Quick—what’s the most offensive thing you could say right now?
Whether it’s shit-gargling cunt, faggot cumtits, or something equally inappropriate, your brain’s basal ganglia helped you figure it out almost instantly. And if you didn’t blurt out what you were thinking, you can thank your prefrontal cortex, which plays a role in impulse control. For people with coprolalia, though, something in that neural circuitry has gone awry, and they can’t help letting loose with profane outbursts.
Coprolalia—from the Greek kopros meaning “feces” and lalein meaning “to talk”—is usually associated with Tourette syndrome , although it appears in only 10 to 20 percent of Tourette cases. It has also been documented in people with brain injuries from stroke, encephalitis, and cerebral malaria.
For people who have coprolalia, the vulgar vocal tics can be mortifying, not least because the utterances can be way louder than their normal speaking voice. You can imagine how uncontrollable swearing, the content of which may not reflect the person’s own opinions, can kill job opportunities and relationships. But research on coprolalia can tell us a surprising amount about ourselves and how we store and process language:
Coprolalics invoke the most inappropriate curses of their eras and cultures, and their vocal eruptions are contextually relevant in their inappropriateness. For instance, phoning an airline to make a reservation, a coprolalic patient blurts out, “there’s a bomb on the plane!” When he meets a black person, the same patient cannot restrain himself from exclaiming, “Nigger!” 
In other words, semantics matter. Earlier researchers weren’t sure if people with coprolalia repeated swear words because of their percussive sounds (see James Harbeck’s post about the phonology of cuss words) or because of their meaning. Putting that debate to rest, with meaning being the clear winner, is the case of a coprolalic man who was deaf before he learned to speak: he’d not only make obscene signs but also finger-spell swear words.
More generally, these studies suggest that the catalogue of what people emit in their coprolalic tics reflects what society at large finds most offensive. Take this list of the most frequently uttered coprolalic words in different countries :
UK: fuck, cunt, bastard, piss, sod, cock, shit, prick
USA: fuck, shit, bitch, asshole, bastard
Denmark: kaeft (shut up), svin (swine), fisse (vulva), kusse (vulva), pik (penis), røv (buttocks), pis (piss), gylie (animal feces), sgu (by God)
Spain: puta (whore), mierda (feces), coño (vulva), joder (fornicate), maricon (homosexual) cojones (testicles), hijo de puta (son of a whore), hostia (holy bread)
Brazil: merda (feces), bosta (feces), filho da puta (son of a whore), bunda (buttocks), buceta (vagina), cacete (penis), caralho (penis), porra (sperm), vai tomar no cu (fuck off)
Words about body parts and bodily functions dominate over religious curse words. The differences in the UK and US lists are also interesting, showing what words their respective populations consider more obscene.
What’s more, coprolalic tics are pretty much never euphemisms, which might mean that we store the most offensive terms separately from the rest of our lexicon. Primal, hardwired connections to an exclusive library of curse words explains why euphemisms feel so much less powerful and provocative—and why fluently bilingual people with coprolalia involuntarily swear only in their native tongue. When we tone down our vulgarity for the sake of decorum or swear in a second language, even one we’ve mastered, we’re using different, more conscious neural pathways.
 Kate E Brown and Howard I Kushner, “Eruptive Voices: Coprolalia, Malediction, and the Poetics of Cursing,” New Literary History 32 no. 3, 2001: 537–562.
 F Cardoso, CC Veado, and JT de Oliveira. “A Brazilian cohort of patients with Tourette’s syndrome.” Journal of Neurology, Neurosurgery, and Psychiatry 60 no. 2, 1996:209–212.