[Francis] Galton was into the idea of improving the human race and believed that statistics could help. He loved [Adolphe] Quetelet’s whole “average man” thing but had one minor problem. In the center of Quetelet’s bell curve were the most commonly occurring traits, not the ideal bodies and minds Galton believed everyone should have. To solve this problem, Galton, through a complicated and convoluted mathematical process (the technical definition of statistics), took the bell curve idea, where the most common traits clustered in the middle and the extremes, and created what he called an “ogive” (he had a habit of making up words) which, as Davis explains “is arranged in quartiles with an ascending curve that features the desired trait as “higher” than the undesirable deviation.” He called this the “normal distribution curve,” and it made the most commonly occurring differences that Galton did not value into deficiencies, and the uncommon ideal bodies and minds that he did value… normal.

[…]

The conflation of the “average” man as normal was a significant step in the history of normality. Statistics did not discover the normal, but invented the normal as that which should occur most often. […] This was a big deal, especially to those who would later find themselves on the wrong side of normal. As Alain Desrosières, a renowned historian of statistics wrote, with this power play by statistical thought the diversity inherent in living creatures was reduced to an inessential spread of “errors” and the average was held up as the normal—as a literal, moral, and intellectual ideal.

Jonathan Mooney gives a brief history of “normal”.