Soldiers and firefighters also certainly skew less wealthy/educated/powerful, so a priori I'd say the same power dynamics apply there (though it's of course possible that the effect is smaller if you dig into the numbers).

It's also entirely possible that some people would feel driven to participate in a medical study out of a sense of duty (e.g. I'm young and healthy and want to put myself at a bit of risk to contribute to our understanding of the virus so that my much more vulnerable (grand)parents have a better chance).