Features
On the Other Hand
Study refutes scientific belief that left-handedness is linked to dyslexia, homosexuality, asthma
BY RACHELLE COOPER
Contrary to popular scientific belief, left-handedness is not linked to dyslexia, poorer spatial ability, homosexuality, asthma or hyperactivity, Prof. Michael Peters, Psychology, has found.
“We've shown on a number of tasks that there's no difference between right- and left-handedness," says Peters, whose study of more than a quarter million people is published in the current issue of Brain and Cognition, co-authored by psychology professors Stian Reimers of University College London and John Manning of the University of Central Lancashire.
The study did show, however, that individuals who didn't favour either hand for writing had significantly poorer spatial performance in a mental rotation task and significantly higher prevalence of homosexuality, bisexuality, hyperactivity, dyslexia and asthma than did individuals with clear left- or right-hand preferences.
The survey was hosted on the BBC Science and Nature website for a number of weeks. It included more than 150 questions about demographics, personality, sexuality, social attitudes and behaviours, as well as spatial and verbal tasks.
Obtaining data on 255,000 people is extremely significant in the study of handedness, says Peters. “Because only 10 per cent of the population is left-handed, you need a huge study group to draw any sensible conclusions."
The researchers were cautious about their study results because of the lack of control over an Internet survey, but when they cross-checked the results, “we were impressed by how similar the prevalence figures we observed were to the figures obtained under more controlled conditions in the lab," he says.
Instead of asking people if they were left- or right-handed, the survey asked subjects to indicate hand preference for writing on a scale of one (left) to five (right). Participants who chose “three" were comfortable writing with either hand. This mixed group made up less than one per cent of the participants, but they produced the most interesting results, says Peters.
“Normally the mixed group is so small in laboratory studies that you can't draw any conclusions from it."
The survey also asked participants to self-identify their sexual orientation as heterosexual, homosexual or bisexual. There was no significant difference in sexual orientation between the left- and right-handed respondents, but both males and females who said they used “either hand" to write were overrepresented in the non-heterosexual categories, especially the bisexual category.
There has been some evidence that right- and left-handed people exhibit significant differences in spatial ability, says Peters, “but we just didn't find that in this study. We did find, however, that mixed-handedness definitely seems to be a factor in spatial ability." Respondents who chose the “either hand" option performed worse on the spatial tasks.
The researchers included health categories and developmental problems in their study because there's a general underlying rationale that left-handedness is a marker for “something not quite right," says Peters. The lefties didn't differ statistically from the right-handed participants in the number of asthmatics, dyslexics or cases of hyperactivity, but individuals in the mixed-hand category had the highest prevalence of these traits.
“Our study shows that much greater attention has to be paid to the definition of handedness," he says.
Peters stresses that just because someone can write with either hand doesn't mean the person is homosexual, bisexual or dyslexic.
“Individuals who are unclear about their handedness make up a disproportionately higher number of dyslexics, but since there are so few dyslexics in the first place, the majority of people with mixed-handedness aren't dyslexic. Similarly, there is a disproportionate number of homosexual males with mixed-handedness, but the largest number of mixed-handed males aren't homosexual."