For Those Of Us Who Use The Wrong Hand

The Bad

Statistics show left-handed people are more likely to be schizophrenic, alcoholic, delinquent, dyslexic, and have Crohn's disease and ulcerative colitis, as well as mental disabilities. They're also more likely to die young and get into accidents. So if evolutionary theory dictates survival of the fittest, why do lefties still exist?
http://abcnews.go.com/Technology/story?id=498707&page=1

Left-handedness has been associated with a wide range of indicators of reduced fitness, from the standpoint of natural selection. Left-handed people, the authors say, have a smaller number of offspring, higher number of spontaneous abortions, lower birth weight, higher number of serious accidents, higher rates of serious disorders, and a shorter life span. Left-handedness has similarly been linked to neural tube defects, autism, stuttering, and schizophrenia.
http://www.narth.com/docs/toxins.html

Research has suggested that left-handed people are more susceptible to a range of problems, including allergies, auto-immune diseases, depression, drug abuse, epilepsy, schizophrenia and sleeping disorders. Left-handers are thought to have poorer spatial skills, and thus to be more vulnerable to car crashes and other serious accidents.
http://news.bbc.co.uk/1/hi/health/4242419.stm

A Dutch team found left-handed women were more than twice as likely to develop premenopausal breast cancer as non-left handed women.

http://news.bbc.co.uk/1/hi/health/4274994.stm

The Good

Then again, as many lefties might point out, being left-handed can also offer intellectual prowess. Tests conducted by Alan Searleman from St Lawrence University in New York found there were more left-handed people with IQs over 140 than right-handed people. Famous left-handed thinkers in history from Albert Einstein to Isaac Newton to Benjamin Franklin seem to underline the point.
http://abcnews.go.com/Technology/story?id=498707&page=3

While there is no suggestion that left-handed people are more violent than the right-handed, it looks as though they are more successfully violent.
http://www.economist.com/science/displayStory.cfm?story_id=3471297

http://news.bbc.co.uk/1/hi/health/4073775.stm

The Indifferent

Analyzing a number of studies, their paper concludes that male homosexuals are about one third (31%) more likely than heterosexuals to be left-handed (2), while lesbians are almost twice as likely (91%) to be left-handed as heterosexual women.
http://www.narth.com/docs/lefthand.html

The fraction of left-handed people today is about the same as it was during the Ice Age, according to data from prehistoric handprints....In the general population today about 12% are left-handed, though populations vary considerably, between 3 and 30%.

http://news.bbc.co.uk/1/hi/sci/tech/3485967.stm

Conclusion

It seems there are two different theories as to why some of us are still left-handed. One theory is that we are gifted with intuition and intelligence. The other is that we are gifted at violence.

I don't think the genetic factors are well understood. Perhaps the reason why you don't have an entirely right-handed society is the same reason you can't create a 'race' of blond-haired blue-eyed people; the way genes interact fortunately doesn't allow for this.

In the argument for the violent theory, it was mentioned that a particular ethnic group in Burkino Faso had a very low incident of violence, and only three percent of this population was left-handed. There is a problem with this, in that it is a culture where your right hand is used for sanitary things, like eating, and your left hand is used for 'dirty' things, like wiping your butt. I think this would tend to produce more 'right-handed' people. I could do an entire blog, just as long as this one is turning out to be, on cultural/religious bias against lefties.

Left-handed people are over-represented among the intelliectually elite. But we are also over-represented among the intellectually challenged. This tends to balance things out.