Observation of my sites would suggest that adsense has a negative effect, but I'm looking for some wider input.
I have quite a few sites, never had a problem till Panda 2.1, subsequently some were hit by Penguin and then the latest Panda 20/EMD debacle.
This is what I see - most of my sites with adsense on them were hit by Panda 2.1 and Panda 20. Sites without adsense seemed to have escaped. I just looked at a couple of sites that I sold a while ago, they were of similar quality to my pandalized sites but are still ranking well.
My site hit with Penguin is quite similar to another site in terms of number of pages, number of backlinks, age etc. In fact his backlink profile looks distinctly spammy (nearly all links are from sites by the same designer/on the same hosting). I was dominating the serps equally with him but since Penguin hit me for six and then Panda 20 hit me for six again. Superficially the main distinctive is that my site has adsense and his is not monetised.
So - how likely is it that Adsense is causing or contributing to my problems? Is it that the presence of Adsense is a trigger but not cause of a penalty? Is it that Google doesn't like some of my sites and just pulls down nearly all sites with the same adsense code?
The reason that this is a burning question is in deciding how to move forward. Is it possible that effectively my adsense code is toxic and will always result in any new sites I develop with adsense getting hit when there's a major update?
Finally to say, I have done very little that encroaches into grey hat territory, no link farms, no buying of masses of links, no duplicate content, no scraped content etc