It's called "your acoustic startle reflex," said Seth Norrholm, a translational neuroscientist at Emory University.
explained that if a sound is loud enough "you're going to duck down your head.
"You get evidence from your parents and your environment that you need to be scared of these things," said Norrholm
While the fear itself is learned, though, humans seem to be predisposed to fear certain things like spiders and snakes because of evolution.
"Back in our ancestral age ... young children learned not to pick up snakes and spiders because they're venomous," said Norrholm
In fact, studies have found that when asked to pick out spiders and snakes from a collection of pictures, both preschoolers and adults react more quickly than when asked to pick out non-threatening items -- like flowers -- from the same collection.
That's believed to happen because of the bias we have carried toward them throughout time.
As we get older, fears are developed because of association.
compares it to a combat veteran who survives an encounter with an IED that was hidden in a shopping bag.
If that vet is redeployed and sees another shopping bag, "he
has a fight or flight response.
Here, an association has been made between the cue and the fear outcome."
It's the same exact response a child has to scary Halloween decorations.
"It's about context," said Norrholm
A young child may not know that a skeleton is a scary, until his
parents say over and over how skeleton decorations are spooky.
How does the brain process fear?
When presented with something that scares you, your brain reacts with its fight or flight response.
For example, if you see a snake while hiking, there are two roadways for your brain, said Norrholm
The high road says 'I've seen this kind of snake before, and I don't have to worry'," said Norrholm
Think of it as the reasoning response that overrides the low road.
"There is some evidence to suggest that thrill-seeking is like anything pleasurable -- gambling, eating, -- it releases dopamine," said Norrholm
Dopamine is a neurotransmitter that helps control our brain's reward and pleasure centers.
"We know that the more you reward something, the more that they do it," said Norrholm
And the more that thrill-seekers seek out the dangerous behavior, the better they are able to engage the cortical high road, and provide the rational context that the thrill-seeking behavior isn't dangerous.
Extreme sports athletes are a great example of this: They continue their dangerous behavior because each time they do it, they survive, Norrholm