Handedness is genetic.

Right handedness is dominant over left, which is why left handedness is so rare.

Basically if a person has one gene for right handedness and one gene for left handedness they will be right handed. But they are able to pass the left handed gene to their children. A person would need two left handed genes to be left handed.

That's a very simplified version of the way it works... other things that are genetic, apparently, are which way you cross your arms, and which hand goes on top when you clasp your hands together.

Just some of the random things I remember from my days as a science teacher!

BW