It’s not necessarily a bad thing that 67% of studies are done on psychology undergrads. 90% or so of medical studies are done on mice. If you find something big, you should go outside your cheapest testing group (be it mice or psychology undergrads), but if something fails to produce interesting results even on them, you just saved yourself a lot of money and effort—and failures will be much more common than interesting finds.
It’s not necessarily a bad thing that 67% of studies are done on psychology undergrads. 90% or so of medical studies are done on mice. If you find something big, you should go outside your cheapest testing group (be it mice or psychology undergrads), but if something fails to produce interesting results even on them, you just saved yourself a lot of money and effort—and failures will be much more common than interesting finds.