> Most of them are American because psychology is a bigger thing in the US than in Europe, or at least it seemed to be regarding well-known theories, so I presume most research happens there.
I'm not sure about the amount of research but the amount of psychology students here in the Netherlands is huge. And it seems to function in much the same way. Often first-years get some form of credit for mandatory participation in experiments for higher-year students and sometimes regular faculty members. I'm also aware of some researchers going out of their way to get a different sample from the population but in the end it's a lot easier to get participants when you make them.
I'm not sure about the amount of research but the amount of psychology students here in the Netherlands is huge. And it seems to function in much the same way. Often first-years get some form of credit for mandatory participation in experiments for higher-year students and sometimes regular faculty members. I'm also aware of some researchers going out of their way to get a different sample from the population but in the end it's a lot easier to get participants when you make them.