[Says Vonnegut:] Whatever [science fiction] knows about science was fully revealed in Popular Mechanics by 1933. Whatever it knows about politics and economics and history can be found in the Information Please Almanac for 1941. Whatever it knows about the relationships between men and women derives from the clean and the pornographic versions of “Maggie and Jiggs.”
In roughly ascending order of importance, if you ask me. The science has always been hand-waving -- god knows I indulge freely in that -- but the history and politics and economics have long been riddled with wishful thinking and contrafactual (not counterfactual) absurdity. But the people -- yeowtch!
Let's face it -- a big part of why SF is written, read,and published has little to do with being true to human behavior, and more to marketing a certain flavor of something to an audience that wants to taste it. It's been thus ever since the label "SF" existed to stick on a story in the first place. Same with romance, same with comics, same with anything that has a Genre With A Cap G label.
The mere presence of a genre label provides us with reading instructions that let us fill in many blanks on our own. Some of those filled-in blanks are things authors fill in themselves with their own heavy lifting. But when a genre label's applied, there's more of a sense that the readers can do that job on their own. Never mind the violence done to the story in the process, let alone the characters.
So if Vonnegut didn't think much of SF, it seems mainly because everyone involved in its production and consumption was locked in a vicious cycle of shooting high but aiming low.
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Other Lives Of The Mind