system
Given the following post, respond as if you're commenting on Reddit:
human
Why are most people right-handed?

Is there any instinctual reason for this?
gpt
The reason we have a dominant hand rather than being all ambidextrous seems to be that it saves brain space to only have fine-motor-control in one hemisphere of the brain, rather than needing two. Most tasks our ancestors needed to do could be performed with one dominant hand just as easily as with two dominant hands, so it gave an adaptive advantage.

So why the right hand? If we look at the genetics of it, then we see that there isn't really a 'gene' for left-handedness. Of course, there isn't just one gene governing these things, but lets simplify it for now and say there is just one gene that governs these things. It has two forms:

C is the natural form of the gene that came about first. It doesn't really do anything, and the whole way the brain is organised is left up to the environment and chance (hence the c). It appears recessive, but it may be slightly codominant so I've put a capital letter.

D gives a specific mental schema for the brain. It tells us which hemisphere all the brain functions should go in. This particular schema works well, and so has an adaptive advantage. It also involves putting the fine motor control in the left hemisphere which controls the right hand (d for dexter).

So a person can be DD (right handed), DC (right handed) or CC (either right or left-handed at about a 50% chance). This accounts for the fact that two righties can give birth to a leftie, and two lefties can give birth to a rightie, but it still seems to run in families. It also accounts for why identical twins can have different dominant hands.

If we look at the proportion of left-handers among people of high intelligence, then we find it's about 20% as opposed to 10% in the general population. However, this is more or less balanced by the fact that about 20% of people of retarded intelligence are left-handed too. Having a CC gene means you are more likely to be an outlier on either side because your brain is less normal, and this could work either well or badly. Example: lefties have higher rates of schizophrenia.

But expression of the D gene clearly gives an adaptive advantage to an individual, so why hasn't the C gene been wiped out?

The answer is that in a right handed society where people fight each other, left-handers have advantages. Take playing tennis. A right-handed person normally plays right handed people, and his technique is built around this. A left-handed person also normally plays right handed people and does the same. So when a rightie plays a leftie, the leftie is playing in her comfort zone whereas the rightie is playing out of his. All other things being equal, the leftie will win. Now apply this to physical combat and you'll see that this gives lefties an advantage in a right-handed society (swap the hands and it would be the other way around). This is enough to preserve the C allele.

We can test this by looking at more tribal societies that have higher levels of instability, fighting and combat. They do have a higher percentage of left handers that does not seem to be based on geographical location.

But nobody has isolated the D/C gene, and it's likely it's actually a large collection, so all these are our best guesses more than gospel truth.