Maya Angelou spoke to laughter’s innate role in our social interactions and well-being when she said, “I don’t trust anyone who doesn’t laugh.” From awkward moments when everyone seems to understand the joke but you, to disagreements on which subject matters are appropriate to joke about, to laughing with your closest friends until your stomach hurts, laughter is generally a social activity and plays an intrinsic role in our social interactions and sense of self [1, 2, 3]. Although we tend to think of laughter as an innately human and indescribable emotional reaction, it has also been observed in chimpanzees, apes, and rats and has been simplified by neuroscientists to a stereotyped series of voiced notes typically about 75 ms long that repeat in intervals after some stimuli [1, 2, 3]. By examining the characteristics of laughter, we can evaluate the theory that laughter may facilitate the parasympathetic response after non-threatening instances of broken expectations.
The parasympathetic nervous system is a branch of the nervous system that restores the body to balance after a stressful event. The parasympathetic system induces processes throughout the body such as decreased heart rate, increased digestion, and increased urinary output. For this reason, its effects are commonly referred to as “rest and digest.” On the other hand, the sympathetic nervous system is the parasympathetic nervous system’s opposite, as it prepares the body to respond to stress. It consists of a similar set of neurological structures that induces the opposite processes, including increased heart rate, decreased digestion, and decreased urinary output, and its effects are known as “fight or flight.” Both the sympathetic and parasympathetic responses are initiated in the brain in response to stimuli. When a person encounters a stimulus, their brain sends signals to the rest of the body via nerves and hormones, eliciting the desired response in each muscle, organ, and gland. For example, if a person encounters a bear while camping, their body initiates the sympathetic response to increase their heart rate and initiate the fight or flight response. Once the person escapes the bear, their brain initiates the parasympathetic response to decrease their heart rate so they can relax again. Because these two systems have opposite effects, they work in tandem to maintain homeostasis, a state defined by the steady and optimal functioning of the body, by responding to internal and external stimuli [4]. One way in which these systems maintain homeostasis occurs following a stressful event. After the sympathetic response is engaged and the sympathetic response is no longer needed, the body engages the parasympathetic nervous system in order to reverse these effects and restore homeostasis.
Aspects of the sympathetic response are engaged during surprise, a phenomenon that involves processing events that are misaligned with previous cognitive and neural assumptions [5, 6]. For example, consider a situation in which someone walking down the street suddenly starts screaming and waving their arms. This broken expectation manifests itself as surprise in the people who are watching the situation unfold. When an expectation is broken, in a social setting or otherwise, the resulting surprise can induce physiological changes associated with distress such as increased heart and respiratory rate. When social expectations are broken but are not threatening, such as when someone suddenly trips but is unharmed, there is no reason for the body to maintain the fight-or-flight state that occurs in response to the surprise. Laughter may serve as a reflex to initiate the parasympathetic nervous system, allowing the body to return to its resting and digesting state.
There is strong evidence to support laughter's ability to initiate the parasympathetic response. Laughter has been associated with stress reduction and improved digestion, both of which are controlled by the parasympathetic nervous system [7, 8]. In fact, simply listening to laughter has been shown to initiate the parasympathetic response, leading to decreased heart rate [9, 10, 11]. Longitudinally, laughter has been associated with decreased stress and generally better health. Bearing all this in mind, laughter is not only a fascinating subject but perhaps even is the best medicine.
In humans, laughter is thought to be a primitive response developed around 2 to 3 months of age and is seen in brief moments of joy [2, 12]. For example, laughter early in development can be observed in the game peek-a-boo. While often played with children of all ages, laughter during peek-a-boo begins at around 4 to 6 months [13, 14, 15]. Interestingly, this is the same period during which infants begin to exhibit object permanence, or the understanding that objects continue to exist even when not seen. Infants of this age therefore have the expectation that objects in front of them will not spontaneously disappear. Thus, when a person “goes missing” during peek-a-boo, a stress response may be induced [15, 16]. When the person returns however, the infant laughs. This timing of laughter supports the theory that laughter serves as a reflex to initiate the parasympathetic nervous system, as once the person returns there is no longer a need to engage the stress response.
Another source of laughter common in young children is tickling, which occurs when non-threatening playful touches are made around ticklish parts of the body [17, 18, 19, 20]. It is thought that tickling-induced laughter is produced reflexively in response to this touch. Additionally, tickling has been associated with activation of brain regions related to the sympathetic nervous system and emotional arousal [21, 22]. As is well established, individuals are rarely able to tickle themselves and self-generated tickling has been shown to activate distinct neural networks from that of others. This suggests that there is an aspect of surprise, or lack of conscious awareness, involved in actually producing a laugh with tickling [19, 22].
While both of these examples appear to support the theory that laughter initiates the parasympathetic response after non threatening instances of broken expectation, why would parents, and often the children themselves, induce this temporary breach in homeostasis? Perhaps the reward of the parasympathetic engagement or “relief” overpowers the harm of the temporary stress [17, 23]. This could be similar to why many people enjoy hard work or exercise, which is unpleasant in the moment but gratifying immediately after. During exercise the body releases more dopamine and beta-endorphins which are both signaling chemicals in the brain, to help the body cope with pain. Similarly, laughter appears to be associated with increased release of dopamine and beta-endorphins. While both dopamine and beta-endorphins are associated with uncomfortable or painful experiences, they are also linked with maintaining a happier disposition and aid in depression recovery. Along those lines, rewarding experiences result in beta-endorphin and dopamine release as well, which may explain some of laughter’s benefits. For example, while tickling often feels unpleasant, it has been shown to act as a reward in rats, to the point where tickling can actually be used in place of other rewards, such as food, to influence their behavior [17]. This connection to beta-endorphin and dopamine release and reward supports the theory that laughter is used to provide relief in stressful situations. Additionally, tickling leads to positive effects on the stress response and helps rats cope with fear and isolation [20, 24, 25, 26, 27]. Laughter through games such as peek-a-boo has shown to be important in infant and child development and bonding [24].
These clearly popular forms of self-inflicted laughter appear to have long term benefits. Some longitudinal benefits of laughter include stress reduction, improved immune response, reduced blood pressure, improved sleep quality, and increased quality of life. It is thought that these benefits can be at least partially attributed to the parasympathetic response, such as through the reduction of stress hormones [26, 27, 28]. These not only help us understand the mechanism and possible purpose of laughter, but provide intriguing evidence that laughter can be used medicinally. Laughter therapy, which can include exposure to humorous stimuli or physically forcing oneself to laugh, has been successfully utilized to help increase quality of life and improve immunity in cancer patients, dialysis patients, and those undergoing long-term hospitalization [28]. The health benefits of laughter have already begun to be incorporated into the medical field: The Humour Foundation in Australia works with “Clown Doctors” in children’s hospitals as a therapeutic measure for young patients undergoing difficult procedures and prolonged hospital stays.This is one instance in which self-medicating may be safe, so feel free to spend a couple more minutes on TikTok today.
The theory that laughter serves to initiate the parasympathetic response after non-threatening broken norms appears to be further supported as individuals age. Throughout life, it is common to continue actively seeking out laughter-inducing stimuli. For example, teens and young adults often turn to internet humor such as memes and TikToks for their comedic relief. In many forms of internet humor, there is a text caption or audio paired with a visual that originates from a different source. Often the captions do not match the original intent of the audio or visual, and it is this unexpected pairing that makes the joke humorous. This incorrect pairing of stimuli could cause confusion and stress response [21]. However, once the humorous intent is realized, the body no longer needs to engage the sympathetic nervous system and thus the reflex of laughter initiates the parasympathetic response.
Also in the category of lifelong humor are inside jokes, which are often connections between words and moments brought up at otherwise unrelated occasions. If a third party does not understand how these two seemingly unrelated objects are in fact related, they will be confused and laughter will not be initiated because the body has no reason to relax. In other words, the person does not find the joke, meme, or TikTok funny. Similarly, if the caption of a meme or TikTok were a simple description of what was occurring, this would likely not elicit laughter, unless of course this was registered as breaking the expectation of what a meme should be. This supports the theory, as laughter would only occur if there is good reason to induce relaxation.
Humor becomes further integrated into social interactions as friends begin to “make fun” of one another. Teasing in this sense parallels rougher play in animals, in which animals such as chimps chase, bite, or slap one another with seemingly no intention to cause actual harm. In fact, laughter-like behavior has been studied in chimps during rougher play [29]. In both cases, seemingly mean or aggressive actions (breaking both innate and socialized expectations of relative peace during social interaction) are acknowledged as non-threatening with laughter and the laughter helps ease the stress of these otherwise aggressive actions. If a chimp were truly aggressive in play, the other would reciprocate violence that could ultimately lead to the opponent’s death [5]. Similarly, humans respond with physical or verbal aggression in response to truly mal-intended insults or aggressive comments. However, in the case of insult for the sake of humor, both parties acknowledge that the aggression is playful and laughter is induced. The connections here suggest that laughter is involved during playful aggression that both parties are intended to enjoy. Play has been shown to reduce stress and be important for proper development and relationships [30]. By extension, it is clear that laughter is an important part of maintaining relationships and participating in social interactions.
Along with seeking laughter-inducing stimuli such as play comes a sense of humor and what jokes or people are deemed funny. Theories on why certain people are funnier than others include the types of associations and ability to spot mistakes, such as relating two otherwise unrelated things in a way that makes sense and causes the reaction of “Haha you’re right!” [22]. Stand-up comedians often deliver characteristic punch-lines which in some way alter the course of the story and thus induce laughter. Proper set-up of audience expectations and delivery of this twist is perceived as funny, while statements that are overly obvious or overly convoluted are not. Humor that points to unsurprising or predictable things, or alternatively surprising things that actually pose a threat, would not be registered as funny according to these findings and the present theory.
People have been seeking laughter-inducing stimuli in the form of comedy long before the creation of TikTok and memes. Less modern examples such as Charlie Chaplin films also focus on breaking expectations as audiences laugh when he trips or lacks understanding in something that is made obvious to the viewer. Were he to trip and be seriously hurt, it would be beneficial for the viewer to evoke the sympathetic response. However, when the viewer knows that Chaplin is not in danger, the reflex of laughter can serve to initiate the parasympathetic response. Once again, it would make sense that people would repeatedly seek out this momentary distress if the reward of initiating the parasympathetic response is greater than the temporary stressor. Not only do these examples offer possible support for the theory, but they demonstrate how humans continue to seek laughter throughout life. Charlie Chaplin was internationally renowned, and today, Netflix comedy specials get upwards of 70 million views.
However, there are situations that do not align perfectly with the model of laughter proposed. In addition to otherwise aggressive situations, humans have been observed to laugh in uncomfortable or nerve-wracking situations [31]. Some circumstances that may incite nervous laughter include when telling personal details and during serious conversations or events such as funerals or telling a lie. In all of these activities, there is some stressful stimulus that breaks from the norms of social situations, as with the other forms of laughter. What differs here, and what complicates the theory, is that these stressful stimuli pose some actual threat. Nervous laughter could still be a way to initiate the parasympathetic response and restore homeostasis during these stressful situations but at seemingly inappropriate times. This may be an inappropriate overactivation of the laughter reflex, similar to when someone jumps at the mere sound of a doorbell. Jumping at the doorbell is a reflexive overreaction to a stimulus that does not warrant that dramatic of a response. Similarly, nervous laughter could be a reflexive overreaction to a stimulus that does not warrant relaxation. Even so, while possibly awkward in the moment, nervous laughter may still benefit someone going through a genuinely stressful event. It has been shown that laughter helps people going through traumatic or generally aversive experiences, which makes sense in the context of the laughter therapy previously discussed [32].
Overall, the theory that laughter serves as a reflex to engage the parasympathetic response after stressful stimuli are registered as non-threatening appears to have many supported aspects. Laughter is a complex process that would be done no justice by oversimplification, but its reflexive nature suggests a stereotyped pathway that has yet to be clearly understood. In order to verify or deny the proposed model of laughter, further research on stereotyping stimuli that cause laughter and measuring neurological and physiological changes before, during, and after laughter would be necessary. Additionally, the possibility of stress response, measures of heart rate, breathing rate, and skin conductance could be taken to better establish a physiological reflex pathway of laughter. The future of laughter research is rich and likely challenging, but if indeed laughter is the best medicine, perhaps this field is no laughing matter.
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