So, firstly, I'm not qualified in science, medicine or research, and as such I'm completely open to being wrong in anything I said/will say. (But I have been following this area, and these specific researchers, with great interest.)
Secondly, since they released their results a couple of days ago I haven't yet read the entire paper, just highlights from mainstream press. I'll be reading the full paper over the weekend if not today, in case you're interested you can find it at http://www.thelancet.com/pb/assets/raw/Lancet/pdfs/S22150366...
Now as to my uneducated thoughts on the matter:
1) Maybe they will figure out a good way to do placebo controls. My mother has never tried any drugs other than weed, which was ~40 years ago in university. She swears that the first time she did it, she witnessed a third arm grow out of her body, but also acknowledges that weed doesn't have such effects. Maybe she actually smoked something other than weed, but that long ago I suspect it was just a very strong placebo. There are many, many anecdotes of for example people drinking non-alcoholic beer and seeming to become drunk, or smoking inactive things like tea leaves and thinking they are high. So perhaps, at least for drug-naieve people (i.e. those who don't already know what psilocybin effects feel like) they might not be able to tell the difference between a placebo and the real thing.
2) Given the background of the people in this trial, who have all failed to see improvement in their depression after multiple attempts, even if psilocybin was helping them only by way of placebo... wouldn't that be great in itself? These people who showed improvements had all tried at least two different types of currently-approved anti-depressants, I suspect most/all had tried therapy, etc. (Will have the exact details once I've read the full report, but one line I saw was that one of them had made 11 failed attempts to get rid of it before.) Given psilocybin made a big difference compared to every other option available, it makes me think that this being a placebo is less likely, given all of their previous attempts didn't work at placeboing (is that a word?) them out of depression.
3) If the success rate seen in this admitedly very small trial were to scale up, my gut reaction would be... cares whether it might be a placebo? I read quite recently that Pfizer tried running trials to see if viagra could increase the sex drive of women (this was shortly after they discovered it's now-used benefits for men) and abandoned them because 85% of the women on the placebo reported positive results, so they couldn't draw any data about actual non-placebo effects. Psilocybin is very low on a scale of risk of negative effects, especially when used in such conditions with experts on hand as trip sitters, if it cures $bigpercentage of people with depression who weren't getting cured by SSRIs or anything else, then I say who gives a flying fuck whether it might be caused by the placebo effect, if it works, use it.
4) Previous anecdotal experience. There are many stories of people finding either temporary, or in some cases even permanent, relief from their depression after using psychedelics (not just psylocibin). I'd hazard a guess that these are less likely to be caused by placebos, since these are people who used the drug without any expectation, i.e. without anyone saying "maybe this will cure your depression".
TLDR: Maybe they can figure out a way to do placebo controlled studies, and in my opinion even if they can't it's not neccessarily the end of the road for this research. Another disclaimer is that it's early in the morning and I now need to go do other stuff so I haven't proof-read this comment. Nor have I had my first cigarette of the day. If you get notified of HN replies, and would be interested to see if I have any different thoughts once I've read the study, I'd be happy to comment here again when I get round to doing so.
