In a new book , The Smile Revolution, the emergence of smiling (it grew slowly and exploded in the 20th century), is said to be attributed to a revolt against tradition, to the rise of dentistry, and especially to what he describes as a “shift in social practices and in sensibilities.” Suddenly, people were spending spare time in non-work, non-religious environments – coffee houses, salons, promenades – which “provided an accommodating framework for face-to-face encounters in which a smiling demeanour prevailed.” Suddenly, in other words, people had something called “spare time,” and needed smiles to fill it.

According to historian Colin Jones, the first recorded smiles were cracked at the end of the 18th century. Before, in antiquity and in medieval and early modern times, the notion of revealing one’s teeth was considered unspeakably vulgar; we didn’t have words or concepts, in most languages, to describe the act.