Pharyngula was musing today on the tendency for those who don't believe in evolution (which is decidedly NOT a belief system, any more than gravity is) to take identical stances on abortion, stem cells, the death penalty, gun control, taxation, etc. This is a very peculiar set of characteristics to share. PZ asks the question:
I wonder what common force ties all those disparate ideas together?
My assumption is that PZ was referring to religion, which isn't totally correct. There are liberal theists who hold a traditionally liberal set of political and ideological positions. PZ is right, of course, that this is a common trend among people with a specific set of ideas about the world. To them it makes perfect sense to be in favor of the death penalty but opposed to abortion, to promote "small" government but demand an enormous military budget.
It turns out that the cognitive scientist George Lakoff addressed this very question in his book Moral Politics: How Liberals and Conservatives Think which I highly encourage people to read (and which Pete endorsed in PZ's comments). The common force that ties these disparate ideas together is a worldview based on a very basic premise: your views on family dynamics. It turns out, as Lakoff describes, that conservatives operate on a Strict Father Model of family dynamics while liberals operate on a Nurturant Parent Model.
These worldviews are formed through childhood experience and then projected into their political ideologies as adults. This has roots in evolutionary strategy since, if a cognitive framework was adaptive during your period of dependency, there's a likelihood that such a framework will continue to be adaptive. Many of the adaptive pressures that our ancestors faced (seeking protection from predators, finding food sources, avoiding parasites and infectious disease) were based on behavioral strategies learned from childhood. These cultural traditions (or memes, if you prefer) imprint themselves on the developing brain and become, in essence, common sense.
Even though views on stem-cell research or gays in the military have no adaptive survival value for those that hold them, the evolutionary cognitive machinery doesn't know that. This "meme machine" continues to imprint the worldviews of adults onto the cerebral hard drives of the young. But what Lakoff has found is that these specific views didn't necessarily have to be taught to children for them to hold them as adults. These views make logical sense based on the foundational premise which was taught. It takes a lot of work to train yourself to spot the inconsistencies and hypocritical aspects of one's imprinted worldview. The good news, as Lakoff points out, is that logic can and ultimately will prevail.
But don't take my word for it, let Lakoff explain it himself in Chapter 2 of his book.