A wide variety of neurological and emotional conditions can affect a person's ability to speak and lead to excessive swearing. For example, people with various forms of aphasia lose the ability to speak or to pronounce words because of damage or disease in parts of the brain that govern language. Many aphasics retain the ability to produce automatic speech, which often consists of conversational placeholders like "um" and "er." Aphasics' automatic speech can include swear words -- in some cases, patients can't create words or sentences, but they can swear. Also, the ability to pronounce other words can change and evolve during recovery, while pronunciation and use of swearwords remains unchanged.
Patients who undergo a left hemispherectomy experience a dramatic drop in their language abilities. However, many people can still swear without their left hemisphere present to process the words. This may be because the right hemisphere of the brain can process whole swearwords as a motor function rather than a language function.
Coprolalia is the medical term for uncontrollable swearing and is a rare symptom of Gilles de la Tourette Syndrome (GTS). Published numbers vary widely, but relatively few people with GTS exhibit coprolalia, and more males than females experience it. It generally begins between four and seven years after the onset of tics, peaks during adolescence and tapers off drastically during adulthood. There have been medically documented cases of deaf people with GTS-related coprolalia using sign language to swear excessively.
Studies have made a connection between GTS, coprolalia, and the basal ganglia of the brain. Medical researchers have begun to theorize that basal ganglia dysfunction contributes to or is responsible for GTS and coprolalia. Coprolalia also has interesting parallels to more typical daily swearing -- both tend to be more frequent among younger males.
For lots more information about your brain and language, check out the links on the next page.