Why are some people left-handed?

The vast majority of people on Earth are right-handed, but 10% of people around the world are in fact left-handed. So why is this, and what do we know about it? Please subscribe for more videos like this!
Attributions:
1. By Chickensaresocute - Own work, CC BY-SA 3.0, commons.wikimedia.org/w/index...
2. By Polygon data were generated by Database Center for Life Science (DBCLS) - Polygon data are from BodyParts3D, CC BY-SA 2.1 jp, commons.wikimedia.org/w/index...
3. By slgckgc on Flickr (Original version)UCinternational (Crop) - Originally posted to Flickr as "Jose Reyes"Cropped by UCinternational, CC BY 2.0, commons.wikimedia.org/w/index...
4. By Marianoc - Own work, CC BY-SA 3.0, commons.wikimedia.org/w/index...
5. By Carlos Zito - Own work, CC BY-SA 3.0, commons.wikimedia.org/w/index...
6. By Senator Stabenow - President Obama Signs the Farm Bill, CC BY 2.0, commons.wikimedia.org/w/index...