And there are many people who are extremely good, but too shy to negotiate a higher wage. And there are people who are mediocre, who are good at making deals and get the higher pay.
And there are people who are born to rich parents - that's life. People differ in intelligence, looks, ambition - anything really.
My point is that disparity in income (let's say in the same organization) is mostly to do with value that a given individual brings to the organization (in the eyes of the management who decide on this). Negotiation is an order of magnitude less important.
