I would agree but I think there is something about a person who believes in themselves enough to negotiate that can be a differentiator. For instance, my coworker is a total slacker, comes in late and leaves early. I come in early and leave late and take on extra work. Someone who is determined enough to feel they are worth more money will probably statistically work harder. I know it isn't the same for all people, but I would bet that the majority of people who ask for more money work harder.