Fight or Flight Response: Definition, Symptoms, and Examples

You may already be familiar with the fight or flight response – a simplified term for how humans and many other animals respond to threat. However, you may be less familiar with how this natural response becomes less helpful when activated too regularly. Below, we will discuss how the fight or flight response is an evolutionary adaptation that helps us deal with immediate threats, but that is not as well-suited to present-day chronic stressors.
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As mentioned above, fight or flight is a simplification of how we respond to threat. The term “fight or flight” was first introduced almost a hundred years ago by Harvard physiologist Walter Cannon (1929). While the phrase is still widely accepted as an apt representation of the human stress response, some scientists have proposed further complexities to the underlying theory. For example, a team of physicians and psychologists questioned, “Does “Fight or Flight” Need Updating?” over 15 years ago (Bracha et al., 2004). In particular, these authors present the case for four responses to stress instead of two, that occur in a specific order. These options are freeze, flight, fight, and fright.

Freeze refers to the initial response to a threat such as a predator, in which an animal becomes hypervigilant and stays still. This response is adaptive because creatures that are still are less likely to be spotted by a carnivorous mammal. As Bracha et al. highlight, freeze is the equivalent to a soldier engaging in a “stop, look, and listen” response to any sign of threat.

Meanwhile, the even less commonly known fright refers to an animal’s final attempt to respond to the threat. Fright is also referred to as “tonic immobility” or, more colloquially, “playing dead,” and relates to the fact that a predator will not continue attacking an animal that is already dead.

In the definition at the beginning of this article, a key component of the fight or flight response is the “endocrine changes” in the body. This phrase refers to the body’s hormones, which are chemical messengers that assist in the proper functioning of various bodily functions. For example, another hormone in the endocrine system that you may already be familiar with is insulin, which is involved in the regulation of blood sugar.

The following hormones are involved in the fight or flight response:

• Corticotropin-releasing hormone (CRH): The hypothalamus in the brain releases this hormone, which reduces appetite and increases anxiety and selective attention. It also triggers the release of ACTH from the pituitary gland.
• Adrenocorticotropic hormone (ACTH): ACTH signals the adrenal glands at the top of the kidneys to release cortisol.
• Cortisol: Sometimes referred to as the “stress hormone,” cortisol increases energy, alertness, and immunity – all helpful if responding to an immediate threat. You can test your cortisol levels with this at-home kit.
• Adrenaline: The adrenal glands also release adrenaline, which increases heart and respiration rate.

Some of the above hormones act as signals for the autonomic nervous system to react in certain ways. The autonomic nervous system controls involuntary bodily activities such as heart rate and breathing and is generally thought of in terms of two branches: the sympathetic and parasympathetic nervous systems.

1. Sympathetic nervous system
Usually, when we discuss the fight or flight response, we focus first on the sympathetic nervous system. One of my professors shared the mnemonic that your sympathetic nervous system has “sympathy” for you when you’re scared, increasing bodily responses that enable you to handle whatever is scaring you. For example, so that your body can fight or run from a stimulus, your sympathetic nervous system increases your heart rate and respiration and decreases digestion.

2. Parasympathetic nervous system
The parasympathetic nervous system plays a role after the perceived threat is no longer salient. Several changes take place, including slowed heart rate and respiration. Essentially, the body returns to its resting state, giving this arm of the autonomic nervous system the descriptive phrase “rest and digest.”

Many of the perceived threats we encounter these days are not physical but rather cognitive – there are plenty of things we worry or stress out about that do not require a physical escape or fight. However, our bodies have still evolved to react to psychological stress in this very physical way, leading to many of the hallmark symptoms of anxiety that we feel. For example, if you are about to give a speech in front of a room full of people, you may feel nervous. Your heart rate and breathing are likely increasing, and you are unlikely to want food (as your digestive system has slowed down). Your body is ready to fight or run if needed – even though this is not helpful in this situation.

You have probably noticed that different people have different stress responses. For example, if you and a friend both encounter a bear while hiking, one of you may stay calm and collected while the other begins to panic. There are many theories about why certain people have stronger or more frequent fight or flight responses than others. For example, one theory is that we are genetically predisposed to have certain stress responses. Neuroscientists have identified a particular gene, catechol-O-methyltransferase (COMT), certain variations of which are associated with a stronger startle response (Montag et al., 2008). Other researchers highlight the effect of stressful events during childhood in altering the HPA axis involved in the stress response (Gillespie et al., 2008).

As we have seen, the fight or flight response begins with the sympathetic nervous system stimulating various bodily responses and ends with the parasympathetic nervous system bringing the body back to rest. Ideally, this process occurs only when helpful, and effectively ends with the body returning to rest. However, due to some of the aforementioned issues, this is not often the case. 

The subjective experience of the fight or flight response is one of feeling anxiety (Robinson, 1990). If your stress response is particularly strong or frequent, you are more likely to experience chronic anxiety and mood problems (Gillespie et al., 2008). At the extreme of a dysregulated fight or flight system, people can experience panic attacks – essentially fight or flight responses to no identifiable threat.
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There is a vicious cycle here since chronic anxiety and mood problems can in turn further dysregulate the fight or flight response. Doctors have also long recognized the detrimental physical health effects of chronic stress, including cardiovascular events (Curtis & O’Keefe, 2002).

1. Deep breathing. Methods for counteracting the fight or flight response generally involve actively doing the opposite of what your sympathetic nervous system automatically triggers. For example, while the SNS increases respiratory rate and breathing becomes shallow in times of stress, researchers have found that we can actively counteract the fight or flight response by taking slow, deep abdominal breaths (Perciavalle et al., 2017).

2. Notice your patterns. It can be helpful to pay attention to when your fight or flight response is more active. For example, I have noticed that I am more likely to be on edge and jittery if I have consumed too much coffee. Noticing this pattern helped me alter my behavior so that I now limit my caffeine intake and calm my fight or flight response.

3. Acceptance. Worrying about your fight or flight response while it is happening might send more signals to the brain that you are in danger, with the result of increasing or prolonging the response. This can be seen in the case of panic attacks, where people think that their panic attack will harm them and as a result, the attack continues. Perhaps counterintuitively, accepting the sensations of the fight or flight response as normal can go a long way towards reducing them (Levitt et al., 2004).

4. Exercise. Researchers have found links between exercise and reduced anxiety (Salmon, 2001). While the reasons for this association are still being explored, one idea is that the mild stress of exercise improves resilience to stress more generally. Other theories focus on the ability of exercise to decrease sympathetic nervous system hyperactivity (Curtis &O’Keefe, 2002).

5. Cognitive-behavioral approaches. Hopefully, having read this article, you see that the fight or flight response is not always appropriate or helpful. Recognizing when your fight or flight response kicks in and reflecting on whether or not it is helpful could help reduce this response in instances where it is not helpful. For example, if you feel yourself getting extremely anxious before a date and are considering canceling, notice this fight or flight response – you are trying to “escape” a perceived “threat.” In reality, you are not in physical danger, even though this is what your body is preparing you for. Reframing how you see the situation and your bodily responses can help calm the sympathetic nervous system.

6. Speak with a professional. In addition to potential mental health issues that a professional might be able to help you with, medical issues could also be playing a role in an overactive fight or flight response. For example, a heart arrhythmia can create a sense of panic. Additionally, beta-agonist medication, often prescribed for asthma, can activate the HPA axis and incite a sense of panic. 

Here are a few more articles that may help you change your thought patterns and increase acceptance.

• ​Bracha, H. S., Ralston, T. C., Matsukawa, J. M., Williams, A. E., & Bracha, A. S. (2004). Does “fight or flight” need updating?. Psychosomatics, 45(5), 448-449.
• Britannica, T. Editors of Encyclopaedia (2019, August 12). Fight-or-flight response. Encyclopedia Britannica. https://www.britannica.com/science/fight-or-flight-response 
• Cannon, W. B. Bodily Changes in Pain, Hunger, Fear and Rage: An Account of Recent Research Into the Function of Emotional Excitement, 2nd ed. New York, Appleton-Century-Crofts, 1929.
• Curtis, B. M., & O’Keefe Jr, J. H. (2002, January). Autonomic tone as a cardiovascular risk factor: the dangers of chronic fight or flight. In Mayo Clinic Proceedings (Vol. 77, No. 1, pp. 45-54).
• Duncko, R., Cornwell, B., Cui, L., Merikangas, K. R., & Grillon, C. (2007). Acute exposure to stress improves performance in trace eyeblink conditioning and spatial learning tasks in healthy men. Learning & Memory, 14(5), 329-335. Elsevier.
• Gillespie, C. F., Phifer, J., Bradley, B., & Ressler, K. J. (2009). Risk and resilience: genetic and environmental influences on development of the stress response. Depression and Anxiety, 26(11), 984-992.
• Levitt, J. T., Brown, T. A., Orsillo, S. M., & Barlow, D. H. (2004). The effects of acceptance versus suppression of emotion on subjective and psychophysiological response to carbon dioxide challenge in patients with panic disorder. Behavior Therapy, 35(4), 747-766.
• Montag, C., Buckholtz, J. W., Hartmann, P., Merz, M., Burk, C., Hennig, J., & Reuter, M. (2008). COMT genetic variation affects fear processing: psychophysiological evidence. Behavioral Neuroscience, 122(4), 901.
• Perciavalle, V., Blandini, M., Fecarotta, P., Buscemi, A., Di Corrado, D., Bertolo, L., Fichera, F. & Coco, M. (2017). The role of deep breathing on stress. Neurological Sciences, 38(3), 451-458.
• Robinson, L. (1990). Stress and anxiety. The Nursing Clinics of North America, 25(4), 935-943.
• Salmon, P. (2001). Effects of physical exercise on anxiety, depression, and sensitivity to stress: a unifying theory. Clinical Psychology Review, 21(1), 33-61.