Placebo = believe that they recieve a credible treatment. 
Occur because of expectations of improvement, demand charectiristics, hope, support, etc. = nonspecific elements.

But how about behavioral research?
Psychotherapy effect is much more complex. What's a placebo control here?

Nonspecific elements hebben veel effect. 

There is no common 'placebo' control group, so it's hard to compare effect sizes of different studies.

I'm going to encourage you a little, to help you get started. :)

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Since you looked us up and read the site enthousiastically, I would like to offer you a big discount.