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.
