A study investigating the number of left-handed people in tribal societies has found that the more violent the society, the higher the number of left-handers in the population. The researchers speculate that this is due to left-handers having an advantage in hand-to-hand combat, as shown by the higher number of left-handed champions in sports like boxing and fencing.
Some researchers have linked left-handedness to neurobiological stress during the early stages of brain development, so it has been a puzzle why left-handedness remains a common trait in the population, when this sort of biological stress has been linked to other, less advantageous traits, such as higher rates of nervous system and immune system disorders.
The advantage in combat may be one way (among, potentially, many others, including better non-verbal intelligence) in which left-handers have an edge on their right-handed peers.
Link to story on nature.com
Mind Hacks 
Hey Vaughan.. I found your posts really interesting! 🙂
cant it be becuse the mother is put under stress during pregnancy so the child becomes left-handed?