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Hemispheric dominance (pseudoscience promoters sometimes call it "brainedness," cf. handedness) refers to the fact that:
- The two hemispheres of the brain are specialized to perform certain tasks ("lateralization")
- Individuals show a preference toward using one hemisphere in many cases.
The phenomenon of hemispheric dominance is a case study in how a grain of truth in science gets blown out of proportion into a pseudoscientific industry.
 Actual science
The current incarnations of left brain/right brain myths originate with misinterpretations of Roger Sperry's split brain experiments and later work built on this original research. Sperry's experiments demonstrated the existence of lateralization and have led to an entire body of research dedicated to it. There are certain well-established instances of lateralization. In 95% of right-handed people language is localized in the left hemisphere, specifically the Broca's and Wernicke's areas. Facial recognition is localized in the right hemisphere, specifically the fusiform gyrus.
However, it should be noted that the study of hemispheric dominance and lateralization is very complex and includes many open questions and inconclusive research. About 5% of right-handed people have their language centers located in their right hemispheres, and approximately a third of left-handed people have them in their left hemispheres while about 20% have bilateral language centers. Similarly, facial recognition impairment can occur as a result of either right or left hemisphere brain damage, and studies done on autistic people have shown that, for reasons as yet not understood, they process faces in entirely different areas of the brain and leave the fusiform gyrus severely underused. In addition, it should be noted that these instances of lateralization tend to occur in functionally specific and neurologically local fashions, rather than making people "left-brained" or "right-brained" as a whole.
Of course, when talking about something like language, we're using a very broad term that can include a variety of different things, like tone, pitch, written words, etc. Language is a good example how this false dichotomy oversimplifies brain function. Mandarin Chinese is a language that relies heavily on tone of voice — there are many words that are identical in spelling but change meaning depending on the pitch they are pronounced with. The right hemisphere is dominant in early processing of words in Mandarin while the left is dominant in late processing. Studies of American Sign Language have shown more distributed hemispheric activity as opposed to the dominance of the left brain in English speakers.
The existence of neuroplasticity and recruiting, i.e. the phenomena by which neurons can "rewire" themselves to some degree to take on a new role or function, serves as another counterexample to a rigid divide between the hemispheres. There have been many cases of a hemispherectomy, in which one hemisphere is surgically removed (usually as a means to treat extremely severe epilepsy, although it has also been done in cases of severe trauma to one hemisphere of the brain), being performed on young people where patients have seen only a mild decline in cognitive function and have gone on to live normal lives. Left hemispherectomies on right-handed adults, however, lead to very severe aphasia; although they can acquire vocabulary and some basic aspects of grammar, since their right hemispheres tend to have a few language functions even before the hemispherectomy, this never progresses to normal speech. Similarly, cases where language learning has been delayed until after the onset of puberty (by far the most famous case being that of Genie[wp]) have shown that, even in the absence of brain damage, language acquisition occurs in the opposite hemisphere than normal.
 Left/right brain woo in popular culture
The pseudoscience has come in because a large number of people with a substandard understanding of the science behind hemispheric dominance pile loads of personality trait woo on top of it. Because of the tendency for linguistic function to be localized in the left hemisphere and visuo-spatial in the right (among other functions as well), the pop psychology version of this reduces each hemisphere to a set of personality traits and massively oversimplifies real neuroscience. In the pop version, the left brain represents all verbal, logical, and analytical thinking whereas the right brain is visual, musical, emotional, and creative. Are you an accountant? Then you must be left-brained! Are you an artist? Then you must be right-brained! Much of the woo based on hemispheric dominance comes from personality tests that will tell you whether you are "left-brained" or "right-brained" based on the bogus analytic vs. lateral thinking dichotomy of pop psychology. They usually ask questions like "Do you wear a watch?" "Do you like to draw or read?" "Do you prefer open-ended questions or ones with only one right answer?" And so on.
Drawing on these personality tests, a number of pop psych woo-meisters have built cottage industries of pseudo-scholarship around the concept of "brainedness." A lot of these are vaguely (or sometimes flagrantly) New Age books or products claiming to "help you get in touch" with your left/right brain. Deepak Chopra endorses a lot of this stuff, so you know how high quality it will be.
This stuff is commonly hawked to artists, middle management types, admen, businessmen, and educators and advertised as a way to "increase creative thinking" through "right-brain thinking." "Whole brain theory" is another common trope with these products. The idea behind it is that left-brained people need to "develop" more "right-brain thinking" and vice-versa to take full advantage of the brain's capacity and to be more successful.
- The left brain/right brain myth, Neurophilosophy
- The left brain/right brain myth, OECD
- The Death of "Right-Brain Thinking", Mind Hacks
- More Left Brain/Right Brain Nonsense, Steven Novella
- ↑ The Split-Brain Experiments, Nobelprize.org
- ↑ Jared A. Nielsen, Brandon A. Zielinski, Michael A. Ferguson, Janet E. Lainhart, Jeffrey S. Anderson. An Evaluation of the Left-Brain vs. Right-Brain Hypothesis with Resting State Functional Connectivity Magnetic Resonance Imaging. PLOS One, August 14, 2013
- ↑ http://brain.oxfordjournals.org/content/124/10/2059.full-text.pdf Face processing occurs outside the fusiform 'face area' in autism: evidence from functional MRI]
- ↑ Luo, Hao et al. (2006) Opposite patterns of hemisphere dominance for early auditory processing of lexical tones and consonants. Proceedings of the National Academy of Sciences Online.
- ↑ Neville, Helen J. et al. Neural Systems Mediating American Sign Language: Effects of Sensory Experience and Age of Acquisition. BRAIN AND LANGUAGE 57, 285–308 (1997)
- ↑ Pulsifer, MB et al. The cognitive outcome of hemispherectomy in 71 children. Epilepsia. 2004 Mar;45(3):243-54.
- ↑ http://www.readcube.com/articles/10.1111%2Fj.1467-1770.1973.tb00097.x Lateralization, Language Learning, and the Critical Period: Some New Evidence
- ↑ This quiz is a typical example.
- ↑ Whole Brain Teaching and Whole Brain Marketing are typical examples of these programs.