I have been testing this “theory”.
Verdict: The less ad’s you have displayed on a website, the more clicks and high payment for clicks you get when it comes to AdSense (I haven’t tested this with other networks, however, I would assume it is the same).
I tested this on 3 different niche sites - Proxy, Finance, and SEO site. Surprisingly the biggest change in earnings and clicks came from the proxy site. I was earning an average of $0.07/click with 4 ads displayed, when dropping it down to only 1 AdSense ad (others were replaced with affiliate ads) I rose to an average of $0.31/click with a 35% increase in clicks. I have to also point out that there was NO significant change in traffic to ANY of the websites I tested out. With a 35% increase in clicks and a 350% increase on the average payment per click, I am happy to leave the 1 AdSense unit on my proxy sites.
The finance site wasn’t as lucky as the proxy site, however, it too did see an increase. 11% more clicks and a 120% increase on payout. The SEO saw 20% more clicks and a 200% increase on payouts. Again, neither saw an increase in traffic.
Why this occurs is simple if you think about. The higher paying ads are shown 100% instead of a mix between high and low. And with only 1-2 ads displayed, a user does not feel the only purpose of your site is to get them to click on your ads.
I think it is more than safe to say the “phenomenon” is real. And boy, am I sure happy I tested it out. Good luck!