Further to my query yesterday
Thanks to everybody who provided assistance in trying to discover the details of the story I was looking for - even if I still haven't managed to pin it down.
The context for me was writing up my commentator's notes from the January workshop in Cambridge as an afterword to the published papers. One of the subjects under discussion was the history of artificial insemination, and an issue that grew out of this was the difference between the whole 'Nobel Sperm Bank' type of idea (seed-bank for GEEENYUSES!) and what couples who are having it for reasons of infertility want, which is a healthy baby perhaps somewhat, but not enormously, more intelligent than they are.
In the story I was thinking of, the perfectly ordinary family in question are assigned a genius child (I don't think it's genetically theirs but part of some scheme). It doesn't fit in at all and evokes all sorts of hostility and misunderstanding, because the gap is too great. (Though the whole thing was, if I remember correctly, done in very simplistic, even crude, terms of what the genius child would be like.)
This led me to think of analogous situations, where people can envisage and understand small steps upward (or in a different direction), but not bigger ones. E.g. children improving on their parents' status, but within comprehensible contexts (foreman rather than shopfloor worker, or a nice office job instead of a manual one). Or marrying 'up': in practical terms (rather than romantic novels) this is usually about a fairly small status change.
And I'm now wondering whether this fits in to the reading/writing debate, and whether there are just some gaps between reader and text that they can't comfortably jump. This would be, presumably, where knowledge of genre and convention and standard tropes, not to mention just reading practice generally, help in getting up the momentum to carry one across.