Why are people left-handed or right-handed? Because of genetics and environment.
There are four types of handedness. People can be right-handed, left-handed, right-handed for some tasks and left-handed for other tasks, and ambidextrous. The majority of the people in the world are right-handed. The number varies from country to country, but it is generally between 85 and 90% of the population that are right-handed. About 10% are left-handed and about 1% are ambidextrous.
The brain has two different hemispheres and each hemisphere controls the opposite side of the body: the right hemisphere controls the left side of the body and vice versa. If you want to raise your left hand to pick up a cup of coffee, the right side of your brain has to send the signal to do it. Nobody knows the reason why, but it has its origins long long ago in evolution. One theory is that it would give us more chance to survive. If we were attacked on our left side and the brain was damaged, we could still push the attacker back because the right side would send the commands to the arms on the left side. Another theory is that at some point in history, our ancestors’ heads twisted around, twisting up the nerves that send the signals. It could have evolved because the brain requires a huge amount of energy and it might be more efficient to allow the brain to carry out two different functions at the same time. There are other theories, but there is no real way of knowing.
The different sides of the brain are also responsible for different tasks. The left side is responsible for logic, sequencing, linear thinking, mathematics, facts, thinking in words. The right side is responsible for imagination, intuition, holistic thinking, arts, rhythm, nonverbal cues, feelings visualization, and daydreaming. Although, it is not quite as clear cut as that.
So, why are people left-handed or right-handed. Hand preference begins from very early in humans. In the womb, babies suck one thumb more than the other. Researchers have discovered that there are possibly 40 different genes that might dictate hand preference. Each one of the genes is quite weak, but when you put them all together, they become strong enough to affect hand dominance. However, biology is not the end of the story. The home people are brought up in and their culture can have a big impact on which hand becomes dominant.
For many years, people thought it was wrong to be left-handed and children were forced to use their right hands. In fact, the word “sinister” comes from the Latin for “left”. And people who remained left-handed had to survive in a world that was not designed for them. Left-handed people have a higher frequency of work related accidents than right handed people because they have to adapt to use the tools.
So, why did people become right or left-handed? It turns out that one of the reasons is because we walk on two legs. When chimps and primates stand on all fours, they don’t have a preference for either hand. However, when they are made to stand upright, they demonstrate a preference. With chimps though, it is pretty much fifty-fifty which hand they will choose. It appears that once we started to walk on two legs, we began to use one hand more than the other, but it was very much 50-50, as with the chimps. Experts know this from looking at stone tools from two million years ago. The researchers made stone tools using their left or right hands and then compared them to ancient stone tools that had been found. They discovered that there was very little difference in the number of left-handed and right-handed people. This began to change and by 1.5 million years ago the tools are starting to show a bias towards right-handedness. Once we get to Homo heidelbergensis, about 600,000 years ago, there is a very clear preference for the right hand. This can be seen with the tools, but it can also be seen in the teeth. Wearing on the teeth changes depending if the thing being eaten is being held with the left hand or the right hand.
Why would more people be right-handed? One idea is that we developed language in the left sides of our brain, which could have just made it more convenient for the left side to also control the dominant hand. However, nobody knows. It might have just been luck. Once one hand starts to be more dominant in society, there are more disadvantages than advantages to being in the minority, so natural selection could have made the gap wider.
So, why are people left-handed or right-handed? Because of their genes, and their upbringing. Although, nobody knows why there are more right-handers than left-handers. And this is what I learned today.
https://medlineplus.gov/genetics/understanding/traits/handedness/
https://www.livescience.com/what-causes-left-handedness.html
https://en.wikipedia.org/wiki/Handedness
https://www.scienceofpeople.com/why-are-you-right-or-left-handed/
https://www.bbc.com/future/article/20141215-why-are-most-of-us-right-handed
https://www.healthline.com/health/left-brain-vs-right-brain#left-brain-vs-right-brain-myth