Although there is no conclusive proof as of 2014, scientists have a number of theories as to why 10 percent of the world's population are left-handed. Evidence points to many genes, responsible for the development of asymmetry in the human body, as causing left-hand dominance.
The gene PCSK6 is one such responsible for handedness development, as explained in a research paper published in PLOS Genetics in September 2013. The gene is not solely responsible, however, and more research is needed to understand the subtle genetic traits required for left-handedness.
One other common theory regarding left-handedness is in how the brain divides tasks between hemispheres. In right-handed people, speech and hand dominance have a high correlation in the left hemisphere of the brain, while in left-handed people the correlation is less significant.