The Forbidden Left Hand

In 1986, National Geographic magazine collected responses from 1,177,507 Americans aged 10 to 86 about which hand they wrote and threw with. Researchers Avery Gilbert and Charles Wysocki sorted the answers by birth year: 3–4% of those born before 1920 were left-handed, against 11–12% of those born after 1950, three times as many, in the same country, answering the same question. source: Hand preference and age in the United States

A giant survey can hide a slip of memory or social embarrassment. That is why the second result matters, obtained without asking anyone anything: I.C. McManus and Alex Hartigan recoded, frame by frame, 391 English people filmed between 1897 and 1913 by the Mitchell and Kenyon film operators, counting which hand each person waved at the camera with. Within the same footage, older people used their left hand more often than younger ones, the signature of a low point right around 1900, measured without relying on participants' reports. source: The decline of left-handedness in Victorian England: arm-waving in the films of Mitchell and Kenyon

Genetics alone cannot explain a shift this fast, from one generation to the next. A 2022 meta-analysis of 21,127 twin pairs finds that genetic inheritance accounts for at most a quarter of the differences in handedness between people; the rest is left open to factors other than genes, the only kind that could have changed between grandparents and grandchildren without a single mutation. source: Handedness in twins: meta-analyses

One documented change between grandparents born before 1920 and grandchildren born after 1950 was the weakening of school discipline: left-handed children were less often made to write with their right hand. That may account for part of the shift; the cited studies do not establish how much or rule out other causes.

Neither historical source gives a year-by-year reading: the 1986 survey groups respondents by birth decade, and the Mitchell and Kenyon footage only covers 1897–1913. So this page shows two firm poles, before 1920 and after 1950, with a gap between them.

The "right-handed," "mixed," and "left-handed" thresholds above come from one convention (Veale, 2014); other studies draw the lines differently, so your exact LQ matters more than the label attached to it.

The genetic study reports writing-hand categories, while this test calculates LQ from ten gestures. Read the two classifications separately.