When one comes home and sees a questionably unsanitary male lurking in the hallway--blood-stained knife, twitching snarl, camo pants, tattoo of El Diabolico covering his face--one is going to be scared, energized. Fight or Flight is going to kick in. But that’s just horror. Terror occurs when one comes home and sees a tiny child draped in a cutesy bear outfit, hair descending beyond her pale white face, motionless, unapologetic, and unexplained, standing in the hallway.

That is creepy. But why does that fill us with terror? The answer is because we have no label for it, which makes our arousal (see: terror) climb. This is what old experimental psychologists call “two factor theory.” The idea that arousal leads to a feeling that needs a label, and vice versa.
In the tattooed butcher situation, we get anger, fear, action. Either the hero or the victim’s blood is spilled, the problem is resolved, and we go home and forget it instantly. In the innocent-looking girl situation, we get multimillion-dollar blockbusters like The Ring, Firestarter, Watcher in the Woods, and on and on. Why? Because we have no box in which to put a creepy little girl and still feel safe when our spouse goes to sleep and we are left to ascend the dark stairs alone.

Juxtaposing the styles of the two has been interesting, and I find myself using a lot of psychology to try to suit the stories to each market. For instance, in our arousal-label thinking, the arousal (e.g., blood pressure, dry mouth, a tingling spine, shallow breathing) is going to be fairly similar regardless of age. What drives the arousal, however, is profoundly different, and theory can assist a good writer in ruining the reader’s perceived comfort for the next several days.
For an adult, terror can sizzle when it is connected to the developmental crisis the adult is experiencing at the time, but because we play different roles in our 20s, 30s, 40s and beyond, that is really, really hard to do. We are just too abstract and cognizant to make many things believable fears. On the other hand, an adolescent, who is still unsure how a metaphysical presence pushing on the floorboards differs from age-related decline in lumber, is perhaps more likely scared when the fear object is simple and personally relevant.

Good, pervasively frightening adult terror often has to have larger implications. Has to grab you in the darkness of your soul and whisper that you are not strong enough to make it through what you are about to see, hear, smell, and touch. The fear might not be death (anyone raising kids can appreciate the escape that death might bring!) but preserving life. It might be about being big enough and strong enough to help the helpless, to save the vulnerable.
Speaking of fear, I think many writers are afraid of dabbling in this domain because it can be hard to perfect the language of terror. Start with the plot and velocity of the story first, comforting yourself with Salvador Dali’s reminder that we should “have no fear of perfection--we’ll never reach it.” Once you have yourself nice and freaked out, then go back and add the décor of nuance of prose, grammar, and presentation, and you’re ready to scream. Add a five-year-old and Teddy Ruxpin staring back at you vacantly in the hall, and your friends will never invite you to a dinner party again.
About Dr. Heath Sommer:

A native of Sacramento, California, Heath earned his Ph.D. in clinical psychology with an additional degree in the family sciences. He is a regular public speaker, adjunct professor of psychology, and CEO of Seasons of Hope. Currently an Idahoan, he spends his time with a mixture of metropolitan intensity and country living, presiding over a clan of junior novelists who are never fully impressed.
The Manufactured Identity at Goodreads, Amazon and Facebook.
Visit Dr. Sommer's website.
Also catch him on Twitter.