2005

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Whether an ad appearing alongside organic search results for a search consisting of a trademark, creates a likelihood of confusion with that trademark, may depend in part on the user’s ability to distinguish what on the search results page is an organic result and what is a paid result.
Publicly available research in the topic

Something for trademark lawyers to do when its slow: search Wikipedia to see what it says about your brands. One reason has to do with preventing genericide (I suspect that the lawyers for THERMOS and PING PONG have been in conact with the folk at Wiki.
There’s also the corporate image thing. Pick any controversial