Put simply, your brain is complex.

It's what defines your humanity and makes you a free thinker. It's where your senses, body movement and behavior originate and are controlled from. It's...why you fall for the creepy scenes in horror movies, involuntarily letting fear take over as you react to something that's scary, no doubt, but that you also know isn't actually real.

Whether you love horror movies or avoid them at all costs, you're certainly familiar with the wave of emotions that comes along with being afraid. And whether it's brought on by a movie or a run in with a big spider, what is fear and why is it so immediately intense?

It's time to discuss the science of what causes fear. And what better way to begin this journey than by examining a pastime that's great at reliably inducing fear: Watching a horror movie.

The frightening psychology behind horror movies

You've got the horror classics: The Exorcist, Nightmare on Elm Street, Halloween, Carrie, Alien, The Shining. Then you've got the horror contemporaries: The Conjuring, Scream, The Ring, Dawn of the Dead, The Blair Witch Project. Even some video games are classified as horror these days.

Whether you're watching something scary on purpose or by accident (*sigh* every horror movie hater has been there), here's what's going on in your brain when that first scary scene hits:

The protagonist steps into a dark, abandoned warehouse, and you know danger is imminent. Fear sets in, and your brain gets to work.

Your amygdala, an area of your brain that helps you take in and respond to emotions, immediately presses the panic button. Because fear isn't just any emotion. It's a powerful, primitive one that your brain and body rely on to maintain your safety. So you better believe that, if sensed, your amygdala places a heavy emphasis on responding to fear.

Next, your amygdala calls upon another area of your brain for help: the hippocampus. The job of your hippocampus is to interpret this fear, helping to put it into context.

In the meantime — just in case — your amygdala and hippocampus work with the rest of your brain to start producing the neurochemicals and hormones that can help your body effectively respond to this fear-inducing situation you've found yourself in.

The scarier the scene (like an amorphous shadow appearing behind the person in the abandoned warehouse), the more strongly your amygdala may react. The cheesier, less believable the scene (the shadow is revealed to be a single zombie walking around 2 mph), the more your hippocampus may try to downplay your fear. This all results in you being more or less afraid while watching any handful of horror movies.

In reality, sensing and processing fear is much more complex than just two areas of your brain balancing one another like a set of scales. Scientists are still learning about fear and the complex chain of events orchestrated by our brains in response to it. And this complexity may also explain why some people like horror movies and others don't. How you sense and respond to fear may be different than the person next to you — explaining why you have your hands over your face but he or she looks wide-eyed with...excitement?

Either way, what we do know is that before your brain has time to fully process your fear — such as how valid or unrealistic it is — it gets a few other bodily systems involved. And it's this response that turns the emotion of fear into the actual physical feelings we associate with being afraid.

Fear isn't just in your head

To say that watching a scary movie can sometimes feel like a physical activity may not be that far of a stretch, because it's not just your brain that reacts to being afraid. As you already know, it's your entire body.

The gasp and the goosebumps. The widened eyes and pounding heart. The intense focus. The hair standing up on your neck and arms.

In response to fear, your brain releases biological molecules that:

• Increase your heart rate and blood pressure
• Accelerate your breathing
• Hyperfocus your attention
• Elicit sweating (Did you know there's more than one type of sweat?)
• Slow your digestive system
• Dilate your pupils
  
  

All of these physiological changes result in your body becoming ready for action. But...what action? When you're simply watching something frightening — full of people and events that you know aren't even real — why does your brain get so worked up? The short answer: Because that's the way it's always been.

The real purpose of fear

When your brain senses fear and quickly sets in motion the events that take your body from 0 to 100, you become better equipped to handle the situation you're in. It's a response that has stood the test of time, and it's called your "fight or flight" response.

Your heart rate and blood pressure increasing means more blood can get to get your muscles. Faster breathing ensures your body is getting plenty of oxygen. Slowing your digestive system allows your body to dedicate more energy to the task at hand. You're ready for fight, or your ready for flight — whichever you choose.

While our hunter-gathering, cave-dwelling ancestors probably relied on this fight or flight response almost every day, it's important for you today, too. You feel it when you come face-to-face with a dangerous animal, have to swerve through traffic to avoid road debris, or as you cower through a haunted house — any threatening event or circumstance that causes you to feel afraid.

And you need to respond immediately to these events because waiting on your brain to fully assess whether the perceived threat is actually real could result in you reacting a few seconds too late.

Hence, the "overreactions" we have to horror movies, as well as why we fall for these scenes time and time again.

We're hardwired to respond to fear, whether real or perceived.

And, while it may be annoying when you're just trying to get through those shadowy-figures-in-a-warehouse scenes, your flight or fight response could be the difference between life and death should you ever have the misfortune of finding yourself in a truly dangerous situation.