So, I promised to follow up on this, and here it is.
All else being equal, we would expect that a person with a will to succeed who is given an opportunity to succeed will do just that. The problem is that the world is messy, that is, that all else is not in fact equal, and here is where we can draw a line between conservative and progressive thinkers (and I use that term loosely).
Conservatives will first point out that not all people have a will to succeed, and indeed, personal and cultural definitions of success vary--but not as much as we might think. Virtually all people desire to live happily and in financial "comfort." This does not, however, mean lounging around the house all day. To the contrary, productivity seems to be a prerequisite to happiness. (Read more on that here.)
Progressives will point out that the all-important all-else-not-being-equal is not just about starting points but also about structural constraints that inhibit initiative along the way. For example, white men with virtually identical resumes to black counterparts are far more likely to get job offers--even if the white men are convicts!
And here is what the social scientific method has to offer: we can systematically control for both starting position and the all-else-not-being-equal's. Moreover, we have been doing this for over a hundred years, amassing quite a bit of evidence which tends to support the progressive worldview over the conservative (though not always perfectly). The conservative viewpoint tends to trust "common sense" over scientific inquiry, and problematically for conservatives, the two match-up infrequently.