system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are people either Righties or Lefties and not all ambidextrous? What makes us either a Righty or a Leftie?
gpt
Some scientists think it has to do with how our brains divide up the labor, such as since most people use left side of the brain to do fine motor skills (aka speaking and handiwork), and left hemisphere of the brain control right side of the body, most of us are right handed.

of course, there's also genetic contribution to this. if both parents of a child are left-handed, there is a 26% chance of that child being left-handed.

There are other theories out there, all with very good evidences, but the top two i mentioned are the most popular one.

Source:

> Banich, Marie (1997). Neuropsychology: The Neural Bases of Mental Function.

> McManus, Chris (2003). Right Hand, Left Hand. Phoenix Paperbacks. ISBN 0753813556.