"Concern for posture, as a matter of etiquette, has been around since the Enlightenment, if not earlier, but poor posture did not become a scientific and medical obsession..."

"... until after the publication of Darwin’s 'On the Origin of Species' in 1859. He posited that humans evolved through natural selection, and that the first thing to develop was bipedalism; in other words, standing upright preceded brain development. This idea was controversial because convention taught that higher intellect distinguished humans from nonhuman animals, and now it appeared that only a mere physical difference, located in the spine and feet, separated humankind from the apes.... With the rise of eugenics in the early 20th century, certain scientists began to worry that slouching among 'civilized' peoples could lead to degeneration, a backward slide in human progress. Posture correction became part of 'race betterment' projects, especially for white Anglo-Saxon men but also for middle-class women and Black people who were trying to gain political rights and equity. Poor posture became stigmatized and defined as a disability. As I show in my book, people with postural 'defects' were regularly discriminated against in the American workplace, educational settings and immigration offices..."

From "Beth Linker Is Turning Good Posture on Its Head/A historian and sociologist of science re-examines the 'posture panic' of the last century. You’ll want to sit down for this" (NYT).

This made me think about the way, back in the 1950s, we girls were encouraged to train ourselves in good posture by walking with a book on one's head. I see there's an entry at TV Tropes, "Book on the Head."

And here's a random poster (from 1946):