In the 1980’s, the eminent Canadian neuropsychologist, MP Bryden, observed that the current knowledge of the brain was based on data from studies of right-handed males. He noted that females and left-handers were systematically excluded from brain studies because researchers considered their data to be variable, adding unnecessary ‘noise’ to the otherwise consistent results obtained from right-handed males. A 2019 paper authored by researchers also based in Canada offers supporting evidence for Bryden’s observation of nearly forty years ago.¹
The authors of the 2019 study examined over one thousand research articles published in 2017 in three major neuroscience journals. The studies in question used a range of brain imaging techniques from functional magnetic resonance imaging (fMRI) to transcranial magnetic stimulation (TMS). Research using these procedures require participants to complete a task while the brain is scanned for metabolic or electrical activity in areas of the brain involved in completing the task.
The authors noted that left-handers are often excluded from studies involving language tasks. Ninety-five percent of right-handers have language centers in the left hemisphere of the brain as compared to 70% of left-handers. Since left-handers are more likely than right-handers to have right hemisphere or bilateral (both-sided) speech representation, left-handers are eliminated to reduce the data ‘noise’ they potentially provide. For this reason, the researchers divided the articles under review into three categories, language-related, sensory or movement-related, and other. Handedness information was reported for over 30,000 participants of which only 3.2% were non-right-handed (a category including mixed- and left-handers). Language-oriented studies were more likely to report participant handedness but were no more likely than studies in the other two categories to exclude non-right-handers in the research.
The authors understood that rates of left-handedness vary worldwide so they undertook a separate analysis of the research conducted in Europe and North America. These are regions of the world with the highest rates of reported left-handedness (10-13%). There were 842 published papers that fit these criteria. The rates of non-right-handed participation rose to 4.2%, a percentage still below the expected population values of 10 to 13%. Unlike the handedness data, the authors found that the percentage of female participants in these studies matched the population rates, both being around 49%. Females were included in expected numbers but non-right-hander participation was well below the overall population rates.
The authors concluded there is a bias against recruiting non-right-hander participation in research using brain imaging procedures. These methodologies have enhanced our understanding of brain mechanisms and operation but, unfortunately, a percentage of the population is being excluded from the process. This exclusion has “unnecessarily hindered scientific understanding of the brain at the whole-population level.” The authors argued that eliminating non-right-handers from research deprives them of the learning opportunities afforded to research participants. Individual difference factors, such as sex or socioeconomic status, are included as variables in brain imaging research. The authors recommended that handedness be considered like any other individual difference variable that may or may not affect functions or structures in the brain. The inclusion of non-right-handers in brain imaging research will contribute to not detract from our understanding of the human brain.
¹Bailey, L.M., McMillan, L.E., & Newman, A.J. (2019). A sinister subject: Quantifying handedness-based recruitment biases in current neuroimaging research. European Journal of Neuroscience, 00, 1-15. https://doi.org/10.1111/ejn.14542