There is absolutely no historical evidence that Marie-Antoinette ever said “Qu’ils mangent de la brioche” or anything like it. So where did the quote come from, and how did it become associated with Marie-Antoinette?
Folklore scholars have found similar tales in other parts of the world, although the details differ from one version to another. In a tale collected in 16th-century Germany, for instance, a noblewoman wonders why the hungry poor don’t simply eat Krosem (a sweet bread). Essentially, stories of rulers or aristocrats oblivious to their privileges are popular and widespread legends.
The first person to put the specific phrase “Qu’ils mangent de la brioche” into print may have been the French philosopher Jean-Jacques Rousseau. In Book VI of Rousseau’s Confessions (written about 1767), he relates a version of the story, attributing the quote to “a great princess.” Although Marie-Antoinette was a princess at the time, she was still a child, so it is unlikely that she was the princess Rousseau had in mind.
Since Rousseau’s writings inspired the revolutionaries, it has sometimes been supposed that they picked up on this quote, falsely credited it to Marie-Antoinette, and spread it as propaganda, as a way to rouse opposition to the monarchy. However, contemporary researchers are skeptical of such claims, having found no evidence of the quote in newspapers, pamphlets, and other materials published by the revolutionaries.