20世紀初めの工場を描いたドキュメンタリー映画から、19世紀イングランドでは左利きが少なかったことがわかったという話（わたしは左利きなので、こういう話はつい読んでしまう）。
Chris McManusとAlex Hartiganはドキュメンタリー映画の中で手を振る人を抜き出し、左右どちらの手で振っているかを数えた（手を振ることは字をかくことと異なり、利き手矯正の社会的プレッシャーにさらされにくいのだそう）。すると
They report in Current Biology that the rate of left-handedness plunged from an estimated 20 percent of children born around 1840 to a mere 3 percent of those born 50 years later. The finding dovetails with a landmark 1992 survey that documented a rise in the fraction of southpaws from about 3 percent of the U.S. population born in 1900 to a steady 11 percent of respondents born in the 1950s or later.
Caught on Film: Lefties Were Rare in 19th-Century England - Scientific American
1840年に生まれた人のうち20%が左利きだったのに、1890年頃の生まれだと3%。
映画の詳細。
The researchers eyeballed the ages of the wavers and found that left waving decreased by age, from 40 percent of those in their sixties to just 7 percent of early teenagers and kids. In contrast, when they searched the Web for contemporary images of people waving, 24 percent of the images showed left wavers. McManus says they estimated handedness by noting that, in today's population, left wavers are about twice as common as true left-handers, who use their left hands for much more than waving.
ただ、左利きの人の割合は18世紀の終わりから減りはじめていた。これは産業革命の影響かもしれないそうだ。
McManus says the number of southpaws seems to have started waning between 1780 and 1830. He speculates that the trend may have something to do with England's Industrial Revolution, which occurred around that time and brought with it the rise of factory work on machines―designed for righties, of course.
