About 10% of the population is left-handed. However, neuroscientific and genetic studies often exclude left-handed individuals. What is the rationale behind this, and is it justified?
Already 15 weeks into pregnancy, ultrasound scanning can detect differences in hand-preference: 90% of the fetuses is sucking the right thumb, a percentage almost exactly the same as the prevalence of right-hand preference in the adult population.
Across history and cultures, left-handed individuals –‘lefties’ - have always gained special attention, ranging from being credited with ‘special mental powers’ to an increased vulnerability for developing psychiatric disorders. Not only does being left-handed have negative connotations on a lexical basis (i.e., the Latin adjective sinister means ‘left’ as well as ‘unlucky’, whereas in the English language the direction ‘right’ is a synonym for ‘correct’), left-handers are also excluded from scientific studies. Why is it that researchers systematically exclude 10% of the population?
Variable lateralization patterns
The most frequently used argument to exclude left-handers from neuroscientific studies is that left-handers have different patterns of ‘lateralization’ of functions in the brain than right-handers. This means that for instance in right-handers it is the left side of the brain that is mainly recruited for a particular function, whereas in left-handers the right hemisphere is the dominant part. Indeed, for a long time it was believed that this reversed ‘cerebral lateralization’ in left-handers applied to language functions such as speech production. However, recent studies show that in both right- and left-handers the left hemisphere is the dominant part of the brain when it comes to language functions. Still, there is more variability in the lateralization of functions in left-handers, and lateralization can even be the reverse of that in right-handers.
Fundamental information on genetics
Instead of seeing left-handers as unwanted noise in the data, Dr. Roel Willems and colleagues recently published an opinion paper in Nature Reviews Neuroscience in which they argue that left-handers are ‘a compelling and widely available but largely untapped resource for neuroscience and neurogenetic studies’. The authors provide an overview of studies showing that the asymmetry of handedness is likely to be of genetic origin. Thus, if we specifically target the differences between left-handers versus right-handers we can obtain important information about the genetic basis of handedness.
Also, handedness and brain lateralization are correlated to some extent, which provides a starting point from which to search for genetic factors common to both handedness and lateralization of brain and other functions. Therefore, studying left-handers is relevant from a very fundamental point of view.
Practical implications: different treatment
Another argument against excluding ‘lefties’ has direct practical implications. For example, in right-handers approach-related motivational tendencies are controlled by the left hemisphere of the brain, whereas avoidance-related motivational tendencies are controlled by the right hemisphere. Recent research has found that left-handers show the opposite pattern. This finding may have direct clinical implications. Studies aimed at increasing approach-related motivations by applying noninvasive brain stimulation to treat depression, could be targeting the wrong hemisphere. That is to say, when brain stimulation is applied to the left hemisphere of depressed patients in order to increase brain activity this could result in worsening of symptoms if the patient is left-handed.
Lefties as research opportunities
In conclusion, depending on the research questions that need to be addressed, including both left- and right-handers in research will give us a unique opportunity to study the origins of not only handedness, but also of the organization of various functions, including those in the brain. This already shows that lefties must not feel left out.