Just an idea I was playing around with that I thought I'd open up to the board:
OK, I'm going to ask this question in two parts - the first is just sort of the backdrop for question 2 and is based on the defining (or at least stereotypical) characteristics of different generations in recent U.S. history; the second is based on the relative importance of the functions in a changing world.
1. Hard to tell if these are accurate, or just the lasting impressions created by the media. Most likely, a bit of both. Generational quirks are probably magnified in pop culture (not everyone in the 20s was a flapper, not everyone in the 60s was a hippie...), but magnified or not, the quirks existed.
The generation that came of age in the '30s and '40s ("The Greatest Generation") seems to be quintessential SJs. This is the duty generation.
The '50s into the early '60s still had some residual SJness, but this also seems like an NT decade. Technological advances exploded, Ayn Rand was writing about the "New Man," and there was a lot of optimism regarding the future.
The later '60s and '70s saw the generation gap between the SJs from The Greatest Generation and their more hedonistic, less duty-bound SP children. The New Age and hippie movement had a very NF feel to them.
The '80s strike me as very SPish. Wheeler dealers on Wall Street, pop music on the radio, bright colors were in style.
The '90s had the obvious NT technological boom, but kept a bit of SP style as well (remember the foosball tables in dot com offices trend?) On television, family shows were decidedly out of style, and shows about groups of friends lounging around were all over TV. Most seemed to combine the introspective self-awareness of the NF temperament (think Dawson's Creek) with the aimlessness of SPs.
Anyway, I think it's too soon to type the current decade, but you get the general idea.
2. Do generational realities actually change the percentages of of MBTI types - sort of an evolutionary process regarding type? Certain functions get more practice, children are raised in a world with different needs, etc...
If so - two more questions: first, given the world we live in (technologically, globally, economically...) which types would likely become more numerous, and which types would we likely see less of? Second, think that would be evolutionary or cyclical?