"Left-handedness is important because more than 10 percent of people have their brains organized in a qualitatively different way to other people," said Ian Christopher McManus of the University College London. "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."

The precise reasons behind lefties' apparent decline, however, have so far remained murky, according to the researchers.

McManus said he was initially interested in geographical differences in the rate of handedness. In examining those patterns, he discovered that the differences in handedness between countries at one point in time also existed within a country over the course of time. "'The past is another country,' as they say," McManus said.

Indeed, although left-handers currently form about 11 percent of the population, earlier studies showed that only about 3 percent of those born in 1900 were left-handed, "a more than three-fold difference requiring some explanation," the researchers said.

To begin to sort out the causes of lefties' decline, the researchers looked to a series of films made between 1897 and 1913 in northern England, in which they identified 391 arm-wavers. They then compared the numbers of left and right-handed wavers in the old movies to a "modern control group" obtained by searching for "waving" in Google Image.

The incidence of left-handed waving in the turn-of-the-century films was about 15 percent, compared to about 24 percent on the web, they report. They then compared their arm-waving data to extensive data on handwriting preferences gathered by other researchers, noting that left-waving is generally more common than left-writing. After accounting for that distinction, they 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."

In the Victorian images, lefty wavers were most commonly older, they found, excluding the possibility that mortality among lefties was greater. "Since Victorian social pressure to wave with the right arm also seems highly unlikely, and there can be no response bias, the most likely interpretation is of a falling rate of left-handedness in the nineteenth century, with a true increase in left-handedness during the twentieth century," they concluded.

McManus and Hartigan: "The decline of left-handedness in Victorian England: arm-waving in the films of Mitchell and Kenyon" Publishing in Current Biology, 18 Sept. 2007, R793-R794. 

Note: If no author is given, the source is cited instead.

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