Left-handed people are at slightly increased risk of a long list of diseases and disabilities, and so the question is why left-handed people still exist as about 10% of the population. The Forey Raymond hypothesis stated that those who were left-handed had one advantage which outweighed these negatives; they were better at fighting. When confronted with a southpaw, one has to change the way they defend and attack. However, the proportion of left-handedness does not appear to correlate strongly with the level of violence in society. A riffing theory posited that we know left-handed people are over-represented in sports, and so perhaps athletes have a higher number of children.
An entirely alternative explanation does away with all of this, focusing instead on the brain’s hemispheres. The evolutionary selection of language and motor control emphasised the left hemisphere in the brain, bringing about a preference for right-handedness (left maps to motor control on the right side). That means that either the left-handers will decrease in number over the next many millennia. Though, game-theory suggests they will exist as around one-tenth of the population. Within an environment of majority right-handers, the left-handers prosper and counterbalance their deficiencies.