“Lefties are smarter than righties!” “Left-handers are more intelligent than right-handers!” These and similar “fun facts” about left-handedness are quite common on social media, but are they actually true? The fact is that more than 30 million Americans are left-handers, and at first glance, it seems somewhat odd that a preference to write or draw with the left and not the right hand should affect how smart someone is.
The idea that left-handedness might be associated with general intelligence (or any other cognitive ability for that matter) seems to be related to the fact that the origin of handedness has very little to do with the hands themselves. It is impossible to judge whether someone is a left-hander or a right-hander just by looking at the hands, as long as the person is not performing any action. Bones, muscles, tendons, and any other parts of the hands of left- and right-handers typically do not show any visible differences. Instead, the preference to use one hand over the other for fine motor tasks such as writing is caused by the brain (Ocklenburg et al., 2013). Thus, it is at least conceivable that genetic or environmental factors that influence brain development in such a way that someone becomes a left-hander might also affect the development of brain areas linked to intelligence.
Along these lines, it is not impossible that left-handedness and intelligence are linked. But are they really? Empirical studies show surprisingly ambiguous results. Some studies find that right-handers are more intelligent (Nicholls et al., 2012), while others find the exact opposite (Ghayas & Adil, 2007). Such differences between studies are not uncommon in psychological science and can often be explained by sample characteristics and the specific methods used to assess handedness and intelligence. Therefore, it is hard to determine the true effect by looking at single studies.
To finally settle the debate on left-handedness and intelligence, Eleni Ntolka and Marietta Papadatou-Pastou, two researchers from the University of Athens in Greece, performed a so-called meta-analysis of published studies on left-handedness and intelligence (Ntolka & Papadatou-Pastou, 2018). A meta-analysis integrates the results of several empirical studies, which has the advantage that the sample size is much larger, increasing statistical power and rendering the analysis less likely to be affected by sample characteristics of individual studies.
Overall, Ntolka and Papadatou-Pastou (2018) integrated the results of 18 studies that had measured full IQ scores in different handedness groups. Altogether, data from 20,442 individuals were included. Three meta-analyses on possible differences in standardized IQ score between different handedness groups were conducted:
- Left-handers compared to right-handers
- Non-right-handers (e.g., left-handers and mixed-handers) compared to right-handers
- Mixed-handers compared to right-handers
The results? There were no differences in mean IQ scores between right-handers and non-right-handers, or between right-handers and mixed-handers. For the comparison between right-handers and left-handers, however, there was a statistically significant effect, showing that right-handers had a higher average IQ than left-handers. Importantly, this effect was tiny and unlikely to have any substantial effect in real life. To illustrate this, the authors gave the following example: Assuming that left-handers had an average IQ of 100, and both left-handers and right-handers had similar variation in their data, then right-handers would have an average IQ of 101.05. Furthermore, the effect lost statistical significance when one study was excluded from the meta-analysis. These findings led the authors to conclude that the absolute magnitude of IQ differences between left-handers and right-handers is extremely small and that any intelligence differences between the two groups in the general population are negligible.
So no matter what social media fun fact pages tell you: Sorry, left-handers, you are not smarter than the rest of the population.
Ghayas S, Adil A. (2007). Effect of handedness on intelligence level of students. J Indian Acad Appl Psychol, 33, 85-91.
Nicholls ME, Chapman HL, Loetscher T, Grimshaw GM. (2010). The relationship between hand preference, hand performance, and general cognitive ability. J Int Neuropsychol Soc, 16, 585-592.
Ntolka E, Papadatou-Pastou M. (2018). Right-handers have negligibly higher IQ scores than left-handers: Systematic review and meta-analyses. Neurosci Biobehav Rev, 84, 376-393.
Ocklenburg S, Beste C, Güntürkün O. (2013). Handedness: a neurogenetic shift of perspective. Neurosci Biobehav Rev, 37, 2788-2793.