I've posted a few more articles on my evidence page, including this one: Metaphysics of the tea ceremony: A randomized trial investigating the roles of intention and belief on mood while drinking tea, by Yung-Jong Shiah and myself. Our objective was to test, under double-blind, randomized conditions, whether drinking tea "treated" solely with good intentions would enhance mood more than drinking the same tea. We used oolong tea.
This was a follow-up to an earlier, similar study testing whether intentionally "treated" chocolate would result in improved mood, also under double-blind conditions. Both studies showed that the treated substance resulted in better mood. The latest study also studied the role of expectation to see if it modulated this intentional effect. It did, to a highly significant degree.
The bottom line is that if you believe/expect that you are consuming a specially treated substance, that belief alone will strongly influence your mood. But if the substance is also intentionally "treated," then it will influence you even more. And vice versa -- if you don't believe, you're less likely to see any effect.
This is related to the sheep-goat effect, long observed in psi studies, and to placebo effects in medicine and to experimenter expectancy effects in a wide range of areas. These effects have not been warmly embraced in science or in medicine despite the evidence that they exist because of a core assumption that underlies much of scientific epistemology: objective measurements are supposed to be completely independent of observation or psychological factors. This assumption works well enough to be useful in many contexts, but it's not universally true.
When core assumptions are found to be incorrect, that's where real progress begins.