Reading Usable Help
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Gordon R. Meyer
Do warnings cause problems?
Steve Silberman's "Are Warnings About the Side Effects of Drugs Making Us Sick?" discusses the little-known nocebo effect. (It's like the placebo effect, but with negative consequences.)
If the documentation you write is littered with warnings and important notices, it's worth considering what the cumulative impact of these inclusions might be. I'm not saying they'd cause a reader to commit an error with grave consequences, but their overuse might result in heightened anxiety and dissatisfaction about the usability of your product.