Is paying participants below min. wage ethical? I am currently doing my PhD, finished all my modules and doing research right now.
My supervisor and I had a good idea for a research paper and I conducted a survey through a crowdsourcing website (e.g. Mturk). The decision was made to pay every participants 0.50 $ and the survey took about 9 minutes. 

Our results are pretty good and I am currently writing the "Sample Collection" section of the paper. However, I am worried: should I include the compensation? Right now it came to my mind that we did pay way below the minimum wage. Was this unethical? Should I include the amount of payment in my paper or just state that we bought some data?
I've had this conversation with various researchers in the crowd-work community, and it is actually a bit of ongoing controversy. Right now crowd-work can be terrible for many workers - way below minimum wage, many tasks are a waste of time and just broken (cannot be completed), tasks take way longer than the poster estimates and ends up wasting workers time, there are few controls from being denied pay for completed task for spurious reasons...and on and on.

So there are communities of people who are trying to improve the situation for crowd workers, and this includes things like trying to improve pay standards, less time-waste, better designed tasks, catering to crowd-worker communities and "guilds" (people often cooperate to find and complete 'good' tasks), etc. Some propose voluntary guidelines for researchers to try to pay at least something vaguely like minimum wage, even for micro-tasks.

With that said, the current state of professional ethics and norms (and applicable laws) - as of the beginning of the year 2017 - is in flux. However, paying crowd-workers to complete online tasks is still regularly paid below anything comparable to minimum wage. So while you weren't on the vanguard of people advancing the standards for crowd-work, you weren't conducting torturous experiments on prisoners and orphans, either.

Therefore, you should be fine to just follow the norms of your field and state compensation (not all fields explicitly mention the amount of pay). Nothing to hide, and I definitely don't think you should try to skirt the issue and claim you paid for data.

That said, for future readers working in crowd-work, you should be aware of a few things:

- There is controversy on how crowd-workers (or "gig workers" more generally) should be classified, protected, and treated.
 - The current state of things is to consider crowd-workers as "not employees of anyone", and so minimum wage does not apply.

 - Researchers are treating crowd-workers as human research participants in some aspects (not subject to minimum wage and typically compensated very little monetarily when possible).

- At the same time crowd-workers are usually not recruited, given informed consent, and debriefed in ways entirely comparable to regular research participants.

- Minimal consideration is given to guiding ethical principals like "Beneficence" when it comes to crowd-workers (or even "Justice" and "Respect for Persons" for that matter). Most IRBs require that you at least pretend to make the participant better off then they were before in some way, such as learning about how science is conducted, but it really strains belief how this can apply to micro-tasks (like image tagging) performed on most crowd-work platforms.

- Some researchers seem to give no real thought to the idea that there are humans working on their research tasks, who are often putting in many hours of labor for them, and often subject crowd-workers to poorly designed, frustrating, time-wasting tasks.

- Some crowd-workers are in fact trying to make a living through crowd-work, and most are not even living in countries with wages so low that the pay is vaguely decent.

This is all very much a part of social advocacy, especially as many believe that crowd-work and gig work are major parts of the future of work in our civilization. Laws have (and will) trail behind these changes in the workforce, and so everything from labor laws to taxation is often a mess and does not apply with great consistency, and these things are highly likely to change in the next decade or two.

Finally, the state of ethics today is still that it is quite typical and normal, and only occasionally "frowned upon", to pay crowd-workers well below anything approximating a minimum wage. It is the norm of today, and you should be honest and forthright about it regardless of how you feel things should have been or should become in the future.