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I am having the same issue currently with one of my clients sites. No robots.txt is or was ever in place. I can not find any issues at all that would cause the problem. Was wondering if you every had any resolution to this? Client also uses Yoast.

Hello, If you are searching for studies about this topic then here is a very good paper you have to read. It's named Impact Of Ranking Of Organic Search Results On The Incrementality Of Search Ads http://research.google.com/pubs/pub37731.html I have read it a few times, there are some interesting findings in it. Try to extract the facts and maybe then draw your own conclusion. And just after this read their conclusion!