You can't do experiments on prisoners without their consent. It's been tried, and the results have been really abusive.
Experiments? That's not as terrible as you suggest. Psychologists carrying on a research to find out why a person betrayed his faction (or worse, his species) aren't that abusive.
And they're much more productive than saying "Die, you traitor!" and putting everything to an end.
There are ethical guidelines scientists, psychologists included, have to follow when they're recruiting individuals for experiments. You can't do an experiment or study on a population of prisoners without consent, so far as I know.
Prisoners have most commonly been used for drug tests, which is now (I believe) illegal.
A case study of an individual prisoner, as you suggested, would certainly be interesting, and those are common.