Some of these excellent comments put me in mind of the following crude but suggestive way of stating the difference between liberals and conservatives: liberals think that the average person is good but dumb, conservatives that he or she is "bad" (in the sense of self-interested) but smart. Liberals trust the intellectual elite (because they are good) to guide the masses (because they cannot guide themselves); conservatives distrust the elite (because the elite are bad and therefore dangerous) and think the masses can guide themselves. So in the social security debate, liberals oppose private accounts because they do not think the average person competent to manage money for retirement but think government can be trusted to manage it; conservatives support private accounts because they give the opposite of the liberals' answers to the goodness and competence questions.
The basic contrast that I have suggested (something of a caricature, I admit) between the liberal and conservative world views has a further implication for the social security debate. Beleving that people are good and therefore never, or at least very rarely, deserve to be poor, liberals favor redistribution of wealth from rich to poor, which a self-financed retirement system would be incapable of bringing about because everyone would be paying for his own retirement rather than for the retirement of others. Conservatives recognize that people can be unlucky, and also (because in the conservative view people are "bad") that the elderly may free ride on their children, and on these grounds support public welfare for the indigent elderly.
Several comments take issue with my suggestion that social security is prone to politicization because the elderly vote disproportionately to their percentage of the population. The commenters argue that the young could vote more if they wanted to and if they don't, it suggests that they are content with the status quo. I disagree on two grounds. The first is that the cost of voting is greater for the young because they are more likely to be employed and therefore to have a high opportunity cost of time--not only time spent voting but also time spent studying the candidates and the issues. Second, children are disfranchised. This creates serious distortions in public policy. For example, it would make better sense to subsidize health care for children than for old people, because in the first case one would be adding to the stock of human capital. I have argued that each parent should be given an extra one-half vote for each of his or her children, in order to redress the arbitrary imbalance of political power in favor of the elderly.
I would like to underscore a point made by Becker. Compelling and providing are separable. There are good reasons to require people to save for their retirement just as there are good reasons to require children to be educated. But in neither case does it follow that the government should provide the required service. That decision should depend on the relative competence of the public and private sectors in producing particular products and services.