Recently I reread Jane Austen's PRIDE AND PREJUDICE, a thing I do rather often. This time through, I got to pondering why this period behavior still rings true when so much of other books does not. And I think I found an answer.
In P&P, the characters do not react to events as twenty-first-century readers would do. When, for example, Lydia runs off with George Wickham, the entire Bennet family feels disgraced, and if Lydia does not marry Wickham, they will cast her off. Her virginity, that all-important asset, would be irreparably damaged. That was the belief of Jane Austen's time, and of the author herself as implied in the novel's authorial stance, and everybody in the book conforms to that. We readers accept it as belonging to the novel's period. Except in the case of a few social fanatics, nobody rejects Jane Austen because she was not a free-thinker about sex and marriage. We don;t expect that of her.
In the 1950's, virginity was again (or still) a much-prized social asset. Take one popular novel of the period: Herman Wouk's YOUNGBLOOD HAWKE. This whole book takes a conservative stance toward sex and marriage. There is a scene in which the hero, Hawke, and his friend Jeanne are having dinner in a Greenwich Village restaurant. The author -- not a character, but the author in descriptive narration --says "the other diners were mostly morose young couples who had the look of living together out of wedlock and growing tired of it." Hawke says, "Look about you, Jeanie, and see how stupid sin can get to be."
This is ridiculous. Not Hawke's belief, or even Wouk's, that "living in sin" is wrong and debilitating. That's a belief of the time, and beliefs of the time inevitably turn up in fiction. But nobody can tell by looking at diners in a restaurant if they are shacking up, married, brother and sister, or just friends. That "morose" look might be due to a bad day at work, a head cold, a depleted bank account, or a quarrel over appetizers about President Eisenhower's foreign policy. The difference between Austen and Wouk is that Austen has her characters react to a violation of their belief in the way that such people would react, whereas Wouk misuses his authorial power to falsify human reactions in the service of his story. That is one difference between good and bad fiction.
Much of science fiction is set in the future. Some of it shows humans with beliefs much like those of today, some with a different credo. The point here is that whatever the characters' beliefs, an author does well to know just where they dovetail with his or her own. The danger is not where they don't match, but where they do. This must be portrayed without violating basic, universal human truths, one of which is that characters in a restaurant cannot accurately read the minds of the other diners.
Unless, of course, your book is about telepaths. But that is, literally, a different story.