Humans possess a cognitive faculty to acquire and practice multiple linguistic systems. As cognitive linguistics makes progress in understanding the organ that produces this multilingual faculty of language, monolingualism emerges as the marked case. Considering the biological foundations of human language, and the input impoverishment necessary for the realization of monolingualism, I propose that monolingualism is a social practice of body modification. Like cranial shape modification in babies, foot binding, or tight-lacing of girls, the social practice of monolingualism takes advantage of the plasticity of human bodies in order to alter the natural growth of children before puberty, obtaining a shape and behaviour that conspicuously mark their bodies and deliver a desired social signal. The social practice of monolingualism forces the faculty of language (the linguistic brain organ) into a state of functional atrophy. I explore the predictions that this proposal makes and discuss them in taking the French state as a case study that provides adequate historical context for the development of such a body modification practice in modern Europe.