Why do people faint in response to a threat?


Have you ever wondered why people faint? What’s going on when a person falls unconscious?

Imagine you’re alone in a jungle and you sense a predator nearby. This event can trigger a series of fear responses in you that are designed to help you survive.

When you first sense the predator, you’ll likely freeze and stand still- minimizing your bodily movements and gestures. This freeze response allows you to evaluate the situation closely and decide whether you’ll run away (flight) or engage with the predator to ward it off (fight).

By making you motionless, the response also helps you avoid detection by the predator since predators are sensitive to the motions of their prey.

But what if the situation is such that neither flight nor fight is likely to work? If you try to run away, the predator will outrun you and if you try to fight, you’ll be easily overpowered.

In such a scenario, an additional fear response is likely to get triggered- fright.1

Fright (playing dead)

Fright is a response in which an organism plays dead by becoming immobile. When flight or fight will not work, playing dead can sometimes help the organism escape and survive. If the predator thinks that the prey is dead, it’s likely to loosen its grip or lose interest. This can provide an opportunity for the prey to escape.

Here’s a short clip of a gazelle displaying the fright response:

The fright response is observed in not just humans but in all other mammals. Same is true for the responses that precede it- Freeze, Flight, and Fight.

Although the animal plays dead in fright, it does not become unconscious. Instead, the animal is super-alert and its heart rate is super-high as it patiently waits for the right moment to escape or launch some other form of defence.

There’s a species of mammals that can sometimes become unconscious in response to acute fear- homo sapiens.

Fainting (losing consciousness)

Threat-induced fainting is a uniquely human stress response in which a person loses consciousness as a response to a specific type of threat. The person experiences a sudden drop in heart rate and, consequently, blood volume (about a 30% drop2) after the heart-rate increases upon perceiving a potential threat during Freeze.

It’s this lack of blood supply to the brain that causes fainting which explains why fainting may also be caused by hunger and neurological disorders in which the supply of oxygen-laden blood to the brain is hindered.

The faintness or dizziness that you might have experienced after sitting for a long time and then suddenly standing up is also the result of a temporary drop of blood supply to the brain.

Anyway, what we’re interested in here is specifically threat-induced fainting which, because it’s unique to humans, demands a special explanation.


It has been observed that among all the phobias that humans experience, fainting is only associated with what is known as the blood-injection-injury phobia.3 Humans faint when they’re exposed to blood, syringes, needles, knives, etc. and when they experience an injury (even minor, non-fatal skin injuries). 

In other words, threat-induced fainting in humans has nothing to do with the predator-prey interaction but everything to with blood, injection, and injury.

It makes sense because if you faint when you’re in the grip of a predator, you might be able to fool the predator into thinking you’re dead and it might loosen its grip but in your unconscious state, you won’t be able to escape once you get the opportunity. 

What’s so special about blood, injection, and injury?

why do people faint

“I am not a threat”

Human inter-group and intra-group conflict was a consistent feature of the Paleolithic times as it still is, though the violence has comparatively decreased. During such warfare, it would’ve been disadvantageous for the warring males to faint in the battlefield for they’d then become easy targets to kill for the enemies.

But for women, children and other non-combatants, fainting could be useful. It could non-verbally signal to the enemies that, “I’m not a threat to you. You can safely ignore me or take me captive.”4

This way they could avoid getting killed and pass on the trait of tending to faint at the sight of an approaching enemy with a sharp object who’s already shed a lot of blood around and probably has blood marks all over his weapon and armour.

Researchers have found that blood donors were more likely to faint when blood was collected by an experienced phlebotomist (fancy name for a blood-collector) rather than an inexperienced phlebotomist.5

Experienced phlebotomists approach you with a sharp object (needle) and get the job done quickly and non-communicatively. Novice phlebotomists are more likely to be slow-moving and talkative.

It’s possible that the blood donors in the study perceived experienced blood-collectors as a threat similar to the threat of a Paleolithic warrior approaching them quickly and non-communicatively with a sharp weapon.

Consistent with this explanation, epidemiological data on contemporary humans suggests fainting in response to blood, injection, and injury is more common in women and pre-pubertal children and significantly less common in post-pubertal men.6


  1. Rogers, S. M., & Simpson, S. J. (2014). Thanatosis. Current Biology24(21), R1031-R1033.
  2. Schwartzman, R. J. (1992). The Autonomic Nervous System: An Introduction to Basic and Clinical Concepts. Archives of Neurology49(4), 341-341.
  3. Marks, I. (1988). Blood-injury phobia: a review. The American journal of psychiatry145(10), 1207.
  4. Bracha, H. S. (2004). Freeze, flight, fight, fright, faint: Adaptationist perspectives on the acute stress response spectrum. CNS spectrums9(9), 679-685.
  5. Kaloupek, D. G., Scott, J. R., & Khatami, V. (1985). Assessment of coping strategies associated with syncope in blood donors. Journal of psychosomatic research29(2), 207-214.
  6. Bienvenu, O. J., & Eaton, W. W. (1998). The epidemiology of blood-injection-injury phobia. Psychological Medicine28(5), 1129-1136.