It occurs to me that this is more of a storyteller's pitfall, since it is all about the business of what stories are. (What they are for is for another time).
IN the theatre, Naturalism is a style of performance and of representation, and a style of writing, that seeks to represent Real Life (RL) - and distance itself from the stylistic, symbolic or mythological. It is accompanied by a style of acting which seeks to represent characters as if they were real, ordinary people. Movement is limited to what is necessary for practical purposes and speech patterns are those used in everyday life.
Plays written in a naturalistic style deal with small, local issues - both action, location and consequences are restricted to a small group - such as a family. The sources of drama are to be found in the everyday lives of the people; their choices, and the consequences of their choices.
The same principle, applied to literature, has far reaching consequences, because outside of the artificial environment of the theatre, there is nothing that obliges a literary treatment to be dramatic. It can describe real, everyday existence as it really is.
Here is the fallacy. In RL, shit happens for no reason. People can go through a whole series of episodes of misfortune or good fortune in RL without explanation, justification or lasting consequences. Bad stuff can happen for no reason at all and ruin your life. Good stuff can happen for no reason at all and change your life forever. That's the way RL is.
When naturalism is used intentionally in literature, this is either because the author has a literary or political agenda, or because the story is a true life (TL) story. It can work, and often does.
A typical TL story is the Sporting Hero or, as I usually think of it, Douglas Bader. The main character is a boy wonder who becomes and ace sportsman. Then he has some terrible injury. He has to struggle against a physical disability and his own self-esteem. By the end, he has once again become an ace sportsman.
Because there really are TL examples of exactly this story, we tend to think of this as an example of RL, but it isn't. Think of how many ace sportsmen there are who have a terrible debilitating injury and never play again. Most of them, probably. RL isn't about stories. The Sporting Hero archetype works because it is life conveniently fitting the pattern of a story.
Naturalism recognizes that life doesn't always follow the pattern of a story. RL is usually patternless. But. And it is a very big but, which is why it gets a whole sentence to itself. But readers expect patterns. We turn almost everything we see into a story if we possibly can. We see connections where there aren't any, we coerce unrelated events into relationships by telling ourselves that so-and-so is just naturally lucky while so-and-other is having a run of bad luck. We expect there to be unknown connections underlying coincidences.
And we read stories because we need them. They make sense of our lives, our language, our culture. Naturalism is not always fallacious, but it is when it is used where a story is expected.
Such as in the modern fairytale setting of heroic fantasy. You could write a heroic fantasy where the main protagonist is a peasant housewife whose son is murdered by bandits, whose daughter is eaten by a dragon and whose village laid waste by a goblin horde, and who herself eventually dies of starvation and exposure. But this wouldn't be a story, it would be real life in a fantasy setting.
It isn't enough to write real characters and put them in a fantasy or imagined setting, and then see what they do. An author needs to put those realistic characters into a story, and then see what happens.
The Naturalistic Fallacy then, is this: since real life is not composed of stories, then a novel need not have a story, just convincing characters, locations and events.
The very best naturalistic writers can convince you that there isn't a story when there is. In a story, all the events are related. There are connections between all the characters. Every action has a butterfly effect, and every item is Chekhov's Gun.
I will talk about the Butterfly Effect and Chekhov's Gun in the next installment of "Writer's Pitfalls".
Remember, there are no rules if you can break them skillfully.