Intelligence
Further information: Handedness and mathematical ability and List of musicians who play left-handed
In his book Right-Hand, Left-Hand, Chris McManus of University College London argues that the proportion of left-handers is increasing 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 increases their range of abilities) and the genes that determine left-handedness also govern development of the language centers of the brain.[35]
Writing in Scientific American, McManus states that,
Studies in the U.K., U.S. and Australia have revealed that left-handed people differ from right-handers by only one IQ point, which is not noteworthy ... Left-handers' brains are structured differently from right-handers' in ways that can allow them to process language, spatial relations and emotions in more diverse and potentially creative ways. Also, a slightly larger number of left-handers than right-handers are especially gifted in music and math. A study of musicians in professional orchestras found a significantly greater proportion of talented left-handers, even among those who played instruments that seem designed for right-handers, such as violins. Similarly, studies of adolescents who took tests to assess mathematical giftedness found many more left-handers in the population.[36]
Conversely, Joshua Goodman found that evidence that left-handers were overrepresented amongst high end of the cognitive spectrum was weak due to methodological and sampling issues in conducted studies. Goodman also found that left-handers were overrepresented at the low end of the cognitive spectrum, with the mentally disabled being twice as likely to be left-handed compared to the general population, as well as generally lower cognitive and non-cognitive abilities amongst left-handed children.[37]
Early childhood intelligence
Nelson, Campbell, and Michel studied infants and whether developing handedness during infancy correlated with language abilities in toddlers. In the article they assessed 38 infants and followed them through to 12 months and then again once they became toddlers from 18–24 months. What they discovered was that when a child developed a consistent use of its right or left hand during infancy (such as using the right hand to put the pacifier back in, or grasping random objects with the left hand), it was more likely to have superior language skills as a toddler. Children who became lateral later than infancy (i.e., when they were toddlers) showed normal development of language and had typical language scores.[38] The researchers used Bayley scales of infant and toddler development to assess all the subjects.
Health
Lower-birth-weight and complications at birth are positively correlated with left-handness.[39]
A variety of neuropsychiatric and developmental disorders like autism spectrum disorders, depression, bipolar disorder, anxiety disorders, schizophrenia, and alcoholism has been associated with left- and mixed-handedness.[40][41]
A 2012 study showed that nearly 40% of children with cerebral palsy were left-handed,[42] while another study demonstrated that Left-handedness was associated with a 62 percent increased risk of Parkinson's Disease in women, but not in men.[43] Another study suggests that the risk of developing multiple sclerosis increases for left-handed women, but the effect is unknown for men at this point.[44]
Left-handed women have a higher risk of breast cancer than right-handed women and the effect is greater post-menopausal.[45]
At least one study maintains that left-handers are more likely to suffer from heart disease, and in a cardiovascular context, are more likely to have reduced longevity.[46]
Left-handers are more likely to suffer bone fractures.[47]
One systematic review concluded: "Left-handers showed no systematic tendency to suffer from disorders of the immune system".[48]
If handedness is entirely genetic, these health problems mean left-handness could be eliminated through natural selection. However, left-handers enjoy an advantage in fighting and sports increasing their likelihood of reproduction.[49]
