Left-Handedness Linked to Certain Neurological Disorders: Study Explores Correlation, Not Causation
A recent study published in the journal Psychological Bulletin has unveiled a potential connection between left-handedness and certain neurological disorders, although researchers are emphasizing that the findings demonstrate a correlation, not a causal relationship. While the study suggests that atypical handedness – including left-handedness and mixed-handedness – is more prevalent in individuals with conditions like autism, schizophrenia, and intellectual disability, experts caution against drawing any conclusions about cause and effect.
The international research team analyzed data from over 200,000 individuals, revealing that people with autism are approximately 3.5 times more likely to be left-handed compared to the general population, where left-handedness occurs in about 10% of individuals. The study also indicated a higher prevalence of left- and mixed-handedness among individuals diagnosed with schizophrenia and intellectual disability.
Mixed-handedness, as defined by psychology experts, refers to the preference for using either hand for different tasks. This is distinct from ambidexterity, which signifies equal proficiency in using both hands. Dr. Julian Packheiser, the study’s lead author and a researcher at the Institute of Cognitive Neuroscience at Ruhr University Bochum in Germany, clarified the core finding: "Alterations in hand preference from the norm (i.e., right-handedness) are associated especially with psychiatric and psychological conditions that affect the language system (e.g., dyslexia or schizophrenia) or affect neurodevelopment through an early age of onset (e.g., autism or ADHD)."
However, the study also highlighted that not all neurological conditions exhibit a link to an individual’s dominant hand. For example, depression and dyscalculia, a learning disability affecting math abilities, showed no significant difference in hand preference compared to control groups.
Furthermore, the researchers found a potential association between atypical handedness and long-term linguistic symptoms, such as stuttering and dyslexia. Dr. Earnest Lee Murray, a neurologist with West Tennessee Medical Group, who was not involved in the study, elaborated on the neurological basis for this association. "In most people, the dominant side of their brain is typically where speech, language, logical thought processing and hand preference is located," he explained. He further added, "People who are left-handed or ambidextrous often don’t have such a dominant one-sided brain — rather, the key portions of language can be divided between both sides."
The study also examined the prevalence of atypical handedness in clinical populations, comparing groups of people diagnosed with specific health conditions to a control population. Dr. Packheiser, in a press release about the study, stated, "We suspected that left- and mixed-handedness could be associated with disorders whose symptoms are related to language. 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 considered factors such as the age at which a person begins to exhibit symptoms of a neurological disorder. The findings suggest that the earlier the symptoms emerged, the more frequently left- and mixed-handedness were observed. This led the researchers to hypothesize that these hand preferences could be associated with diseases that manifest very early in life, as the dominant hand is determined at a young age, possibly even before birth. They cited research indicating that thumb-sucking in the womb at 10 weeks gestation is strongly correlated with hand preference later in life.
Dr. Murray emphasized the potential implications of the study for understanding brain organization and development, stating that the connection could "help experts potentially understand the way brains are organized and how they develop." He also suggested that the study has "implications when understanding ways of finetuning the neurorehabilitation of patients with one-sided brain conditions, such as stroke, multiple sclerosis or traumatic brain injury."
However, experts are cautioning against overinterpreting the results. Jonathan Alpert, a Manhattan-based psychotherapist and author, who was not involved in the study, emphasized the limitations of the findings. "Correlation doesn’t imply causation — and being left-handed does not mean someone will develop autism or schizophrenia," he stated. "Handedness is just one of many factors that can be associated with neurological traits, and these findings should be interpreted with caution to avoid unnecessary alarm or stigma." Alpert called for further research to better understand the relationship, if any, between handedness and mental health.
Dr. Packheiser, the lead author, also reiterated that the results are "purely correlational." "We have no causal evidence that developing a psychiatric disorder linked to language or neurodevelopment causes a change in handedness, or whether changes in handedness cause any disorder," he explained.
The researchers stressed that these findings should not be cause for concern for individuals or parents of children who prefer to use their left hand. "Individuals from the general public should therefore never be alarmed if their child prefers to use the left hand," Dr. Packheiser said. However, he also emphasized the importance of further research to better understand mental disorders, particularly in light of their increasing prevalence in the population.
In conclusion, while the study provides intriguing evidence of a correlation between left-handedness and certain neurological disorders, it is crucial to recognize that this is not a causal relationship. More research is needed to fully understand the complex interplay between handedness, brain development, and the manifestation of neurological conditions. The current findings should be interpreted with caution and should not be used to create unnecessary anxiety or stigmatize individuals with left-handedness or mixed-handedness. The study serves as a valuable starting point for future investigations into the intricacies of brain organization and the factors that contribute to both typical and atypical development.
