Left handedness & Right Handedness:

Advantages of left-handedness:
Some studies have shown that "...left-handers also tend to have unusually good visual-spatial skills and the ability to imagine spatial layouts."Santrock goes on to point out that mathematicians, musicians, architects, and artists are more commonly left-handers than would be expected."Also, in one study of more than 100,000 students taking the Scholastic Aptitude Test (SAT), 20% of the top-scoring group was left-handed, twice the rate of left-handedness found in the general population (10%).Left-handedness may also reduce the risk of developing arthritis.However, Hardyck and Petrinovich reviewed a large literature and found no overall differences in mental ability in right-handers and non-right-handers.
Other reported associations include with higher creativity, a larger corpus callosum, certain right brain regions being larger, shorter transfer time of information between hemispheres, and higher socioeconomic status.

Disadvantages of left-handedness:

Although there is little association with children's school performance in regards to handedness, some studies have shown problems in language development in left-handers.Research has shown left-handers are more likely to have problems with reading and they also "...don't do as well on phonology (the sound system of language) tasks..." when compared with right-hander. Also, in left-to-right languages, if left-handed writers move their left hands across what they have just written, they may smudge their writing. This situation is reversed in right-to-left languages such as Hebrew and Arabic.
Other reported associations that may have decreased evolutionary fitness include shorter adult height, lower weight, puberty at a later age, possibly a shorter life expectancy, increased risk of accidents, increased risk of certain neurological and immunological disorders, and decreased number of children.