I've been wondering if there is any correlation between Adsense ads and the number of newly registered users on websites that require registration to participate (like forums, blogs, social websites, etc.).
For example, are there any studies discussing new membership numbers before / after adding or removing Adsense from a website? It would be interesting to know if such a correlation exists and if Adsense has negative/neutral/positive impact.
I don't mean the impact of adding or removing Adsense code on organic Google search results -- I'd rather focus on usability/user-reaction point of view.