What causes children to be left handed when neither the parents nor the grandparents are?
There are many different theories regarding left handedness. Approximately 10-13% of the population is left handed. In addition, males are more likely to be left handed than females.
One evolutionary theory states that this is related to the warrior and his shield. The heart is on the left hand side of the body so it would make sense to use the left hand to hold the shield to protect the heart, therefore making the left hand the dominant one.
Another theory is the brain hemisphere division of labor. This theory is perhaps the most accepted theory. Normally, one side of the brain is used for speaking and the other half for writing. Having one side of the brain responsible for all fine motor skills would be more efficient than dividing it up between both hemispheres.
Genetics may have something to do with left handedness, however, even if both parents are left handed, the chances that their will be left handed is approximately 26%.
There are many theories on what impacts the development of handedness in children. One theory proposed by Norman Geschwind suggested that exposure to high levels of testosterone in-utero impacted the handedness of the child. The Gerschwind theory suggests that high levels of testosterone leads to left handedness. This is because testosterone suppresses the growth of the left cerebral hemisphere. Due to this, more neurons traveled to the right hemisphere. The better developed right hemisphere then takes over as the center for language and handedness. Since the right side of the brain controls the left side of the body, the child becomes left hand dominant.