對《左手人的產生》用英文重寫了一下，由於英文邏輯性較強、資料多。這一寫感覺基本明白了左撇子的產生原因。希望讀者能讀英文，後麵有中文版。
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.
左手人的產生原因
人類的主要特點，是有個聰明的大腦袋和一雙靈巧的手。但幾乎所有的人都會偏用其中的一隻手，而且其中隻有～10％的人天生的偏用左手。至今令人困惑不解的是，左手人似乎與遺傳和環境的影響沒什麽關係，或說，科學至今無法解釋左手人是如何產生的。
雖然隻占10％，但著名的左手人所占比例卻偏高很多，如超級天才愛因斯坦，達爾文，富蘭克林，牛頓，艾倫圖靈（人工智能之父）；工商業巨頭福特，比爾蓋茨，洛克菲勒，紮克伯格；前八位美國總統中，有五位是左手人，福特，裏根，布什，克林頓，歐巴馬。另一方麵，左手人的健康問題則多一些，包括鬱抑、精神分裂、酗酒、等；左手人的壽命較短。
左手人有以下明顯的特點：
一、男女的比例，左手男明顯多於左手女，現代統計的結果是1.23:1，而幾十年前資料說是約2：1。左手女的比例在現代化社會中似乎在升高。不同的數據與年代、文化、種族、地域等都有關，如白人中左手人比例略高。最新的統計資料顯示，左撇子最多的三個國家為：荷蘭13.23%；美國13.1%；英國12.24%。而中國則是左手人比例最低的國家之一，三個不同來源的資料分別為：1％，2％，3.5％，取決於左手人的不同定義。
二、雖說現在世界上左手人的比例大約10％，但曆史上不同來源資料有很大差異，從1％到27％，即便同一種族文化的人群，在戰亂地區，左手人出生的比例較高，而在安定地區，左手人的比例則較低。
三、凡以手臂為主的運動項目，左手人常顯優勢，包括乒乓球、羽毛球、網球、擊劍、拳擊、等等，如網球的世界高手中，左手人占40％；而其它非手臂為主的運動項目，則無明顯差異。
這些特點隻有根據陰陽理論可以得到全部答案，左陽右陰，男偏陽女偏陰，動則生陽，春夏間陽氣多，這些是陰陽理論的基本知識。並且，陰陽平衡是健康的基礎。
因此，左手人的產生，根據陰陽理論可以作如下解釋：在胎兒發育期間，如果胎兒上肢氣血的陽多於陰，則導致這些胎兒的左手發育優於右手。
因為男性比女性的陽氣略盛，故左手男的比例較高；同樣由於其上肢的陽氣多，左手人在上肢運動的項目占優。動蕩的環境，會促進陽氣的增加，導致左手人的出生率增加。值得注意的是，荷蘭人的平均身高世界第一，荷蘭人的左手人比率也是世界第一，兩者都與陽氣有關。另一方麵，中國為一個偏保守的國家，也反映在左撇子比率很低的數字上。
（這意味著一個國家／地區的左撇子比例，非常可靠的反映了該國的陽氣水平，表現在該國的活力、創新力、國人男子的陽剛氣、等）
該理論同時也解釋了所有其它現象：由於陽氣是功能性的，故左手人往往創造力較強；同樣由於陰陽平衡略有偏差，導致左手人的健康問題較多。
每一個生物體的陰陽量大體都是平衡的，包括上下平衡，左右平衡。由於人類有個複雜的大腦，因而頭部的陽氣總是偏多，而四肢便會略偏陰，可能是導致右手人多的主要原因。另外，一個人身子的左右也會傾向於陰陽平衡，因為心髒（陽性）偏左，右手人便會左右比較平衡。其它動物的全身陰陽會較平衡，在貓、狗、和馬中，人們發現雄性動物多會先使用左前肢，而雌性動物多會先使用右前肢，至今也是唯有陰陽理論才能解釋。
根據這個理論，推測左手人的產生可能有三類：一是局部性的，即隻有上肢陽氣偏多；二和三是全身性的，陽盛，或陰虛。以下分別給予說明：
一、陽盛，即個體陽氣偏多，表現有“麵赤時煩，聲高氣粗，喜涼怕熱。”身體性能亢奮，精神茂盛。
二、陰虛，體內陰液不足，形體消瘦，胃弱，五心煩熱（心和四肢手腳心），不耐高溫。易患虛勞、失精、不寐、便秘、閉經、憂思過度、房事不節。
三、上肢陽氣偏多，這還隻是個假說，或許能用科學方法證明。如果有一種儀器，能非常精確地測出身體不同部位的溫度，那麽這類人上肢的相對溫度，可能會比右手人上肢的相對溫度略微高一點點。
左手人是天生的，那麽便是胎兒在發育過程中出現上述情況。出生後由於飲食和環境等的影響，陰陽平衡可以有所改變，但習慣性左手一般就會繼續使用左手，故成年的左手人不一定有上述情況，隻是比例會偏多很多。實際上，對胎兒作仔細超聲波檢查，可以看到多數胎兒會較多的活動右手，吸右手拇指；而少數則相反，這少數胎兒便會發育為左手人。
基因分析至今並未找到與左手有關的遺傳原因。但如果根據上述假說，應該先將左手人分類。例如：對於陰虛者，會有可能找到有關基因突變；但這些基因的攜帶者可能也隻有部分發育成陰虛個體，而這些個體又可能比正常人有較高比例發育成左手人。
理論的重大意義是可以指導實踐，如果左手人的健康狀況與其左手有關，那麽調整陰陽就是改善患者健康的重要一環。例如，不少左手人性欲偏高，陽盛者和陰虛者都由於腎陽偏多，與腎陰比例失調，導致易勃起和衝動。陽盛者往往到處尋求欲女（如克林頓、拿破侖），或許可以通過調理，適當泄點腎陽。天生陰虛者精瘦，必需常注意補陰，絕對不能縱欲。
