Editing Handedness (section)

=== Language dominance ===
One common handedness theory is the brain hemisphere division of labor. In most people, the left side of the brain controls speaking. The theory suggests it is more efficient for the brain to divide major tasks between the hemispheres—thus most people may use the non-speaking (right) hemisphere for perception and gross motor skills. As speech is a very complex motor control task, the specialised fine motor areas controlling speech are most efficiently used to also control fine motor movement in the dominant hand. As the right hand is controlled by the left hemisphere (and the left hand is controlled by the right hemisphere) most people are, therefore right-handed. The theory depends on left-handed people having a reversed organisation.<ref name="book">{{cite book |last=Banich |first=Marie |url=https://archive.org/details/neuropsychologyn00bani |title=Neuropsychology: The Neural Bases of Mental Function |publisher=Houghton Mifflin |year=1997 |isbn=9780395666999 |author-link= |url-access=registration |name-list-style=vanc}}</ref> However, the majority of left-handers have been found to have left-hemisphere language dominance—just like right-handers.<ref>{{cite journal |last1=Rasmussen |first1=T |last2=Milner |first2=B |date=1977 |title=The role of early left-brain injury in determining lateralization of cerebral speech functions |url=https://pubmed.ncbi.nlm.nih.gov/101116/ |journal=Annals of the New York Academy of Sciences |volume=30 |issue=299 |pages=355–369 |bibcode=1977NYASA.299..355R |doi=10.1111/j.1749-6632.1977.tb41921.x |pmid=101116 |s2cid=10981238}}</ref><ref>{{cite journal |last1=Carey |first1=David |last2=Johnstone |first2=Leah |date=2014 |title=Quantifying cerebral asymmetries for language in dextrals and adextrals with random-effects meta analysis |journal=Frontiers in Psychology |volume=5 |page=1128 |doi=10.3389/fpsyg.2014.01128 |pmc=4219560 |pmid=25408673 |doi-access=free}}</ref> Only around 30% of left-handers are not left-hemisphere dominant for language. Some of those have reversed brain organisation, where the verbal processing takes place in the right-hemisphere and visuospatial processing is dominant to the left hemisphere.<ref>{{cite journal |last1=Cai |first1=Q |last2=Van Der Haegen |first2=L |last3=Brysbaert |first3=M |date=2013 |title=Complementary hemispheric specialization for language production and visuospatial attention |journal=PNAS |volume=110 |issue=4 |pages=322–330 |doi=10.1073/pnas.1212956110 |pmc=3557046 |pmid=23297206 |doi-access=free}}</ref> Others have more ambiguous bilateral organisation, where both hemispheres do parts of typically lateralised functions. When tasks designed to investigate lateralisation (preference for handedness) are averaged across a group of left-handers, the overall effect is that left-handers show the same pattern of data as right-handers, but with a reduced asymmetry.<ref>{{cite journal |last1=Karlsson |first1=Emma M. |last2=Johnstone |first2=Leah T. |last3=Carey |first3=David P. |title=The depth and breadth of multiple perceptual asymmetries in right handers and non-right handers |journal=Laterality |date=2 November 2019 |volume=24 |issue=6 |pages=707–739 |doi=10.1080/1357650X.2019.1652308 |pmid=31399020 |s2cid=199519317 |url=https://research.bangor.ac.uk/portal/en/researchoutputs/the-depth-and-breadth-of-multiple-perceptual-asymmetries-in-right-handers-and-nonright-handers(15fe822a-a733-41f7-b817-70760ca7001d).html }}</ref> The majority of the evidence comes from literature assessing oral language production and comprehension. When it comes to writing, findings from recent studies were inconclusive for a difference in lateralization for writing between left-handers and right-handers.<ref>{{cite journal | vauthors = Papadopoulou AK, Samsouris C, Vlachos F, Badcock N, Phylactou P, Papadatou-Pastou | title = Exploring cerebral laterality of writing and the relationship to handedness: a functional transcranial Doppler ultrasound investigation | journal = Laterality | volume = 29 | issue = 1 | pages = 117–150 | date = November 2023 | doi = 10.1080/1357650X.2023.2284407| pmid = 38112692 }}</ref>