Munchkin approach to earning money from scientific studies (if near a big university): Find out if the psychology department (and related social science departments) requires students to participate in experiments for course credit. If not, or if the student numbers are low relative to the number of researchers, there will be arrangements to pay participants. If you are near a suitably large department, as long as you show up reliably and follow the instructions without being "difficult", participation in such experiments can be a decent source of income. And less risky than medical testing in most cases. Central London (UK) is one such place where this seems to work - there are a number of "professional subjects" who work the circuit.

What consequences does this have for the scientific studies? Well I said this is a munchkin approach to earning some cash.... it certainly doesn't make the participant "sample" any less WEIRD