While cats don't write or perform other activities that might make it obvious whether they favor one paw or another, a team of psychological researchers from Northern Ireland's Queen's University Belfast suggest that a cat's gender may determine whether it is a righty or a lefty. According to this research, female cats tend to favor the use of their right paw, while male cats tend to use their left paw more often than their right. This preference may only become apparent when the cat in question is trying to accomplish an agile task with its paw, including using its paws to grab something it can't get at with its teeth.
This research suggests that cats may use any paw for normal tasks that don't require particular dexterity such as scratching a post or batting at a toy. It is only for particularly delicate tasks that cats tend to display their gender-specific handedness. In the study, the researchers put a chunk of tuna in a jar that was too small to fit a cat's head. They then observed which paw the cat used to get at the tasty treat and, using this information, were able to determine the gender split in cat handedness.