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2007-09-18 04:22:07
Andrea Thompson
LiveScience Staff Writer
LiveScience.comMon Sep 17, 2:45 PM ET
Lefties have been bouncing back in recent decades, following a decline in the beginning of the 20th century, a new study shows.
While lefties currently make up about 11 percent of the population, earlier studies found only 3 percent of those born in 1900 were left-handed. These claims led Ian Christopher McManus of the University College London and his colleagues to learn more about this change.
"Left-handedness is important because more than 10 percent of people have their brains organized in a qualitatively different way to other people," McManus said. "That has to be interesting. When the rate of a [variable trait] changes, then there have to be causes, and they are interesting as well."
To tease out the cause of the lefties' decline in Victorian times, the researchers looked at a series of films made between 1897 and 1913 by early filmmakers Sagar Mitchell and James Kenyon, whose body of work forms the largest collection of early non-fiction films in the world.
McManus and his team identified 391 arm-wavers in the films and compared the number of lefties and righties in the old movie to a more modern "control group" formed by searching for "waving" with Google Images.
About 15 percent of the people in the old films waved with their left hand, compared to 24 percent of lefty wavers in the Google search, the study, published in the Sept. 18 issue of the journal Current Biology, found.
The researchers compared these results to previous studies of handedness in writing and found that left-handed waving is more common than left-handed writing.
After correcting for this bias, the researchers found that "the earlier Victorian rates of left-handedness are broadly equivalent to modern rates, whereas rates then decline, with the lowest values for those born between about 1890 and 1910," McManus said.
The lefties in the Victorian films were typically older, which rules out the possibility of higher rates of mortality among lefties.
The most likely reason that lefties dropped in numbers at the turn of the 20th century was possible social influence brought about by universal education and the industrial revolution. These two factors would have forced lefties into the spotlight as they learned to write in the classroom or clumsily used machines built for right-handers.
"That would have exacerbated the stigma that any visible minority can experience, and the result could have been that left-handers found it more difficult to find marriage partners, marrying later, and hence having fewer children so that fewer of the relevant genes went into the gene pool," McManus told LiveScience. "And we do have evidence that around the turn of the 20th century left-handers had fewer children than right-handers."