We all have that friend or relative who is super easy to spook. (And if you don't, check out the guy in the video above.)
But why is it that some people will jump a mile high if you utter the word 'boo' while others won't bat an eyelid? According to Professor Ottmar Lipp of Curtin University, it all comes down to the individual's startle response, otherwise known as the startle reflex.
"Startle is a very interesting reflex. To start from the basic physiology, it is a brainstem reflex, and depending on which source you are relying on, is mediated by two to three synapses. In other words, it's actually very simple," Lipp said.
"Basically, if someone says boo or slams a door, what we do is jump, and some people are more jumpy than others.
"To psychologists, startle is very interesting. It’s a reflex, but it is also affected by psychological processes, especially emotion."
In other words, the startle reflex is just that -- a reflex, and it just so happens some people's reflex is more sensitive than others. However, unlike, say, the automatic reflex that occurs when someone taps your knee, the startle response (and how sensitive it is) can also be affected by your emotional state.
"If I put you in a situation where you feel unpleasant, anxious or nervous, you might have a heightened startle reflex," Lipp said. "Whereas if I put you in a state where you are feeling pleasant and I am showing you materials you like and find pleasant, the startle reflex is usually reduced.
"Probably the best way to explain it is to imagine if you are watching a movie. Say you are seeing a suspense thriller, think 'Psycho' or the like. You're watching and the person is standing there under the shower, the killer comes with the knife... and then someone slams the door. What do you do? You jump out of your skin.
"Whereas if you were watching a comedy and there was hilarity all over the place -- people are laughing, pies are flying -- and then someone slams the door, your chance of being really startled is reduced.
"So even though the physical stimulus was the same, the reaction was different, because the reflex is dependent on the emotional state you are in."
This son really has it in for his easily startled Dad.
And what of those YouTube videos where the same person (like the unfortunate Dad above) gets scared again and again, regardless of emotional state?
"I would say those people naturally have a more sensitive startle response, but I would also predict if their son does that over and over again, the severity of the response will decline," Lipp said.
"One of the major characteristics of the startle response is the reaction to unexpected stimulus. If you do it over and over again, it tends to decline... it is a reflex, after all. You get used to it, basically."