A recent article in The Economist highights the impact Mechanical Turk is having on how behavior research is conducted. According to the article, "the ability to run experiments quickly, cheaply and globally promises to transform psychologists’ understanding of human behaviour. Studies that would once have required months or years can now be done in days."
In the past, graduate students were the subjects of many behavioral research studies because they are cheap, and easy for academics to recruit. In fact according to a study conducted by Joseph Henrich and his colleagues at the University of British Columbia, a random American undergraduate is about 4,000 times more likely than an average human being to be the subject of such a study.
This heavy reliance on graduate students limited the diversity of subjects participating in these studies. Mechanical Turk makes it feasible to reach a diverse, on-demand participant pool. As the article notes, several classic psychological studies are being re-run with this more diverse population to see if the results vary.
You can read the full article here.