A placebo is a preparation which is pharmacologically inert but which may have a medical effect based solely on the power of suggestion, a response known as the placebo effect or placebo response. [wikipedia definition]

An example of this could of be when non-alcoholic alcohol were put into the big brother house a few years ago and yet the housemates still act as if they were drunk.

So in other words, it's not so much the alcohol effecting the way they are but them adapting themselves to how they feel they should be acting from drinking it.