A widely-held belief among left-handers is displayed in a popular t-shirt saying, “I’m left-handed, what’s your superpower?” The superpower refers to cognitive abilities. Left-handers believe they are smarter than right-handers. Exploring the intelligence differences between left- and right-handers has a long research history. When I reviewed this published literature for my book, Laterality: Exploring the enigma of left-handedness, I found little evidence for intelligence differences between left- and right-handers. Studies conducted in the 21st century have used large databases with thousands of participants and have failed to find support for the claim that intelligence varies by handedness type.
A previous blog post detailed the development of standardized IQ tests for children and adults in the early 20th century.¹ These tests have multiple sub-tests that assess different cognitive abilities from vocabulary mastery, to memory, to math skills, and non-verbal understanding of visual patterns. The results of the different sub-tests are combined into an overall numeric IQ score. A recent study used a sample of over 25,000 children from a database in the USA called the National Collaborative Perinatal Project (CPP), 1959-1973.² Infants were enrolled in the project and given a physical and neurological exam shortly after birth. They were followed until age 7 when each child was tested for the side of the writing hand and given a standard IQ test.
The results indicated that 90% of the children were right-handed and 10% left-handed. The overall IQ scores of the left- and right-handed children did not differ. The authors conducted a further analysis of the relationship between handedness type and giftedness, a superpower left-handers claim. Giftedness is defined by standard IQ scores. IQ scores follow a normal or bell-shaped distribution with most people (68%) falling between tested IQ scores of 85 and 115 as shown in the illustration. Only 2% of individuals have IQ scores of 130 or above and are called gifted. The tiny percentage of those with IQ scores of 140 or above are considered highly gifted. These classifications along with the IQ score range associated with each category are shown in this table.
The authors compared the handedness of children in the normal IQ range (85-115) to that of the children in the gifted (greater than 130) and highly gifted (greater than 140) categories. There were no significant differences in the percentages of left- and right-handers in any of these groups. The data revealed that 1.9% of right-handed children and 2.3% of left-handed children had IQ scores in the gifted category while 0.24% of right-handers and 0.28% of left-handers scored in the highly gifted range.
The authors acknowledge that these data are fifty years old. Children born in 1959 are now in their sixties. However, the standard IQ test and the handedness measurement technique employed in this study are still in use today so it is reasonable to compare the results to contemporary data. These findings add to the body of data showing no tested IQ differences between left- and right-handers. Also, left-handers are no more likely than right-handers to achieve tested IQ scores above 130.
¹Handedness and IQ: What’s the story? April 29, 2019.
²Lu, N., Rigsby, D.C., Kelm, S.A., Rapoport, E., & Adesman, A. (2020). Clinical correlates of laterality among school-age children in the United States. Laterality: Asymmetries of Brain, Behaviour, and Cognition, 25, 620-639. https://doi.org/10.1080/1357650X.2020.1788051.
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