While our political process is certainly responsible for making this polarization more well-defined, and entrenched, I would argue that there is a natural process that accounts for much of (though not all of) the clumping itself. That is; when examined, there is a good reason why people should find themselves on particular sides of certain issues, if they are on a particular side of another seemingly unrelated issue. The reason is the underlying worldview, or vision.
There appear to be two main conflicting, dominant visions of the world, and of humanity, with very little in between*. In the book Fooled by Randomness Nassim Taleb calls them the Utopian vision (typically that of the left) and the Tragic vision (typically that of the right.) Thomas Sowell has a whole book dedicated to the topic, A Conflict of Visions, where he classifies the visions as the constrained (Tragic) vision and the unconstrained (Utopian, or Vision of the Anointed) vision. If you want an in-depth treatment of the subject, definitely check out Sowell's book (Taleb mentions the visions only in passing in order to classify his own worldview as part of the Tragic vision.) I will summarize the two visions as succinctly as possible.
The Utopian Vision
- Sees scientific knowledge and empiricism as the be-all, end-all means for determining truth. And that the elites of humanity have the ability to effectively 'know everything'
- Optimistic view of humanity and it's ability to significantly improve (if not eventually 'perfect') its condition by learning all there is to learn, and coming up with solutions to problems.
- Believes in centralized processes (i.e. top-down economic systems), and entrusting decision-making (on behalf of society) to experts who 'know' things.
- Favors the theoretical and hypothetical. Experimentation as a means to (hopefully) stumble upon solutions eventually.
- Epitomized by: Godwin, Condorcet, Rousseau, and Thomas Paine.
The Tragic Vision
- Believes in scientific empiricism, but also believes that there are limits to what any one person, or elite group of people, can actually ever 'know'.
- Believes that whatever it is experts do know often doesn't translate to being able to 'solve' a particular problem.
- Sees humanity as fundamentally flawed, and incapable of ever 'solving' many of the problems that plague it, as many of them are inherent to our nature, and insoluble; we're only able to choose methods of reducing the harm done.
- Believes in de-centralized processes (i.e. the free market), and the efficacy of individuals making decisions on behalf of themselves and their own determined best interest, based on their own personal experiences.
- Favors proven processes, established through years of experiences, and hardships of millions.
- Epitomized by: Burke, Hayek, Friedman, Adam Smith, Karl Popper.
Again, see the work of Sowell for a clearer picture, but hopefully this gives you a basic understanding of the two visions. Once you are equipped with these definitions, you quickly start to realize why people's views on seemingly unrelated subjects tend to congeal and polarize. It's because when you see the world and humanity in a certain way, then the way in which you see all the issues involving humanity and how it relates to the world will be derivative of that overall vision. So a liberal will generally be against a strong national defense, for they believe other nations can always be reasoned with (despite lessons of history that say otherwise), and they will believe in higher taxation in order to centralize decision-making in the hands of the intellectual elite, and leave fewer decisions in the hands of the individual. On the surface these two beliefs seem to be mostly unrelated, but in the framework of the visions laid out, you can see that they actually flow from the same source.
Given this, 'centrism' seems to be the result of a muddled kind of thinking. Although centrists like to portray themselves as the epitome of 'free thought', with no allegiance to any superficial affiliation, it turns out that they often have no allegiance to internal coherence or consistency among beliefs, either. Although I consider myself a strong adherent to the Tragic Vision, I still recognize that those who ascribe to the Utopian Vision at least have an internally coherent perspective. That is, given their starting point, most of their beliefs make sense within that context, and don't necessarily contradict each other. The same is rarely true of 'centrists', who often go out of their way to look for middle ground, when often there is no natural reason that there should be any middle ground to be found. Which is not to say that one can never find himself on the side of an issue that differs from the prevailing vision which he prescribes to. Simply that more often than not you should find that your views tend to be consistent with one another, because the basis upon which they are established are all the same. However, this assumes that you are thoughtful when deciding what it is that you believe, which of course is often not the case with people.
* That is to say virtually all views can be classified as falling under one or the other.