Yeah, the author used some strong language. He obviously should have started off each sentence with "In my humble opinion".

"Effective" is a value word, so no study can prove it without first defining it (which would probably require 4 or 5 studies itself). However, I believe he's merely referring to cognitive dissonance, which can be briefly stated as the reason we tend to resolve mental stress by ignoring logic.

Also he said it "can" have a horribly diminishing effect. He might only have been claiming 1 in 10922. That's a pretty weak proposition, ergo there are dozens of studies that proved it. In my humble opinion.