Reasons taken from wikipedia: 1. A person can be "compensatory" left-handed due to some kind of damage to the brain, more often to his left hemisphere. The activity of the right hand is mainly regulated by the left hemisphere, and therefore, in the event of injury (may be birth) or illness, the right hemisphere can take over the corresponding functions. 2. Testosterone: One theory is that exposure of a fetus to high doses of testosterone before birth could result in a left-handed baby. This is the Geschwind theory, named after the neuropathologist Norman Geschwind, who developed it. The theory continues to evolve to prove the link between higher testosterone levels and the consequent predominance of the right hemisphere with autoimmune disorders such as dyslexia and stuttering. Dyslexia is four times more common in men than in women and appears to be associated with left-handedness. As a speech therapist, even if a left-handed person was born without stuttering, retraining can lead to it. 3. Ultrasound Theory: A popular theory that ultrasound examinations can affect the brains of unborn children. The theory has very limited application (only in humans and only in developed countries in the last few decades) and cannot explain or predict anything. 4. V. A. Geodakyan's evolutionary theory of asymmetry: right-handedness and left-handedness are not pathologies, but normal, adaptive phenotypes for a stable and changeable environment, regulating the behavioral plasticity of society. 5. Archaeological evidence suggests that after initially using both hands the same way (symmetry), an asymmetry occurred in favor of using the right. 6. In 2008, British scientists found the "left-handed gene". The LRRTM1 gene plays a key role in the formation of speech and emotions. 7. In 2012, Canadian researcher S. Coren found that those who prefer left-hand writing are more likely to be born in late marriages than to young parents.
I am the only left-handed person in my family on both sides. When, as a child, I began to take a spoon in my left hand, my parents did not shift it to their right and swear at me, for which I am very grateful to them. At the university, at a streaming lecture, it turned out that I am the only one out of 100 people "left-handed in all respects" and still the only one in the family, the teacher suggested a violation of intrauterine development, although I have no more "deviations". Wikipedia goes on to say that "left-handers are most common among identical twins, sexually challenged people, and certain groups of people with neurological disorders such as epilepsy, Down's syndrome, autism, mental retardation and dyslexia." As a defectologist, I will note that if a child has the above violations, then left-handedness can be caused by concomitant pathologies of the brain, and it will be a drop in the sea of everything else and no obvious inconvenienceswill deliver. The fact that left-handed people still live and, after the abolition of retraining, are increasing in numbers does not allow us to say that left-handedness is one of the criteria of natural selection. But there is artificial selection that seriously prevents us from living. About 2,500 left-handed people die each year from using right-handed items. The world is really intended for right-handers, but manufacturers of goods have recently begun to take this factor into account: pens, rulers, computer mice for left-handers are actively entering the market. But most of all, the question: "How do you write with your left hand like this?" I'm also wondering how you write right? ;)
Left-handed or right-handed is a tendency towards logical or abstract operations in the mind. This is due to the strength of the planets in the natal horoscope chart. A good astrologer can point this out. There is no selection here. Many people are prone to doing most of the work with their left hand.