The idea that our mental terrain decides what information survives in our heads and what gets killed off finds a parallel with a venerable biological theory: Darwin's theory of natural selection. When we see lions and polar bears, we marvel at how well these animals have adapted themselves to the savannah and the polar ice caps. But no committee of lions sat down and decided that the African savannah would be home. No electoral bear-college voted for a leader who called for polar bears to move to the Arctic. These animals did not adapt to their environment, even though we use that figure of speech. Their environment adapted them to it.
Information, in this analogy, is our lion or polar bear. What gets inside our heads depends on the conditions our minds offer. If our minds offer savannah, lions thrive and polar bears die. If we have arctic chills, the polar bears live and the lions die.
This is why offering Obama's birth certificate carries so little weight with people predisposed to buy the theory that the president was not born in the United States: It's like setting a lion down on a polar icecap and expecting it to drive the polar bears into extinction. (Passionate Obama supporters similarly find convoluted ways to reject all criticism aimed at him. This isn't a Republican or Democratic thing; it's just the way our minds work.)
The natural-selection analogy is admittedly imperfect. Animals do not significantly alter their environments, but our minds both shape information and are shaped by it. Accurate information, moreover, is not completely powerless to change people's minds; over time, it becomes harder and harder to deny reality. Finally, partisan commentary is increasingly organized and strategic, and probably shapes the long-term terrain on which our national conversation is carried out.
But the natural-selection analogy is a useful antidote to the surprise we profess when substantial numbers of people subscribe to smears, misinformation and even outright prejudice: Yes, it's true that propagandists spout misinformation to serve their own ends, but if our minds were inhospitable to such garbage, the lies would die as quickly as that lion abandoned on an icecap.