I did some tests with my existing websites to see how AdSense might have an impact on Google search result rankings.
The site I was using for this test ranked pretty well, for a medium competitive keyword. Roughly 5% of the blog was dedicated to display AdSense ads. The first 4 weeks nothing happened, after 8 weeks the ranking slightly dropped. After the drop in rankings the site went straight to the number one spot in Google.
I did a second experiment with a site which ranked for a keyword I was not able to monetize, so I was willing to sacrifice this site to the Google gods. I added a lot of ads, this time around 15-20%. I have to admit that blog started to look horrible. There were ads all over the place. Within the first month the sites lost 2 positions on the Google search result page. After the 5th week the site completely disappeared from the index. Google gods have taken my offering.
This test was in no way scientific. The drop in rankings could have been caused by many different things, such as an Google algorithm update
However, I still believe that too many AdSense ads on your blog will have a negative impact on how people perceive your site as well at the end how G ranks your blog.
Having the AdSense code present on your blog means you give Google a lot of information on how visitors behave. If G sees that your visitors don’t really stick, Google might punish your site.