This article from the UK Times summarising some recent research on handedness was published last year but I thought it might still be worth bringing to GeneExpressors' attention so that it can be enriched with their unique insights. I've long been fascinated by left-handedness and its 'feast or famine' aspect which is quite similar in pattern to males' general distribution curve (i.e. both groups have a disproportionately high number of both retards/psychopathological cases/learning disability cases and prodigies/gifted). See this for a list of groups found to have elevated prevalence of left handedness (it's quite a varied list).

To recap the state of knowledge, one theory which has tried to account for these patterns is that there are 2 kinds of left-handers - genetic left handers and 'pathological' left-handers (made at birth). The disproportionate number of high musical/mathematical ability and high IQ left-handers (as documented by the research of Camilla Benbow) are therefore said to belong to the former group while the retards/alcoholics, etc belong to the latter. The latter group become left handed because the left brain becomes damaged at birth and therefore faculties there have to shift to the right hemisphere and the child ends up left-handed while exhibiting their pathologies due to the diminished left hemisphere function. The research summarised in the article that I've noted, however, argues that left-handedness is a 2nd handedness mutation that occured late in the piece in human evolution and might account for both the 'feast' and the 'famine'. This seems to make sense insofar as it seems a bit of a stretch to blame brain damage on the left hemisphere for the elevated lefthandedness among groups like schizophrenics, autistics and dyslexics. The theory noted by the Times article argues that:

Somewhere along our evolutionary history ... we departed from chimps and other apes to develop a gene for right-handedness (he called it D, for dextral). This was allied to the development of language, which happened in only one half of the brain — the left hemisphere. McManus, who is right-handed but whose mother was left-handed, explains: “It would not have worked to put language in both hemispheres, because the corpus collosum (the bundle of nerve fibres connecting the two halves) is relatively slow and inefficient. The connections within each hemisphere are fast and reliable, which is essential for language.

“Crucially, the left hemisphere controls the right-hand side of the body, so the right hand became more dextrous at tasks such as hammering stone tools.” ... If handedness accompanied language, he adds, then we would not expect any other animal species, bereft of language, to develop handedness.

Then, McManus says, between two million and 5,000 years ago a second gene arose (called C, for chance), which allowed left-handedness to emerge. The second date is bounded by the fact that artworks over the past five millennia that depict people engaged in skilled activities, such as writing or throwing a weapon, display roughly the 90-10 percentage split in handedness that we see today.

What happened to make some of our forebears left-handed, after millions of years of right-handedness? The primary role of this second gene, McManus says, was to tweak brain structure so that the left hemisphere could accommodate other faculties apart from language. This tweaking — caused by inheriting one C gene and one D gene (one from each parent) — would furnish a person with, possibly, a better brain. This gene combination also happened to shift hand dominance in a minority of individuals from the right to the left.

“In CD brains, instead of having language in the left hemisphere only and non-language things in the right hemisphere, some of those faculties flip over to the other side,” McManus says. “You get things side by side that you wouldn’t find in a right-handed brain; they are built in a different way which I think makes them advantageous.

“Imagine, for example, having spatial faculties in the left hemisphere — this means that symbols and language are next to each other. This is particularly good for doing mathematics. Although we are not sure, the proportion of left-handed mathematicians could be as high as 20 per cent; there is certainly enough evidence to believe that left-handedness and mathematical talent may be linked.”

However, having too much crossover between brain hemispheres — caused by a CC inheritance — may lead to “higgledy-piggledy brains”, according to McManus. This genetic combination may lie at the heart of why autism, dyslexia and speech disorders such as stuttering are overrepresented in the left-handed population (the conditions are also much more common among boys).

PS The article also notes that East Asians exhibit a lower incidence of left-handedness. I wonder whether this has more to do with increased conformism and less tolerance of left handedness. I am left handed and I remember when I was in third grade of primary school in Malaysia one of my teachers spent a term trying to get me to write with my right hand. I was the stubborn type, so he didn't succeed. Incidentally I went through a period of stuttering around ages 9-10 but after that period, it basically vanished without a trace and I became active and proficient in public speaking/debating in high school. Perhaps my mutant left-handed brain was going through a period of tweaking/adjustment?;)

Update
On the comments board, Zizka asks:
"I think that there is a general question here which would apply also to bipolar disorder and schizophrenia. If they are hereditary and bad, why are they still in the gene pool?" He argues: "while full-blown schizophrenics are not functional, many schizophrenics in remission (or before the disease expresses) are extraordinarily talented and capable. Not just functional, but exceptional.(I have personally known at least two of them.) Much the same is true of "mania" (= bipolar disorder in its happy mode)."

Research along these lines has been pursued by Dr David Horrobin who has found elevated incidence of either schizophrenia or schizoid personality traits or elevated incidence of relatives who are schizophrenics among high achievers and 'creative types'. Given the elevated incidence of left-handedness among schizophrenics, it may be that there is some common mutation that connects all these conditions along with autistic/semi-autistic characteristics (or perhaps people diagnosed as having autistic traints are actually schizoid). This article has a summary of Dr Horrobin's book 'The madness of Adam and Eve' which outlines his own 'mutation' theory to explain his findings:

Horrobin - who is medical adviser to the Schizophrenia Association of Great Britain - argues that the changes which propelled humanity to its current global ascendancy were the same as those which have left us vulnerable to mental disease.

'We became human because of small genetic changes in the chemistry of the fat in our skulls,' he says. 'These changes injected into our ancestors both the seeds of the illness of schizophrenia and the extraordinary minds which made us human.'

Horrobin's theory also provides support for observations that have linked the most intelligent, imaginative members of our species with mental disease, in particular schizophrenia - an association supported by studies in Iceland, Finland, New York and London. These show that 'families with schizophrenic members seem to have a greater variety of skills and abilities, and a greater likelihood of producing high achievers,' ...

According to Horrobin, schizophrenia and human genius began to manifest themselves as a result of evolutionary pressures that triggered genetic changes in our brain cells, allowing us to make unexpected links with different events, an ability that lifted our species to a new intellectual plane. Early manifestations of this creative change include the 30,000-year-old cave paintings found in France and Spain.

The mutation Horrobin proposes involves changes to the fat content of brain cells. 'Sixty per cent of the non-aqueous material of the brain is fat. Humans have bigger heads than chimpanzees because their heads are full of fat.' By adding fat to our brain cells, we were able to control the flow of electrical signals more carefully and make more complex connections within our cortexes.