Let's say you are recruiting two groups of people, English monolinguals and bilinguals. If you're having a hard time recruiting enough bilinguals, the natural, free-market thing to do would be to offer to increase how much they are compensated to attract more.
Assuming that both groups complete similar activities in the same time frame, is it unethical to pay one group more than another?
Would the answer be the same if the quality wasn't the part of the design? For example, if your monolingual group is older on average, is it unethical to post an advertisement for an increased rate for young monolinguals to bring your group averages back to comparable?
I have never seen a research group do this, presumably because of how time-consuming it would be to do, but I haven't found anything to suggest that it is wrong to do.
I'm asking here because I briefly discussed this idea with someone, and she seemed to think the IRB would take an issue with it but we didn't got much further.