Left-handers historically had less to fear at the dining table than in the classroom. A classroom is a power struggle; the more tenuous the hold teachers have on their pupils, the less inclined they are to tolerate even the slightest deviation from any rule. Good teachers exude authority; their less gifted colleagues resort to authoritarianism: sit quietly at your desks, take out your notebooks and write. That need for conformity was one reason that left-handers used to be forced, often quite brutally, to write with their right hands.

But since most people before the 19th century hardly went to school at all, the average left-hander lived a relatively untrammeled left-handed life in the village, on the farm and even in the factory. The popular image of persecuted left-handers across history is a gross exaggeration.

What did change for southpaws, around the turn of the 20th century, was the blossoming of modern psychology. One of its basic insights was that we humans — like other animals — interact with the world in ways that change our mental constitutions. Think of Pavlov’s dog, which started salivating at the mere sound of a bell once it learned that the ringing signified feeding time.

If the child’s mind was a tabula rasa — a clean slate upon which, as Mao Zedong once put it, “the most beautiful characters could be written” — then a person’s character and mind-set would not be immutable and God-given, but shaped and honed in the environment. Nature would take a back seat to nurture. The idea of normality took root, and with it a nasty premise: If minds could be shaped, it followed that they could be well-shaped or ill-shaped with respect to some norm, naturally the greatest common denominator. Abnormality — undesirable by definition — could be remedied, since minds were putty. Left-handedness became an illness that needed curing.

Pundits of psychology promoted this dismal view without mercy. In his 1937 handbook “The Backward Child,” the British child psychologist Sir Cyril Burt depicted left-handers as fumblers and bunglers who “squint” and “stammer” and “flounder about like seals out of water.” A decade later, Abram Blau, head of child psychiatry at Mount Sinai Hospital, condemned left-handedness as “an expression of infantile negativism” leading to rebellious stubbornness, secretive superstition, parsimony, obsessive cleanliness and other unpleasant traits — all due, it was believed, to an unloving “refrigerator mother.” As a result, “turning” a left-handed child became an act of mercy and the duty of every responsible parent and teacher. Many older left-handers today remember those times all too clearly.