A patient repeatedly performs an unusual movement. Before revealing the answer, decide what would tell a mannerism apart from a stereotyped posture.

Ask whether an underlying goal or purpose is still discernible in the movement. A mannerism is a recognisable, purposeful action — shaking hands, writing, walking — just performed in a strange or exaggerated way; the goal is intact. A stereotypy has lost that goal-directed quality altogether, becoming a repetitive movement or posture for its own sake, with no purpose left to identify. The book itself notes that the exact boundary between the two can be genuinely difficult to draw in practice, so this is a judgement call in ambiguous cases, not always a clean line.