Why are some people left-handed? – Daniel M. Abrams

Whether left- and right-handed people display such behavioral differences has yet to be determined. However, some fascinating studies show that when people are asked to repeatedly contract muscles of their left hand before telling a story, they tell a story with more negative content than those who tell a story after contracting muscles in their right hand. These findings suggest that hand preference in people may influence some aspects of behavior. 

Other studies have shown that people with weaker hand preference are more prone to depression, dyslexia, and schizophrenia. In animals too, it is not just the direction of hand preference but its strength that is associated with particular patterns of behavior. More research is needed to clarify these relationships.