I agree that the distinction between, to quote you, "society denying people opportunities directly, and people not taking opportunities that they have because of some self-fulfilling belief" is very important. I also agree that, as you observe, it is important precisely because understanding the causes of some problem should, in a rational world, inform the policies selected to address that problem.
The counterpoint I would offer to your statement about the "left wing" analysis of such situations is that, to use similarly broad strokes, the "right" has a tendency, quite strong at times, to completely dismiss the importance, or even existence, of "self-fulfilling beliefs", and correspondingly prescribes exactly the kind of "just get on with it" solutions that I mentioned before. Their political opponents, still observing the problem, are then bound to highlight that, given that the problem does still exist, and that obvious direct causes can be ruled out (assuming they can), then the "self-fulfilling beliefs" must be important.
Although you seem to hold similar opinions to me regarding the importance of understanding these effects and addressing them if they are found to be important, I would suggest that your instincts appear to question whether or not these effects even exist at all. Taking your first paragraph:
> It is very typical on the left for a personal account of the experience and psychology of being X to also be a political argument. Even if the author didn't intend it that way, it's certainly being used in the context of a political argument now. Note the replies by monk_e_boy and rayiner who take the statement on face value and argue that the payoff to education/savings is very small for poor people.
This, to me, looks like an implicit rejection of the importance of these "self-fulfilling effects". By claiming it is political, or is being used in a political way, you appear to be suggesting that her experience is either not real, or is unrepresentative, and is simply being isolated to gain traction for some policy or other. Most importantly, this suggestion appears to be made on the basis that it is unquantifiable experience, rather than from the actual content.
Rejecting anecdotal claims is unscientific is reasonable, and you grant that she may not be directly making "claims" herself. What I would caution against is your (apparent) assumption that the use of her experience to highlight an issue (by the editors of the guardian) can be quite safely rejected simply by virtue of the fact that her experience is difficult to quantify. The most appropriate response, it seems to me, is to demand rigorous research on the subject. If, however, I have misunderstood you, and this wasn't your intention, then please accept my apology in advance.