Americans are dumb because they think they can be. And to a certain extent that's true. But I'd seriously doubt they're genetically more stupid than anyone else.
Even though I'd say americans are more stupid than most developed countries people, my own countrymen are becoming quite stupid too. I think it's mostly because of an ignorant collective feeling that the good situation we have now won't change, therefore we can relax.
Though this, more than ever, separates people as the ones that DO want to learn stuff will excel at it, considering there's so much information out there.
I'm quite sure that it's mostly about lack of interest, because even the ones that were really good in my class didn't seem to be particularly intelligent when I spoke to them about other stuff... They just read the stuff, remembered it, and then forgot it =) (Oddly enough, the ones that were lazy and had worse grades were the more intelligent ones
which may go against my argument..hrmm...)
We become stupid because we allow ourselves to be it.