If you're like most of the rest of us, the speed at which you upload is way slower than the speed at which you download. That makes sense if you think of the Internet as a publishing medium, with most of its participants as recipients. It makes much less sense if you think of the Internet as a participatory medium.
So, I canvassed some friends and colleagues who know a lot about this stuff — I'm leaving them unnamed so my mistakes won't besmirch them — about whether this asymmetry is baked into the technology or whether it's a matter of choice and policy. The answer turns out to be complex. But the bottom line is that we download faster than we upload because of economics, not physics. The economics are those of telephone and cable companies. Their business models assumed the value of the Net came from the content they deliver to us. The business models got written into a physical infrastructure that favors downloads over uploads. So, here we are.
It could have been otherwise. If only the access providers hadn't built a business and an infrastructure on the assumption that we really just want to lean back, relax, and scarf down gigabits of tasty content.
Physics of course sets the boundaries for any business model. If you're uploading and downloading through the same "pipe," increasing upload speeds requires decreasing download speeds. To oversimplify incredibly complex technology, it's like having a single tunnel for one-way traffic: If you now make it two-way (instead of, say, putting in another tunnel), you've reduced the one-way capacity. When the Internet started almost all of us downloaded far more than we uploaded — simply visiting a Web page requires downloading it. Asymmetry reflected our typical usage pattern. Most of us still pull down more than we put up, but with the rise of P2P networks, voice over IP, etc., we're doing much more uploading than we used to. And if the access providers made static IP addresses more affordable, we could host our own Web sites and be uploaders to everyone who visits.
Nevertheless, users generally aren't picking up their pitchforks and demanding higher upload speeds ... perhaps because when we can't upload very quickly, there's little market for services that require a lot of uploading, so people don't build upload-heavy services, so we don't see good reasons to demand faster uploading...
The answer to the question of asymmetry gets more complicated when you look at the various ways of connecting. (There are some details in the longer version of this post.) For example, asymmetry seems to have been engineered into the way the providers deployed coaxial cable, but would be easier to turn on with optical fiber.
Nevertheless, given that physics requires an apportioning of bandwidth between up and down, economics could route around physics: If a provider were committed to providing symmetry without lowering download speeds, it could have (for example) put in a second line when it put in the first. The trade-offs were made by the access providers, largely based on business concerns. For example, one reason we have asymmetric fiber may be that the providers have been charging such high premiums for symmetric access that they don't want to lose that extra money, even as the technology makes symmetry more do-able.
So, as was said by David Isenberg (who gave me permission to credit him): "Asymmetry is a belief system. The purveyors of connections looked at the Internet and saw TV, then acted according to what they saw." That's the infrastructure they built. That's the infrastructure we're currently stuck with. If we can get unstuck, we might at long last get ourselves some symmetry...or have broad enough broadband that symmetry doesn't even matter.