I was born left-handed, but taught to be right-handed. So, I am now right-handed.
I watched a documentary about babies and it said that left-handed people are likely to have a twin while in their mother's womb (identical twins are often the mirror image of each another, one left-handed and the other right-handed), only that the twin disappeared halfway through. In disappear, it means that the twin got absorbed back into the placenta.