Why can negative instructions like "do not mention X" backfire, and what works better?

Negative instructions ("do not use jargon," "never mention pricing") are often less reliable than positive ones because the model still attends to the mentioned concept, and long lists of prohibitions are easy to lose track of across a generation. A classic failure is that naming the forbidden thing can make it more salient. More robust approaches: state what to do instead ("use plain language a 12-year-old understands"), constrain the output format so disallowed content has no place to appear, and enforce hard rules with structure or post-processing rather than trusting the instruction. When a negative rule is essential (safety, compliance), make it explicit and prominent, keep the list short, and back it with validation or a separate guardrail step instead of relying on the prompt alone.