[bryanrasmussen]

I think the U.S last century had maybe decades (maybe a bit longer or a bit less than a decade in some cases) that changed what had gone before drastically, as such the feeling came about that every generation was different than the one before and should be categorized.

So, last century U.S, the roaring twenties, the great depression, WW2, then I think the 50s were probably seen as something of a return to normalcy after the war but in some ways quite different because now the U.S was really in charge, followed by the 60s, I don't think the 70s was that big a divergence but lots of big thinkers seem to think it was because of the end of Vietnam, the end of Nixon.

I think the last big decade shift were things were significantly different (last century) was the 80s which began the dismantling under Reagan of the New Deal which kept up in the 90s - however by this time the idea that generational shifts were important had taken hold and somewhat trivial things like changes in musical tastes were taken as examples of such a shift.

Then of course there has been a technological change in this century by widespread use of computers and the internet.

[bryanrasmussen]

no longer editable: when I say "Then of course there has been a technological change in this century by widespread use of computers and the internet." I meant that would be seen as another generational shift where the new generation was different than the preceding.