To ensure that you and I don't end up swimming in different pools, let me try to spell out quickly where we agree. First, the modern infosphere is dramatically more diverse in the number and range of perspectives now available. (Taking your cue, I'm referring to the whole panoply here: the Web, cable, talk radio, and so on.) Second, ordinary people have far more control over the perspectives they are exposed to, thanks both to the diversity of media platforms and to the long tail of viewpoints they support. Third, that infosphere is now far more densely interconnected—on the Web, of course, but also on cable. (Bill O'Reilly is just a couple of clicks on the remote away from Keith Olbermann, after all.)
I agree completely that you can use these three developments to build an ideological cocoon for yourself if you so choose. But you can also use them to expose yourself to an incredible range of ideas and perspectives—to challenge your assumptions, fact-check arguments, understand where your opponents are coming from, and stitch together your own informed worldview out of those multiple realities. I realize that description sounds ridiculously high-minded. (Even the most urbane Web polymath goes for a little partisan red meat every now and then.) But let's think of it, for our purposes, as the caricature on the other side of the spectrum, the opposite of the dittohead who doesn't believe anything unless he hears it straight from Rush's mouth.
What we're trying to figure out is which pole has a stronger magnetic force in this new world: the dittohead or the polymath. In such a connected environment, truth should be able to spread more quickly through the system, assuming people have an interest in truth. But if people are more driven by selective exposure—finding online information that confirms what they already believe—then the system will let them keep truth at bay, assuming their beliefs are untrue. (By the way, I loved the sections of your book on the science of selective exposure.)
You invoke the 20 percent to 40 percent of Americans who don't believe the science of global warming as evidence that the forces of selective exposure are stronger than those of truth-seeking. But the percentage of Americans who have both heard of and believe in human-caused climate change has been growing steadily for the last 15 years. Many more Americans now pursue green lifestyles—in their choice of cars, in the products they buy, and in the food they eat. So there's no question the science is making progress and winning converts at a steady rate. But just like the political struggles that dominated the '50s and '60s—which were about facts as much as values, contrary to what you claim—the conversion process takes time. It's frustrating that the change can't happen overnight, but no more frustrating than it was listening to bigots invoking the pseudosciences of sexism or racism in the '60s.
I suppose the great, untestable question on global warming is this: If we could rewind the clock and somehow build an international scientific consensus about global warming in, say, 1950, would the American public have embraced the reality of the threat and the need for change more quickly? We'll never know, of course. It would be interesting to compare the spread of information in the post- Silent Spring era, to see whether the environmental science of that period reached a broad public consensus faster than global-warming science has in recent years. If you—or any of Slate's readers—know of studies along those lines, I'd love to hear about them.
But we do have one clear social experiment that we can look at on the dittohead-vs.-polymath question. If you and I had been having this debate back in 1990, right as the new infosphere was coming into being—talk radio ascendant, online communities starting to take shape—presumably your prediction would have been that the forces of selective exposure in this new world would drive people into those different pools of information, confirming and amplifying their existing beliefs, strengthening their alliances to their initial tribe, and growing further away from those with different perspectives. My prediction, on the other hand, would have been that the connective, diversifying properties of this new world would express themselves in the opposite direction: people breaking free from the party lines and creating more eclectic political worldviews, stitched together from the diverse experiences that they can now encounter on the screen.
What actually happened during that period? Through all the swings back and forth between the two parties, the single most pronounced trend since the early '90s is the steady rise of Americans who consider themselves independent voters, unaligned with either party. (They have tripled in size during that period, by some measures.) Yes, the new information paradigm has been a boon to people who believe only what O'Reilly (or Michael Moore) has to say. But all those independents make me think that the common ground—the space that connects the pools—has become an even more popular place to be.