Not specific studies, just a whole lot of life experience.
Everyone has life experience. Welcome to the club. Well conducted studies are usually more reliable than individual life experiences when answering questions such as this one.
for instance, I've never worked a job where there have been dual wage rates - have you, and if so, where?
And you know there have not been dual wages...how? Did you survey all your fellow workers? Or have you just worked the kinds of jobs -- such as McDonald's -- where you know within a few cents what your fellow workers are making? It's because most of us don't know what our fellow workers are being paid that well conducted surveys tend to be more accurate than individual impressions.