Is it ethical for a psychology professor to expect volunteer work in the lab in exchange for a letter of recommendation? The rationale is that they are unable to judge a student's suitability for graduate school without evaluating their lab abilities. I can see the point, but it's difficult for a working student to volunteer for an entire four month long semester on the professor's schedule, which varies considerably.
Yes. It would be unethical to trade lab work for a guaranteed good letter of recommendation, but I see nothing wrong with your scenario. I think it's acceptable for a professor to only write letters for students they've worked with if they has open lab positions. I also think it's good to require a student commit to at least one month of research at minimum.
It can be hard for many students to juggle/balance life, school, and work. Doubly so for those who need to work through college. But research is the backbone of many grad programs and it's important to get as much experience as possible. The research requirement can get in the way of everything else, but it's also a way for you to try research before applying to grad school.
EDIT: Another way of thinking about it: Would it be an issue if a professor required a student to take their course (or do research) before writing them a letter of recommendation? Of course not! Wanting a letter from such a professor would require you to spend time going to lecture and doing the course work. The only difference here is that one month of research is probably more beneficial and less work than taking a class.
To me the proposal sounds a bit fishy. I'm assuming voluntary = unpaid, though!
So IMHO that would be rather on the unethical side: there's a substantial risk of a) the work not being that voluntary and b) the letter being influenced by
That being said, it is perfectly sensible and ethical that a professor should write letters of recommendation only for students they know well enough. But if the student is paid in the usual way for the lab work, any risk/suspicion of the volunteering being payment for a nice recommendation is immediately gone.
To put this in some context, I'm not from psychology. I'm from a field and a culture where students working in the lab are paid (not that much, but at least something), and where quite some part of the university regulations are making sure that professors cannot abuse their position.
I wouldn't ask "for work in exchange for a LoR", but I'd certainly not be writing a recommendation for a student that I don't know personally in some detail. I.e., they should at the very least have taken several classes with me, more probably have worked as TA a few terms or worked with me in another type of assistantship, or done a thesis with me (or which involved me in some way). I might make an exception for a student that came to my attention in some other way, but I can't really remember any case where that would have applied (and not one of the above reasons too).