A 1986 study by Camilla Benbow et al found that gifted youth were more than twice as likely to be left-handed than those in a control group (although they were also twice as likely to have allergies and four times as likely to be myopic, for what that is worth), and Norman Geschwind and Albert Galaburda confidently concluded in 1987 that “non-right-handed populations are over-represented in all populations with high talent”, a sweeping statement indeed. A 2008 study by John Santrock found that 20% of the top-scoring group in SAT testing over a particular period was left-handed, as compared to an incidence of left-handedness of around 10% in the general population. A (rather small) 2006 Australian study concluded that left-handers may be faster at processing multiple stimuli than right-handers. Another study, claiming that left-handers were statistically over-represented among members of the high IQ organization Mensa, has been largely discredited and shown to have been based on flimsy and unreliable data.
Other studies, however, such as C. Hardyk and L.F. Petrinovich’s detailed study on left-handedness in 1977 and another by Clare Porac and Stanley Coren in 1981, have found no significant overall difference in mental ability between left-handers and right-handers. A 1987 study by Stephen Williams found the average performance of left-handers in high school to be significantly lower than that of the right-handers. A 2008 British study concluded that left-handed and mixed-handed 11 and 14 year old schoolchildren performed slightly worse on average than right-handers, especially the girls, although left-handed boys dominated the very top and bottom of the ability range. A large Australian study by Mike Nichols in 2011 suggests a small but significant cognitive deficit among left-handed five year olds (of about the same magnitude as that caused by being born premature, although the study does not necessarily link the two). This same study also suggested that left-handed children spent less time on educational activities and more time watching television.
Some studies have tried to link left-handedness with certain areas of knowledge. For example, research by Marian Annett indicated significantly more left-handers among mathematics students than among students of other subjects, arguing that this is consistent with her genetic right-shift theory of left-handedness. John Santrock's 2008 study, mentioned above, also suggested that more mathematicians, architects, musicians and artists - all of whom, it is argued, require good visual-spatial skills - are left-handed than the population as a whole.
Other studies, however, such as those by Douglas in 1967 and B. Jones and J. Bell in 1980, indicate absolutely no correlation between mathematical ability and left-handedness. Another study, by John Peterson in 1979, pointed out relatively low rates of left-handedness among science students in general.
Many websites try to make a causal connection between left-handedness and intellectual genius, citing Aristotle, Leonardo da Vinci, Albert Einstein, Isaac Newton, Marie Curie, Alan Turing, Albert Schweitzer and others in support of this contention. Unfortunately, even if such a cherry-picked list did in fact prove anything about genius - and it does not - it is not clear how this genius is supposed to be connected to left-handedness, nor what it is supposed to say about other left-handers.
Even more unfortunately, though, the evidence of the left-handedness of most of these individuals is suspect to say the least. For instance, there is little or no reason to suspect that Aristotle was left-handed, his main contribution to the subject being “The good things are in the right side and the bad in the left”. Leonardo da Vinci definitely did use his left hand, although recent research suggests that this may have been because his right hand was injured or disfigured in some way. Albert Einstein was certainly right-handed in most aspects of daily life, the only "evidence" of left-handedness or mixed-handedness being that his brain, preserved after his death, is slightly more symmetrical than the average, a trait often (but not always) associated with left-handedness and mixed-handedness (see the section on Handedness and the Brain). Neither is there any evidence to suggest that Isaac Newton was left-handed (other than unreliable populist books like Ed Wright’s A Left-Handed History of the World).
Marie Curie arrived in America nursing a bandaged and injured right hand, but there is no other reason to think her in any way left-handed. Alan Turing was certainly gay and a maverick, but there is no evidence of his having been left-handed. It is even claimed in all seriousness on several cat websites that Albert Schweitzer, always seen writing with his right hand, only did so because his cat liked to sleep on his left, and that he was really left-handed! Ironically, the inventor and polymath Nikola Tesla, a man of genuine genius who rarely appears on such lists, probably was left-handed, although he claimed that he later taught himself to be ambidextrous.
A 2006 study by Christopher Ruebeck et al concluded that left-handed college-educated men earn 15% more than their right-handed colleagues, although this wage differential apparently did not extend to women, nor did it extend to men with a lower level of education. Even the authors were not quite sure what to make of these apparently significant, but somewhat puzzling, results.
Perhaps predictably, another report by Joshua Goodman of Harvard, concludes just the opposite, that left-handers earn 6% less than right-handers, and are more likely to work in manual jobs.