Bagpuss, yes, and it's not always easy to separate the two. Roughly: If I tell a group you'll get a treatment and we expect that it works for you and it does, we call it Placebo. If I tell a group you'll get a treatment but we don't know yet what it does positively or even negatively and we want to find out and if that group does better than a group which get that same treatment but don't know we watch their performance, it would be called Hawthorne effect.

And then there is the Pygmalion effect (experimenter expectation, Rosenthal effect)...

Wolfgang