Aside from just size though the major problem is the gender disparity of ~80% women in most of the studies. I didn't see mention of ethnicity but I imagine it's overwhelmingly white.
Yeah. This is a big problem, because your average psychology student is a very particular kind of person - young, middle class woman that is bad at maths and simply wants to get a degree from something.
Before I get called out as -ist, let me explain. Psychology is one of the few "default" subjects - i.e. something you pick when you don't know what do you want to major in. It's also a stereotypical feminine subject, hence huge gender gap. The other popular "default" choice is economics, which tend to draw those proficient at maths.
The point being, psychology students are a very particular subset of the population, and thus it's difficult to generalize results from them to everyone.
Before I get called out as -ist, let me explain. Psychology is one of the few "default" subjects - i.e. something you pick when you don't know what do you want to major in. It's also a stereotypical feminine subject, hence huge gender gap. The other popular "default" choice is economics, which tend to draw those proficient at maths.
The point being, psychology students are a very particular subset of the population, and thus it's difficult to generalize results from them to everyone.