I wish to take this opportunity to explain much more about trends in handedness and correlates of handedness.
First, let's look into the causes of left-handedness. A website from Australia gives the best summary I could find (http://www.betterhealth.vic.gov.au/bhcv2/bhcarticles.nsf/pages/Left-handedness). Causes listed include genetics, gender, fetal development, modelling, brain damage (that's right), and adjustment due to injury.
Genetics research suggests that several genes may have an influence on handedness, not surprisingly. So far, however, only one gene which has an effect on handedness has been identified. As reported in Anything Left-Handed (http://www.anythinglefthanded.co.uk/research/left-handed-research-current.html#sthash.eUDaBMwR.dpbs), this gene is known as LRRTM1. (I am guessing that my family has this allele plus a few other ones linked to left-handedness.) However, according to Wikipedia, a large study of twins in 25.732 families indicates that handedness is only 24% genetic, which is probably surprisingly low to most of us (http://en.wikipedia.org/wiki/Handedness).
On gender, it is consistently found that males are somewhat more likely to be left-handed than females. This finding seems suggestive that testosterone may have the effect of increaasing the proportion of lefties in males. This issue overlaps with fetal development also, as hormones in utero might influence brain development in ways which affect handedness. However, fetal development effects might include other hormones aside from testosterone.
On modelling, this could be a factor in the fact that left-handedness, and right-handedness, tends to run in families, although right-handed parents sometimes have left-handed children and vice versa. In my case, my parents were right-handed models. However, I may have identified more closely with my 2 older brothers who were left-handed role models.
A small number of researchers think that all left-handed people were meant to be left-handed, but brain damage in some fetuses or young children switches their handedness preference. Clearly, this theory is controversial. However, there is some evidence that people who have brain damage are more likely to be left-handed, which might account for some of the negative findings regarding left-handed people, such as left-handed people being more likely to be schizophrenic or mentally retarded.
Adjustment due to injury, of course, is pretty much the opposite of forcing people to be right-handed; it is nature forcing people to be left-handed. I am reminded of the extreme example of former Anaheim Angels pitcher Jim Abbott, who was born with no right hand, and thus, exercised his left hand by being a left-handed pitcher.
Yet another cause of left-handedness is suggested by Chris McManus as reported in Anything Left-Handed (http://www.anythinglefthanded.co.uk/research/lefthanded-research.html#sthash.klJhFAdF.dpbs). According to McManus, older mothers are more likely to have left-handed children. With the percentage of older mothers increasing, it is to be expected that there is something of an increase in left-handedness.
McManus, who is from England, goes onto give some startling statistics about the increase in left-handedness in England over the years. Only 3% of those born before 1910 were left-handed in England, but now, 13% of men and 11% of women in England are left-handed. McManus cites the primary reason for the increase in left-handed people as a decrease in discrimination against left-handed people, clearly good news. This trend is apparently occuring in many parts of the world, such as the U.S. but not everywhere. For example, in the U.S., the famous (or infamous) study which concluded that left-handed people don't live as long as right-handed people, by Diane Halpern and Stanley Coren, failed to take into account the increasing proportion of left-handed people in the U.S. over time, and thus reached a flawed conclusion. That is, more of the younger generation were left-handed, leading the researchers to conclude that they were dying before reaching old age.
This most informative article also states: "Evolution may be in the process of delivering a new age of genius and creativity, with left-handers leading the way. Professor Chris McManus of University College, London has been researching the subject and is about to publish a new book called Right Hand, Left Hand. He is convinced that the proportion of left-handers is rising and left-handed people as a group have historically produced an above-average quota of high achievers. He says that left-handers' brains are structured differently in a way that widens their range of abilities and the genes that determine left-handedness also govern development of the language centres of the brain."
On the downside about lefties, the article concludes: "Unfortunately, it is not all good news for left-handers. They tend to be over-represented at both ends of the intellectual scale and as well as geniuses the group also produces a disproportionately high number of those with learning handicaps. There have been suggestions of links between left-handedness and dyslexia, stuttering and child autism among others."
Overall, the evidence is clear that left-handedness is increasing, which should result in an increase at both ends of the intellectual scale, as left-handed people are far more variable than right-handed ones on measures of intellect. Overall, studies of IQ in left and right handed people tend to indicate little difference, with left-handers usually having a 1 point advantage in IQ over right-handers on the average according to Wikipedia, but this does not tell the entire story. Anything Left-Handed lists the following advantages for lefft-handers: Sports advantages, higher percentage of intellectual giftedness, earning more money among left-handed men who have gone to college (probably due to either trying harder or better emotional skills), seeing better underwater (right-brained visual skills), multitasking better (using both sides of the brain more effectively, which also results in lefties "thinking faster"), better episodic memory, better at playing video games (again, the faster working, multiasking brain), recovering better from strokes (again, the two sided brain function thing), better at visual skills (a right brain function) and more likely to be an artist, and finally, better at learning to drive. That's quite a list.
One famous study conducted by Camilla Bendow of Johns Hopkins University in 1985, found that over 20% of top scorers on the Scholastic Aptitude Test for college admissions, were either left-handed or ambidextrous, twice what would be expected by chance. There were over 100,000 participants involved in this study, making for a huge and reliable sample with likely very valid results (http://www.thefreelibrary.com/The+left+hand+of+math+and+verbal+talent.-a03743623). Benbow also found that gifted children were far more likely to suffer severe allergies and strangely, to be nearsighted. Other research linking left-handedness to allergies was also cited in this article. As it happens, I suffer very bad allergies (had a bad sneezing attack just a few nights ago), am considerably near-sighted, and of course, am left-handed, the perfect combination for giftedeness.
Another study found that "true left-handedness" as measured by throwing left-handed, being left-eyed and left-eared, was correlated with several measures of superior intellect (http://www.independent.co.uk/life-style/health-and-families/health-news/lefthanders-more-creative-but-forgetful-710626.html). Indeed, Dr. Alan Searleman found that "true left-handers" tended to have higher fluid intelligence, better vocabularies, and were more likely to have an IQ over 140. He also found that lefties had poorer memories, yet were better ar problem solving than right-handed people. He concluded that his findings may explain why left-handed people seem to be over-represented in creative professions such as music, art and writing.
One final research finding of interest has to do with behavioral inhibition. A study in the UK found that left-handed people, whether through experience or due to brain differences, tend to be more inhibited. This is a plus-minus situation. On the one hand (the left), it means that lefties tend to be more anxiety prone and shy compared to right-handed people, but on the other (the right), right handed people tend to have more impulsivity problems (http://www.anythinglefthanded.co.uk/research/left-handed-research-current.html#sthash.eUDaBMwR.dpbs.)
The optimist in me suggests that what we have ahead of us, then, appears to be an age in which left-handedness is more accepted, more common, and thus, peoples' right-brain functions -- not only those of left-handed people but also right-handed people as well -- are allowed to blossom, to the benefit of humanity and hopefully, our precious planet. This is not so much about handedness as about the balance between left and right brained functions, and the fact that society has allowed left-brain functions to dominate for too long.
I will finish with a website which lists left and right brain functions, at the bottom of the site. These are too numerous to mention here, but in general, the left cerebral hemisphere is more verbal and analytical, strategy and detail oriented, while the right cerebral hemisphere handles everything else including visualizing, emotion, meanings, the "big picture," imagination, etc. (http://www.amoils.com/health-blog/right-handed-or-left-handed-should-it-matter-amoils-com/).