I’m left handed. I have been since as long as I can remember. I started writing with my left hand. However, the idea that a left-handed person is left-handed at birth and will remain left-handed for the rest of his life has bothered me for several reasons.
For one thing, I'm only left-handed in writing and eating (with utensils). I throw with my right hand, I use a computer with the mouse on the right and keyboard on the left (just like nearly everybody else), I swing bats and clubs with a right-handed grip (though I rarely play baseball or golf), and I do just about everything else with my right hand. This is not really a rare thing, but I think it proves something about the way people adapt.
For the cases of extreme handedness, in which people do nearly everything with a selected hand, I believe that I can supply a hypothesis, but I can't be sure that there is no predetermination at all. But my hypothesis is that people who are explicitly handed probably learned some motor skill when they were very young, probably still babies, and then they learned another skill with the same hand and continued using one hand for nearly every task that they had to complete. I use this hypothesis because I have noticed that when I newly learn certain skills, my dexterity for those skills is somewhat predetermined if I have learned other skills that are similar to the one that I am currently learning, but if the new skill is completely unlike anything I have ever done, I am not sure which hand to use.
I'd like to extend this hypothesis to point out a fallacy: many people might say that the first time they ever tried something, it seemed completely natural for them to use a specific hand for it. This may be true, and there may be some predetermination involved before they further develop their motor skills. I can never prove to anyone that there is or isn't, but I can hypothesize that there isn't. I believe that when someone attempts a new activity for which it seems natural to use one hand, this is because they have developed motor skills for that type of movement from some other skill, so they have better control with one hand. I can point out an example in my life: eating with my left hand. I don't remember exactly when I learned to eat with utensils, but it might have been when I was about three or four. This was before I learned to write. I learned to write in kindergarten. I don't remember exactly how it happened, but it seems that I had developed a feel for how to hold a pencil unconsciously, but when asked how I held it, I didn't know which hand I used. I eventually figured out that I was left handed. My explanation for this is that my development of motor skill as a result of learning to eat with my left hand led me to write with my right hand.
Although I cannot prove my hypothesis about handedness, I am still figuring out the dexterity of my own hands (and even feet) through drumming. Recently, I have started learning new styles of drumming which include the active and precise movement of the right and left feet in coordination and also precise control of a see-saw motion of the sticks. I currently have little to no experience with that latter, so this should be fun to figure out. More importantly, though, I will be learning to write with my right hand. I predict that if I practice continuously for a matter of years, my writing might come to equal that of my left hand. We’ll see how it turns out.
(Also, feel free to draw whatever symbolic conclusions about life that you can from this. Predestination, freedom, nature vs. nurture, etc. It's fun.)