From a working paper by Kesten Green and Scott Armstrong
We were unable to find evidence that consumers have benefitted from government-mandated disclaimers in advertising. Indeed, experiments and common experience show that admonishments to change or avoid behaviors often have effects opposite to those intended. We found 18 experimental studies that provided evidence relevant to mandatory disclaimers. Mandated messages increased confusion in all, and were ineffective or harmful in the 15 studies that examined perceptions, attitudes, or decisions.
Be careful in jumping to the conclusion that the reason is that the government mandated it, rather than that they are often poorly executed. Lobby groups may in fact spend a lot of time making sure the messages are ineffective. Note that when private enterprise has low incentives to communicate clearly, they don’t do any better. Credit card agreements are a great example, via Planet Money. The before-and-after examples are striking.
Edward Tufte has noted that smoking warnings are usually formatted as if they are meant to be ineffective, with an over-reliance on bold and underlining which actually overwhelms the message itself. I think this derives from non-experts in communication being responsible for what is, in essence, a marketing effort. Typing in bold caps FEELS LIKE YELLING, but that doesn’t mean yelling is effective in changing behavior.