I’ve just been directed to a wonderful 2007 case study from the British Medical Journal that reports how middle aged doctors can mistake street slang for symptoms of schizophrenia.
Detailed and repeated assessment of [the patient's] mental state found a normal affect, no delusions, hallucinations, or catatonia, and no cognitive dysfunction. His speech, however, was peppered with what seemed (to his middle class and older psychiatrist) to be an unusual use of words, although he said they were street slang.
It was thus unclear whether he was displaying subtle signs of formal thought disorder (manifest as disorganised speech, including the use of unusual words or phrases, and neologisms) or using a “street” argot. This was a crucial diagnostic distinction as thought disorder is a feature of psychotic illnesses and can indicate a diagnosis of schizophrenia.
We sought to verify his explanations using an online dictionary of slang (urbandictionary.com). To our surprise, many of the words he used were listed and the definitions accorded with those he gave.
The article also contains a brief test where doctors can test themselves to see if they can distinguish between slang and thought disorder symptoms.
It’s probably worth noting that traditional British and, particularly London slang, could easily seem like thought disorder to the uninitiated as it is heavily based on word play and substitution.
For example, “I was having having a ruby when I caught Susan having a butchers at my missus’ new barnet” probably makes perfect sense to lots of British people, but if you’re not familiar with cockney rhyming slang, it could be mistaken for a language impairment.
I have noted that British sarcasm can cause similar difficulties during discussions with Americans.