Have you ever watched a video of someone dangling far above the ground and found that your own hands started sweating? If not, watch this:

That’s an infamous Ukrainian urban climber known as “Mustang Wanted”. He’s part of a global “rooftopping” craze that has seen daredevils scale everything from The Shard in London, to Shanghai’s tallest building or an abandoned skyscraper-sized radio tower in a Russian forest.

The climbers never use safety ropes, and often dangle themselves off tiny ledges or bars. The videos and selfies they produce regularly go viral, and leading climbers have gathered huge social media followings.

If your palms are already sweating, you’re not alone. There’s even a reddit forum dedicated to these sorts of videos: r/sweatypalms.

People often have a physical response to watching others in peril, even though they themselves are in no physical danger. There are various reasons why your body responds like this in the absence of any real threat.

The brain empathy network

Do you wince when someone is punched in a movie or squirm when someone is shamed or humiliated on screen? These reactions are triggered by empathy: feeling the same thing we believe someone else is feeling. Empathy allows us to put ourselves in someone else’s shoes.

When someone says “I feel for you” they might be talking quite literally. Brain imaging studies have shown that there is a great deal of crossover in brain networks when we experience pain ourselves and when we observe others in pain. For instance, people shown videos of patients being injected in the mouth showed activation in many of the same parts of the brain as if they themselves were being injected in the mouth.

So when we watch videos of people cycling down incredibly steep precipices or dangling from precarious overhangs, part of our physical nervousness on their behalf is because we are imagining ourselves in their situation and how scared we would be.

Aliefs versus beliefs

Another contributing factor might be the fact that the sympathetic nervous system, which co-ordinates your “fight or flight” response, may not differentiate a great deal between real and not-real. Visual information that conveys a threat might be translated directly into feelings of anxiety or urgency, which in turn trigger responses such as muscle contractions or increased heart-rate.

This has often been my experience after watching a horror film. Even when the movie itself has been laughable, with poor special effects and unconvincing acting, I often find myself double checking that the doors and windows are locked before going to bed.

Tamar Gendler, a Yale University psychologist, has proposed that we have two cognitive states for reacting to events in the world. The first is our beliefs – those things that we explicitly believe to be true. I believe with considerable confidence that the protagonist of the movie will be okay in the end and that zombies will not subsequently come into my house and eat me.

Gendler suggests however that there is also a second cognitive state: our “aliefs”. These states are triggered by associations, rather than consideration, and can be either conscious or unconscious. We feel uneasy even though there is no direct threat to us.

Although our aliefs may differ from our beliefs, they trigger many of the same physical responses as a real threat such as trembling, sweating and anxiety. This is why I sit rigid on the edge of my seat while watching the climber dangling precariously from the top of the skyscraper with no apparent support. I believe he will be fine and that this video signals no threat to myself, but I alieve that some threat is occurring and my sympathetic nervous system responds accordingly.

Eccrine glands

Some observers have noted that their palms seem to sweat most specifically when watching climbing videos but not other scary videos where people are equally in danger. There is no real research into this but one possibility, if true, is that our palms sweat when watching climbing videos because of our evolutionary past.

Eccrine glands are the major human sweat glands, distributed all over the body but found in highest concentration on the hands and feet (an average of 370 sweat glands per square centimetre on the palm). They provide cooling from evaporation during thermoregulation but also produce “emotional sweating” in response to stress, anxiety, pain and fear, independently of ambient temperature.

Sweating in the palms is thought to have evolved as a fleeing reaction in mammals. Surprisingly, it has been shown to increase friction and prevent slipping when running or climbing in stressful conditions.

These different theories converge to help explain why your palms might sweat when watching videos of others in perilous situations, particularly if those situations involve climbing or fleeing. The empathy network of the brain allows you to put yourself in someone else’s shoes, imaging how you would feel if you were in their situation. The alief network senses the supposed threat and activates the sympathetic nervous system. This in turn triggers physical threat responses such as nervousness and sweating.

Where the video shows someone climbing, for instance, this sweating may be especially evident on your palms and the soles of your feet, as eccrine glands that evolved to provide extra grip for climbing under stressful conditions are triggered by the alief that some climbing-related threat is afoot.