Why Do People Freeze During Stressful Situations and Faint at the Sight of Blood?

Q: What’s behind the “deer in the headlights” phenomenon? I understand fight or flight is necessary during dangerous situations, but why do people “freeze” when they’re scared? It seems like the worst thing to do when your life is in danger. – Mindy, New York

Dear Mindy,

freeze response and predator
A successful freeze response in action. Hachi knew that deer were close by but had difficulty locating them in the absence of movement. (Don’t worry, she is not allowed to chase wildlife. Lucky for her. These guys look tough.)

If you correctly ascertain the location of the snake and the best escape route, you win! You get to make whoopee and pass your winning disposition – including your successful stress response – to the next generation. Evolutionary psychologists believe that freezing in response to certain threats works well enough that critters who possess the tendency procreate more often than those who do not.

One benefit to freezing lies in the fact that most predatory animals, including human males, respond more readily to movement than other visual cues. In this picture of Hachi the Iron Dog (right), the deer in the background stood motionless in the presence of predators (Hachi and me). Had we gotten close enough, they would have run. But the first order of business was to become invisible by squelching movement. You’ll note that Hachi, who knew there were deer in the vicinity, looked utterly lost.

A second benefit to freezing is that most predators do not eat dead animals. Chowing down on raw roadkill can lead to deadly illnesses, and so that behavior has been selected out of species that are not equipped to handle rotten food. Freezing, in the manner of Br’er Possum, can trick a predator into avoidance or loosening its grip long enough to escape.

Makes sense so far, right? Well, there is one counter-intuitive stress response that seems to be unique to humans: fainting at the sight of blood or needles. Here, too, evolutionary psychologists have taken a stab at the benefits of such odd behavior.

Paleolithic Pain

Fainting at the sight of blood and injections is common enough that psychologists recognize it as a unique phobia. While other phobias result in increased heart rate and blood pressure, this one is different. At the first sign of blood, injury, or injection, heart rate and blood pressure increase but then sharply drop. The sudden decrease causes fainting, but why do we do this? What could possibly be evolutionarily adaptive about it?

Common wisdom says that the drop in blood pressure and cessation of movement minimizes blood loss. That’s reasonable, but it doesn’t explain why the behavior belongs to humans (possums, for example, don’t faint, they become immobile – there’s a physiological difference).

The answer may lie in one of the most important differences between humans and other animals: since caveman days, we have lived in much more complex social structures than other animals. Recently, evolutionary psychologists have reasoned that fainting became adaptive as soon as humans started declaring war and stabbing each other with pointy objects. In warfare, non-combatants who fainted (especially women) stood a better chance of survival because they might be overlooked by pillagers. Men, however, might not benefit from fainting during combat.

Ergo, if the evolutionary shrinks are correct, one would expect Blood-Injury-Injection Phobia to occur more often in women. Interestingly, the Diagnostic and Statistical Manual states that 55-70% of those who suffer from this phobia are women. Coincidence? You be the judge.

Modern Mayhem

Your sense that freezing is dangerous, Mindy, may stem from the fact that you are writing to me from New York, where a person must be ever-vigilant for fast-moving threats that don’t exist in the wild. You wouldn’t want to be immobilized when the crosstown bus is bearing down on you. Chances are, you won’t freeze for too long. (At least not more than once.) Freezing is part of a hierarchy of responses. An animal will typically be immobile for no longer than it is safe to do so. When staying still is no longer safe, other responses tend to kick in, such as fleeing or fighting, usually in that order.

If you worry that your natural reactions are a dangerous anachronism, take comfort in the fact that you can train to overcome your inborn tendencies. Just ask any firefighter, cop, or soldier. If you don’t want to wear a uniform, you can always take up martial arts. On the other hand, you might find that dodging New York City cabs is training enough.


American Psychiatric Association: Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, Text Revision. Washington, DC, American Psychiatric Association, 2000.

Bracha, S.H. (2004). Freeze, flight, fight, fright, faint: Adaptationist perspectives on the acute stress response spectrum. CNS Spectrums, 9(9), 679-685.

Korte, S.M., Jaap, M.K., Wingfield, J.C., & McEwen, B.S. (2005). The Darwinian concept of stress: benefits of allostasis and costs of allostatic load and the trade-offs in health and disease. Neuroscience and Biobehavioral Reviews, 29(1), 3-38.