on March 20th, 2013 at 02:46 pm
"Eutopia," by Poul Anderson.
But you won't find out why until well into the tale :)
Historical note: though it's in many respects a standard crosstime travel story, the twist was considered shocking enough when it was written that it was in Dangerous Visions for that reason alone. And it's one entirely integral to the AH assumption -- Poul Anderson didn't just throw it in to be either PC or shocking.