A 19-year-old university student, with no psychiatric history, has an unusual, exaggerated bow whenever she greets a lecturer, a habit her friends say is 'just her thing' and find endearing rather than concerning. Before revealing the answer: is this a mannerism in the clinical sense, and what does this case illustrate about interpreting mannerisms in isolation?

Yes, this fits the description of a mannerism — a goal-directed action (greeting someone) carried out in a stilted, exaggerated way. What this case illustrates is that a mannerism, on its own, is not diagnostic of anything: mannerisms occur in people with no mental illness at all, sometimes as a feature of personal style, adolescence, or social anxiety. A mannerism only becomes clinically significant when interpreted within a wider clinical picture, not as an isolated finding.