There are three common variations of this condition. I'll outline each of them briefly:
1. SNS with co-morbid cluelessness
Examples: Aislinn (Wicked Lovely), Abbey (The Hollow)
If these characters were in a movie, they'd be the ones innocently walking straight into the clutches of the villain while the whole theatre screams, "No! Don't turn that blood-smeared doorknob!" These are the characters who are always the last to know, who figure things out long after everyone else, who can have the answer staring them in the face for hundreds of pages without triggering the little lightbulb in their heads. And when they finally do get it, the reader's reaction is pretty much, "Well, duh."
2. SNS with co-morbid recklessness
Examples: Bella (Twilight), Nora (Hush, Hush)
These are the characters whose SNS is characterized by stupid behaviours which often seem like nothing more than a way to drive the plot forward. The way these characters behave is pretty contrived... unless it's common to jump off of cliffs to try to trigger auditory hallucinations or to give away your new winter coat in skid row in order to get directions. This variation of SNS also manifests as girls trying to be with guys of questionable safety. The reader usually knows the guy is bad news. Sometimes the girl does, too. But that won't stop her from trying to be with him if she has this form of SNS.
3. SNS with co-morbid denial
Examples: Ever (Evermore), Jessica (Jessica's Guide to Dating on the Dark Side)
This variation is similar to the first, in that the evidence is staring the character in the face. However, rather than being completely clueless, these characters are in denial. They see what they want to see. They're often written as skeptics, who drive readers crazy because they're in denial for so long that you just want to scream, "He's a vampire! Deal with it, already!" Some characters (e.g., Jessica) recover from this form of SNS quickly when presented with sufficient evidence, while others will stubbornly cling to their skepticism until the reader has become convinced the denial is nothing more than a poorly used literary device.
The problem with characters suffering from SNS is that they're often used (to frustrating effect) in order to drag out the narrative. They're also somewhat insulting; they're a sign that the author doesn't think very highly of his or her readers' intelligence. A mystery isn't much fun if the reader has already figured everything out; watching the slow narrator put together the pieces of the puzzle isn't usually entertaining (although there are exceptions).
What's your opinion of SNS? Is it a condition that does little harm? Or is it something that authors should strive to eradicate? What other characters can you think of who suffer from one of the three variants of SNS?
*My intention is not to mock any particular author. I'm just pointing out a pattern I've noticed in my recent YA reads. I actually enjoy some books with characters suffering from SNS!