The number of people born left-handed plummeted temporarily around the turn of last century, according to recently released documentary footage of factory workers in northern England between 1900 and 1906. Researchers recorded the number of people waving to the camera with their right or left hand—a proxy for handedness—and compared the results for different age groups.

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.

Researchers have hotly debated whether the puzzling discrepancy stemmed from reduced social pressure on kids to switch from left to right, a tendency for lefties to die earlier than righties or plain old fibbing by survey respondents. "What we needed was some way of getting at actual handedness of people born before 1900," says study co-author Chris McManus, professor of psychology and medical education at University College London (U.C.L).

McManus and his colleague, psychologist Alex Hartigan, also of U.C.L., got their wish in the form of a 90-minute excerpt of footage shot by early filmmakers Sagar Mitchell and James Kenyon, from which they identified 391 hands waving at the camera. Unlike handwriting, they note, waving is a spontaneous gesture that is not influenced by social pressure.

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.

Researchers hope that figuring out handedness will help them better understand brain organization and the causes of conditions such as dyslexia, stuttering, autism and schizophrenia. Such problems often coincide with abnormal lateralization, or left-right brain specialization.

Many lefties have flipped lateralization, with the right side of the brain controlling speech and language instead of the left. "If the rate of handedness has changed, the implication is the rate of brain lateralization has changed," McManus says.

In July, British researchers reported they had pinpointed a gene linked with left-handedness and schizophrenia.

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.