A deliberately provocative short story by SF great Theodore Sturgeon which argues in defense of consensual adult incest*. First published in Harlan Ellison's 1967 anthology Dangerous Visions.

Sturgeon's is perhaps the most dangerous (certainly the most unsettling) of the visions to be found in the anthology, despite the polished, self-consciously literate prose with which he presents it. The opening paragraph is worth quoting in full:

The Sun went Nova in the year 33 A.E. "A.E." means "After the Exodus." You might say the Exodus was a century and a half or so A.D. if "A.D." means "After the Drive." The Drive, to avoid technicalities, was a device somewhat simpler than Woman and considerably more complicated than sex, which caused its vessel to cease to exist here while simultaneously appearing there, by-passing the limitations imposed by the speed of light. One might compose a quite impressive account of astrogation involving the Drive, with all the details of orientation here and there and the somewhat philosophical difficulties of establishing the relationships between them, but this is not that kind of a science fiction story.

The premise of the story, it is eventually revealed, is that there is a sort of forgotten paradise of a planet, Vexvelt, which is shunned by the rest of the galaxy despite its rich resources and ideal climate. The protagonist, Charli Bux, learns of Vexvelt's existence without knowing why it is shunned. He reaches the planet, is taken into the household of a Vexveltian man, and falls in love with one of the man's daughters. Charli is revolted and outraged to later find her having sex with his host, her father. The father then sits down with Charli to engage him in a Heinleinesque Platonic dialogue:

     "Tell me, then: what's wrong with incest?"
     "You breed too close, you get faulty offspring. Idiots and dead babies without heads and all that."
     "I knew it!...Isn't is wonderful? From the rocky depths of a Stone Age culture... all the way out to the computer technocracies...- you ask that question and you get that answer. It's something everybody just knows. You don't have to look at the evidence....
     "Sex is a pretty popular topic on most worlds. Almost every aspect of it that is ever mentioned has almost nothing to do with procreation.... But mention incest, and the response always deals with offspring. Always! To consider and discuss a pleasure or love relationship between blood relatives, you've apparently got to make some sort of special mental effort that nobody, anywhere, seems able to do easily- some not at all."
     "I admit I never made it. But then- what is wrong with incest, with or without pregnancy?"
     "Aside from moral considerations, you mean. The moral consideration is that it's a horrifying thought, and it's a horrifying thought because it always has been. Biologically speaking, I'd say there's nothing wrong with it. Nothing. I'd go even further..."
And Sturgeon does go further, to argue that a society in which people are free to act on their incestuous urges may be psychologically healthier.

In arguing that incest is biologically harmless, Sturgeon is only half wrong. The offspring of close relatives are more likely to suffer defects due to doubled copies of harmful recessive genes, but such defects are not as common as people seem to think- and the same is true of non-incestuous mating within genetically homogeneous groups. No one attaches moral significance to the fact that the children of two Ashkenazi Jews are more likely to suffer from Tay-Sachs, or that two parents of West African descent are more likely to bear children with sickle-cell anemia. Moreover, the biological consequences of inbreeding are irrelevant in an age of reliable contraception and prenatal testing.

Sturgeon's suggestion that incest could be socially and psychologically beneficial is an extrapolation from Freudian theory. Freud speculated that men** in modern societies are unhappy because civilization requires that they suppress their primal urges- chief among those, the desire for sexual access to all women, including (especially) their own mothers. Sturgeon's story looks at Freudian thought from an unexpected direction: If the malaise and mental illness of civilized men stem from the repression of their incestuous desires, shouldn't a society in which incest is freely practiced be healthier and happier?

Sturgeon's intention in "If All Men Were Brothers" is partly to shock- the Dangerous Visions anthology was conceived as a collection of SF stories too incendiary to be published elsewhere. But Sturgeon also has the more serious purpose of challenging the reader's preconceptions. He takes a practice everyone knows is wrong and asks, "Why, in particular, is this wrong? Whom does it harm?" The story asks these questions regarding incest, but the point is more general: Sturgeon calls for an intellectual fearlessness, a willingness to ask uncomfortable questions, to follow ideas to their logical conclusions, however unsettling. Or, as Sturgeon, in his afterword to the story, more modestly wrote:

I hope the yarn starts some fruitful argument.

*It should be stressed that the story deals with consensual, adult incest- Sturgeon does not sanction child abuse, incestuous or not, nor does the story mention any sexual contact between adults and children.
**And women- but Freud developed this idea mostly with respect to men. And gender neutrality makes for awkward writing.
†Sturgeon presupposes a utilitarian worldview in which all moral questions can be reduced to empirical questions.

Freud, Sigmund. Civilization and its Discontents. Stranchey, James, translator. New York: W.W. Norton & Co., 1961.
Sturgeon, Theodore. "If All Men Were Brothers, Would You Let One Marry Your Sister?" Dangerous Visions. Ellison, Harlan, ed.
     New York: Doubleday & Company, 1967. 344-389.