Handedness and Neurological Disorders Linked
A meta-analysis links left and mixed-handedness with disorders such as dyslexia, schizophrenia and autism.
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The fact that left-handedness resp. mixed-handedness are strikingly common in patients with certain neurological disorders, such as autism spectrum disorders, is a frequently reported observation in medical practice. The reason why handedness is associated with these disorders is probably because both are affected by processes in early brain development.
Various studies have explored this link for individual disorders and have sometimes been able to show it, and sometimes not. A meta-analysis carried out by an international research team from Bochum, Hamburg, Nijmegen and Athens shows that left and mixed-handedness is particularly common in people who suffer from a disorder that manifests itself early in life and is associated with linguistic symptoms. These include dyslexia, schizophrenia and autism. They published their findings in the journal Psychological Bulletin.
Symptoms as starting point
The research team re-evaluated existing meta-analyses from a new perspective. “We suspected that left- and mixed- handedness could be associated with disorders whose symptoms are related to language,” explains Dr. Julian Packheiser from the Institute of Cognitive Neuroscience at Ruhr University Bochum. “Language, like handedness, has a very one-sided location in the brain, so it stands to reason that the development of both and their disorders could be linked.” The researchers also suspected that left-handedness and mixed-handedness could be associated with diseases that occur very early in life. This is because handedness is also determined at a very early developmental stage.
“Both hypotheses have been confirmed,” says Professor Sebastian Ocklenburg from the Medical School Hamburg. For example, left-handedness and mixed-handedness are statistically significantly more common in people with dyslexia – a reading disorder – than in healthy individuals. Autism, which in severe cases is accompanied by communication disorders, and schizophrenia, in which patients sometimes hear voices, are also associated with both linguistic symptoms and a higher incidence of left-handedness and mixed-handedness.
Link between handedness and neurodevelopmental disorders
Consistent with the hypothesis, the earlier the symptoms manifest, the more frequent the accumulation of left and mixed- handedness was found to be. “In people with depression, which on average occurs around the age of 30, we couldn’t show any connection,” points out Julian Packheiser.
The researchers believe this is evidence that handedness and various neuronal developmental disorders are influenced by partially overlapping processes in early brain development.
Reference: Packheiser J, Borawski J, Berretz G, et al. Handedness in mental and neurodevelopmental disorders: A systematic review and second-order meta-analysis. Psychol Bull.151(4), 476–512. doi: 10.1037/bul0000471
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