People being left handed or right handed is generally caused by a pattern of behaviour that they were used to when they were children, like for some children their parents who are generally right handed may make it like a norm or a custom to place things and materials in the hands of a child and when a child grows accustomed to using things like pencil crayons or even holding things with their right hand which is being imbibed on them by their parents they adapt to the practice which on the long-run become part of them.
However there is a strange thing if a child is exposed to using two hands that is the both right hand and left hand then the child may end up being two handed instead of either left handed or right handed.
The body of a child or the the mentality and mind of a child is built to adapt and learn behavioral tendencies from their parents and if a child's parent is right-handed it will be difficult for that child to be left handed because the parent will pass onto them the right hand trait
Some children tend to naturally blend to a type of hand from a young age but their parents seem to always correct this like for example if a child hold a pencil in a left hand if the parents doesn't pay cognizance to this change the child may become left handed for life because it was a known he adopted in serendipity but because he was not actually stopped by the parents or affected by the parents use of hand then the child may go on to be left handed even when the parent is right handed.
But in situations where a parents controls the usage of hand on their children they may affect the way the child picks up things with their hands so generally a parent determines 95% chance in determining what hand a child will adapt to while growing up for example I am left handed but my parents are right handed this is because they did not pay attention to my adaptation of using left hand in fact they encouraged it because they kept holding my left hand because they saw that it was what I embraced as a child so I grew up left handed with my right hand totally being useless in picking up things and actually doing things and even I use my left hand in writing so parents are the main factor that contributes to the fact that people are left or right handed