. There are obviously more left-handed men than women. Most recent statistics says 1.23:1, and the data was about 2:1 decades ago. The number of left-handed women seems to be rising in modern society. The percentage of left-handers change related to age, culture, ethnicity, geography, etc. For example, the ratio of lefties is slightly higher in white people. Recent survey shows that the top three countries with more lefties are: Netherlands, 13.23%; USA, 13.1%; UK, 12.24%. China is one of the countries with lowest percentage of lefties, and three sources said that lefties in China to be 1%, 2%, and 3.5%, respectively, depending on how you define lefties.
2. Although the proportion of left-handed people in the world is about 10%, the data from different sources in history were varied greatly, ranging from 1% to 27%. Even in the same ethnic culture, the proportion of left-handed people born in war-torn areas is higher than in the peaceful area.
3. For the arm-based sports, the left-handers often have advantages, including table tennis, badminton, tennis, fencing, boxing, etc. For example, 40% of top tennis players were left-handers. On the other hand, in non-arm-based sports, there are no significant differences in handedness.
All these characteristics can only be explained according to the Yin and Yang theory. The basic knowledge of Yin/Yang is as follows:
Yang: left, upper, male, warm, motile, energetic, creative, etc.
Yin: right, lower, female, cool, peaceful, quiet, conservative, etc.
Moreover, the balance between Yin and Yang is the foundation of health.
Therefore, the cause of left-handedness can be explained as: during the fetal period, if Yang of the upper limbs of the fetus is excess over Yin, this will lead to better development of the left hand over the right hand in the fetus.
The theory provides perfect explanation to the facts about left-handedness. Because men possess slightly more Yang than women, the proportion of left-handed men is higher than women. Also because of the excess Yang in their upper limbs, left-handed are superior in upper limb sports. When there is more Yang in the environment, e.g., a turbulent environment that promotes the increase of Yang, a higher birth rate of lefties would occur. Interestingly, Dutch people are the tallest in the world, suggesting a higher level of Yang in Dutch population, and this is reflected with highest percentage of left-handed Dutch in the world. On the other hand, China, with a conservative culture, has low rate of left-handers.
(This means that the percentage of lefties in a country is a very reliable index of Yang Qi in that region.)
The theory also explains other phenomena observed in left-handedness. Since Yang is more volatile, left-handed people tend to have stronger creativity; also because of slight deviations in Yin-Yang balance, left-handed people have more health problems.
The Yin and Yang in every organism are largely balanced, including balance of upper and lower, left and right. Because humans have a huge complex brain, the Yang in the head is always in excess, so the limbs possess slightly more Yin, resulting in the right hand to be common. In addition, the left and right sides of a person tend to balance in Yin/Yang. Since the heart (more Yang) is on the left, the use of right hand will resulted in better balance. In most animals, the Yin and Yang of the whole body are more balanced. In cats, dogs, and horses, it was found that male animals tend to use the left forelimb first, while female animals use the right forelimb first (2). These observations fit the theory of Yin/Yang perfectly.
According to the theory, it is speculated that there may be three types of left-handed people as explained as follows:
1. Yang excess, that is, the individuals have too much Yang. Typical appearances of these people are: red face, high voice, preferring cool than warm. Their physical performance is exuberant and the spirit is flourishing.
2. Yin deficiency. They appear to be drier, lack of Yin liquid in the body, thin build, weak stomach, and not able to endure hot temperature. They are more susceptible to labor, more likely to have nocturnal emission, excessive anxiety, etc.
3. The upper limbs are imbalance in Yin/Yang, and have more Yang than Yin. This is only a hypothesis, and may be tested by scientific methods. If there is an instrument that can accurately measure the temperature of different parts of the body, the relative temperature of the upper limbs of such people may show slightly higher than that of the right-handed.
The handedness is likely developed in the womb (3), influenced by Yin/Yang fluctuation in that little compartment, which is determined by the mother’s environment, diet, and emotion. The balance of Yin and Yang can be changed after birth due to the environment, but strong left-handed people will be continuing using their left hand, and some weak lefties may be “corrected” to righties, especially in old times and in some more conservative societies. Therefore, the above-mentioned three types should exist in fetuses, but are likely present in a high proportion of left-handed people.
Genes are unlikely involved directly in generation of left-hander (4). However, some genes may have indirect effects as suggested by this hypothesis. For example, some DNA mutations may contribute to Yin deficiency, resulting in a large percentage of these mutation carriers to develop Yin deficiency, and among these Yin deficient people, a significant proportion would develop as lefties.
The great significance of the theory is that it guides practice. If the health status of a person is related to the left-handedness, then adjusting the Yin and Yang is an important part of improving the health of the patient. For example, most left-handed people have higher sexual desires. Both Yang excess and Yin deficiency will result in an excess of kidney Yang over kidney Yin, leading to easily erection and impulsivity. Persons with Yang excess often seek women everywhere, which can be adjusted through reducing their Yang. Congenital Yin deficiency people are lean. They must pay more attention to conserve their Yin, and absolutely cannot indulge. Also, those left-handed with health problem may have more difference in temperature in their different body parts, as they may have greater imbalance in Yin and Yang within their body. Promoting the Qi movement in their bodies to even the temperature would improve their health.
References
1. Goodman, Joshua (2014). "The Wages of Sinistrality: Handedness, Brain Structure, and Human Capital Accumulation". Journal of Economic Perspectives. 28 (4): 193–212.
2. Callaway, Ewen (2009). "Is your cat left or right pawed?". New Scientist. Retrieved 6 November 2016.
3. Hepper, Peter G. (2005). "Prenatal thumb sucking is related to postnatal handedness". Neuropsychologia. 43 (3): 313–315.
4. de Kovel, C., & Francks, C. (2019). “The molecular genetics of hand preference revisited”. Scientific reports, 9(1), 5986.
左手人的产生原因
人类的主要特点，是有个聪明的大脑袋和一双灵巧的手。但几乎所有的人都会偏用其中的一只手，而且其中只有～10％的人天生的偏用左手。至今令人困惑不解的是，左手人似乎与遗传和环境的影响没什么关系，或说，科学至今无法解释左手人是如何产生的。
虽然只占10％，但著名的左手人所占比例却偏高很多，如超级天才爱因斯坦，达尔文，富兰克林，牛顿，艾伦图灵（人工智能之父）；工商业巨头福特，比尔盖茨，洛克菲勒，扎克伯格；前八位美国总统中，有五位是左手人，福特，里根，布什，克林顿，欧巴马。另一方面，左手人的健康问题则多一些，包括郁抑、精神分裂、酗酒、等；左手人的寿命较短。
左手人有以下明显的特点：
一、男女的比例，左手男明显多于左手女，现代统计的结果是1.23:1，而几十年前资料说是约2：1。左手女的比例在现代化社会中似乎在升高。不同的数据与年代、文化、种族、地域等都有关，如白人中左手人比例略高。最新的统计资料显示，左撇子最多的三个国家为：荷兰13.23%；美国13.1%；英国12.24%。而中国则是左手人比例最低的国家之一，三个不同来源的资料分别为：1％，2％，3.5％，取决于左手人的不同定义。
二、虽说现在世界上左手人的比例大约10％，但历史上不同来源资料有很大差异，从1％到27％，即便同一种族文化的人群，在战乱地区，左手人出生的比例较高，而在安定地区，左手人的比例则较低。
三、凡以手臂为主的运动项目，左手人常显优势，包括乒乓球、羽毛球、网球、击剑、拳击、等等，如网球的世界高手中，左手人占40％；而其它非手臂为主的运动项目，则无明显差异。
这些特点只有根据陰陽理论可以得到全部答案，左陽右陰，男偏陽女偏陰，动则生陽，春夏间陽气多，这些是陰陽理论的基本知识。并且，陰陽平衡是健康的基础。
因此，左手人的产生，根据陰陽理论可以作如下解释：在胎儿发育期间，如果胎儿上肢气血的陽多于陰，则导致这些胎儿的左手发育优于右手。
因为男性比女性的陽气略盛，故左手男的比例较高；同样由于其上肢的陽气多，左手人在上肢运动的项目占优。动荡的环境，会促进陽气的增加，导致左手人的出生率增加。值得注意的是，荷兰人的平均身高世界第一，荷兰人的左手人比率也是世界第一，两者都与陽气有关。另一方面，中国为一个偏保守的国家，也反映在左撇子比率很低的数字上。
（这意味着一个国家／地区的左撇子比例，非常可靠的反映了该国的陽气水平，表现在该国的活力、创新力、国人男子的陽刚气、等）
该理论同时也解释了所有其它现象：由于陽气是功能性的，故左手人往往创造力较强；同样由于陰陽平衡略有偏差，导致左手人的健康问题较多。
每一个生物体的陰陽量大体都是平衡的，包括上下平衡，左右平衡。由于人类有个复杂的大脑，因而头部的陽气总是偏多，而四肢便会略偏陰，可能是导致右手人多的主要原因。另外，一个人身子的左右也会倾向于陰陽平衡，因为心脏（陽性）偏左，右手人便会左右比较平衡。其它动物的全身陰陽会较平衡，在猫、狗、和马中，人们发现雄性动物多会先使用左前肢，而雌性动物多会先使用右前肢，至今也是唯有陰陽理论才能解释。
根据这个理论，推测左手人的产生可能有三类：一是局部性的，即只有上肢陽气偏多；二和三是全身性的，陽盛，或陰虚。以下分别给予说明：
一、陽盛，即个体陽气偏多，表现有“面赤时烦，声高气粗，喜凉怕热。”身体性能亢奋，精神茂盛。
二、陰虚，体内陰液不足，形体消瘦，胃弱，五心烦热（心和四肢手脚心），不耐高温。易患虚劳、失精、不寐、便秘、闭经、忧思过度、房事不节。
三、上肢陽气偏多，这还只是个假说，或许能用科学方法证明。如果有一种仪器，能非常精确地测出身体不同部位的温度，那么这类人上肢的相对温度，可能会比右手人上肢的相对温度略微高一点点。
左手人是天生的，那么便是胎儿在发育过程中出现上述情况。出生后由于饮食和环境等的影响，陰陽平衡可以有所改变，但习惯性左手一般就会继续使用左手，故成年的左手人不一定有上述情况，只是比例会偏多很多。实际上，对胎儿作仔细超声波检查，可以看到多数胎儿会较多的活动右手，吸右手拇指；而少数则相反，这少数胎儿便会发育为左手人。
基因分析至今并未找到与左手有关的遗传原因。但如果根据上述假说，应该先将左手人分类。例如：对于阴虚者，会有可能找到有关基因突变；但这些基因的携带者可能也只有部分发育成阴虚个体，而这些个体又可能比正常人有较高比例发育成左手人。
理论的重大意义是可以指导实践，如果左手人的健康状况与其左手有关，那么调整陰陽就是改善患者健康的重要一环。例如，不少左手人性欲偏高，陽盛者和陰虚者都由于肾陽偏多，与肾陰比例失调，导致易勃起和冲动。陽盛者往往到处寻求欲女（如克林顿、拿破仑），或许可以通过调理，适当泄点肾陽。天生陰虚者精瘦，必需常注意补陰，绝对不能纵欲。
