Natalie Wolchover is a scientist turned science writer who blogs over at Facto Diem. Here's part of her post on handedness:

What causes the brain to sometimes switch up or mix around? There is no clear consensus on the matter, just a lot of interesting hypotheses. First of all, left-handedness is barely genetic. A child of two left-handed parents has only a 26% chance of being left-handed: higher than the 1-in-10 incidence in the general population, but not that much higher. Some scientists think left-handedness is caused by more testosterone than usual flooding the fetus at a critical moment during gestation. This would explain the higher incidence of left-handedness in males than females, and it could also relate to the putative evidence of a correlation between handedness and sexual orientation. However, the way in which testosterone influences brain lateralization isn't established.