Most people who've spent time on a college campus have seen fliers advertising the payoff for participating in, for instance, psychological experiments: do a questionnaire, get $10, a free meal, a gift certificate, yada yada. How do scientists decide what they’re going to pay for your body and mind?

One option is the "inconvenience unit," the National Institutes of Health's way of calculating how much recompense a clinical trial subject should get. For each unit of inconvenience--pain, embarrassment, annoyance, and so on--you get $10, in addition to an hourly wage for your time spent in the clinic.

But the NIH doesn't precisely define or quantify a unit of inconvenience. Christine Grady, acting chief of the NIH Clinical Centers' department of bioethics, is currently compiling the first-ever study looking at how many inconvenience units various procedures have gotten over the years. "An MRI is maybe 5 or 6, a blood draw maybe 1 or 2, a questionnaire, according to how invasive or sensitive it is, maybe 2 or 3," she estimates.

So if you're looking to cash in on questionnaires in an NIH trial, best to find the ones that inquire about your sex life. The snapshot above is from the Kinsey sex survey.

Of course, some people get involved in studies because they want to be a part of something bigger. The folks who volunteer to lie in bed for three months with their feet elevated slightly above their heads so that scientists can study the physiological effects of being in zero gravity tend to be passionate about space flight. They get paid about $5,000 a month, which isn't quite peanuts. But spending that much time off your feet causes significant muscle atrophy and bone loss, so participants must get checked six months after the study ends and again after a year.

That's dedication.

Many scientific studies, though, pull from the vast pool of Psych 101 students required to participate in studies for class credit. The things people will do for class credit are…well, let's just say we're surprised.

For one, they consent to have their arms shaved and let bedbugs crawl around on them. For another, they listen to interminable knock-knock jokes with the punny bits taken out. And let’s not forget the study where college students were directed to wear bathing suits and then made to feel self-conscious about their bodies.

Hope that course credit was worth it.

The Rolls Royces of science experiments--relatively low risk and decent payoff--are Phase I drug trials conducted by pharmaceutical companies. "They want to control the environment very carefully so they can isolate the effects of the drug," Grady explains. "You can find studies where the requirement is to come in for two weeks, and the payment is $2,000." Drug trials can also come with perks. While you're hanging out in Pfizer's clinical trials center, for instance, you’ll get arcade games, a pool table, and widescreen TV.

Drug company Phase I trials are the domain of those people who claim to make a living as scientific subjects (check out a Wired feature on them here). There is a system to it: lining up the trials, making sure you have a two-week cleanup period between studies so you're clean, and so on.

One thing to keep in mind if you try to become a professional guinea pig: The studies are designed to pay so little that they're not a great deal for anyone. While getting thousands of dollars to take some mysterious drug in a research center may sound cush (depending on how you feel about uncertain health risks), the hourly rate is actually quite low.

That's because overpaying patients is something researchers try hard to avoid: Ethics committees discourage any amount that might make people hurting for cash act against their best interests. If the people likely to apply to the study are elderly or unemployed folks desperate for money, or if the community the center is in is poor, the going rates for research subjects' time go down, to a level where they'll actually have to weigh the costs and benefits before getting involved.

In the days before university ethics committees became ascendant, studies sometimes included subjects who had more than a passing acquaintance with the researchers yet were still treated in a manner that seems a bit shocking today.

In a 1949 study on pain, women in child labor consented to have their hands burned between contractions. The women, as the methods section described, were either nurses themselves or the wives of physicians and other "professional men." They undertook participation in the study in order to help others.

They were not paid for their labors.