Animals have been subjected to a lot of unconscionably cruel experiments in the name of science. Darwin’s investigations into ticklishness in apes is a cheery exception. In The Expression of the Emotions in Man and Animals (1872), Darwin carefully describes the reactions of chimpanzees to tickling, comparing his observations with those of scientists who have tickled other apes. He notes that they emit a laughlike panting and that the whole of their expression, in general, is like humans when we laugh: The corners of their mouths are “drawn backwards,” their lower eyelids become “slightly wrinkled,” and “their eyes sparkle and grow brighter” (p. 131). He also notes how the armpits of young chimpanzees are especially ticklish, something which he had also observed in his own children.
Then, Darwin makes a connection between his apes’ response to tickling and the uniquely human capacity for humor: “The imagination is sometimes said to be tickled by a ludicrous idea; and this so-called tickling of the mind is curiously analogous with that of the body” (p. 199). Instead of considering humor as an unprecedented human response, Darwin proposed that it should be considered an elaboration—what we would now call an “exaptation,” using the terminology of modern biology (Gould & Vrba, 1982)—of the response that can be evoked in human and non-human apes alike through tickling. Here, he was following his own famous dictum from The Descent of Man: “the difference in mind between man and the higher animals, great as it is, is certainly one of degree and not of kind” (1871, p. 102).
For Darwin, the key to understanding the nature of humor lay in an appreciation of its precursor: the tickle. A similar idea was proposed around the same time by the German psychiatrist Ewald Hecker in his book The Physiology and Psychology of Laughter and the Comic (1873). As such, it has come to be known as the Darwin-Hecker hypothesis (Fridlund & Loftis, 1990). Recent decades have seen advances in our understanding of humor in the fields of comparative primatology, affective neuroscience, and cognitive psychology, which lend support to Darwin and Hecker’s proposition. A view of humor is emerging which offers new and exciting insights into why some things reliably “tickle” our minds and how gifted jokesters—from those we know personally to professional comedians—exploit this.
This fits within broader attempts to understand human imaginative culture through the lens of evolution and the evolved nature of the human mind (Carroll et al., 2017). Cultural products gain popularity and stay with us because they appeal to cognitive mechanisms often evolved for other purposes (Dubourg et al., 2024). Humor is one of those mechanisms, and understanding its nature is key to understanding comedic culture, from everyday jokes to Hollywood movies. The palette of things capable of making us laugh is wide: In the right artistic hands, even as weighty a subject as our own death can be made light enough to tickle our minds rather than distress us. An evolutionary approach in the vein of Darwin’s, informed by an understanding of where humor sprang from in the first place, helps explain how.
The Deep Roots of Humor
Primatologists call the reaction that tickling evokes in chimpanzees “the relaxed open-mouth display,” or simply the “play face” (van Hooff, 1972). Taking a note out of Darwin’s book, contemporary researchers have systematically tried tickling various primates and have documented the play face across all the great apes, which also includes orangutans, gorillas, and bonobos (Davila-Ross, 2007). The play face is perhaps the most comprehensively studied example of what biologists call a “play signal” (Winkler & Bryant, 2021). Crucially, it can be evoked not just by tickling but also other play behaviors: playful wrestling, playful chasing, or playful biting. These behaviors are staples of play not just among primates but across virtually all animals who engage in play, which includes most mammals and some birds (Owen, 1975).
What unites these play behaviors is that they are mock threats. Animal play typically means play fighting, where the players take turns violating each other’s physical boundaries in ultimately harmless ways. Such play is adaptively functional in that it allows animals to explore what their own bodies can do and endure, training them in the skills needed for actual fighting, while also serving as a medium for amicable social bonding (Gray, 2019). Tickling fits the bill: It is an invasion of one’s personal space, targeting the most sensitive areas of the body, which are generally also the most ticklish ones (Harris, 1999), but it is ultimately unharmful. Darwin noted how, if we feel a tickle to be genuinely threatening, it stops being pleasurable: “a young child, if tickled by a strange man, would scream from fear” (1872, p. 199).
When playing with violating each other’s physical boundaries through wrestling, chasing, biting, or indeed tickling, play signals like the primate play face serve to ensure that all parties involved know that all violations are benignly intended and interpreted, ensuring that no misunderstandings occur that could accidentally escalate the play fighting into actual violence (Pellis & Pellis, 1996). Across all the great apes, the play face is accompanied by a recognizably laughlike panting (Davila-Ross, 2007). Human laughter is distinct only in that we exclusively laugh on out-breaths whereas the other apes laugh on both in-breaths and out-breaths: “ha-ha-ha” vs “ah-ah-ah” (Provine, 2001). Interestingly, when human infants first start laughing, their laughter is closer to the other apes’ (Sauter et al., 2018).
The continuity of humor with play is not limited to its outward expression. Jaak Panksepp (2004) has argued that play is governed by a coherently operating brain system, one among seven basic emotional brain systems that he posits is shared among the mammalian clade. This system is associated with the neurotransmitter dopamine, making play inherently rewarding, and endogenous opioids, which have a pleasantly palliative effect that keeps any startles or minor pains sustained during play from eliciting an actual defensive fight-or-flight response (Panksepp & Biven, 2012). It is largely subcortical, relying on deep and ancient brain structures, as evidenced by the finding that animals who have their entire neocortex surgically removed still retain a love of play (Panksepp et al., 1994).
In humans, humor detection—the ability to see why something is funny—seems to critically rely on higher brain regions in our neocortex, the part of our brain associated with higher thought and abstract mental processes (Martin, 2006). However, humor appreciation—actually finding funny things emotionally rewarding—relies on many of the same subcortical structures and neurotransmitters as play, including dopamine and endogenous opioids (Mobbs et al., 2003; Manninen et al., 2017). In other words, it seems that humor and play, below the hood of our neocortex, may rely on the same basic emotional circuitry. This supports Darwin and Hecker’s proposition that humor taps into the same basic emotional response as tickling does, namely our subcortical mammalian play system.
Slipping on Banana Peels
Strikingly, humor also retains a continuity with play in terms of its eliciting stimuli—the kinds of things that tend to strike us as funny. Scholars have argued about what characterizes the things we find funny since the ancient Greeks, but the most popular theory among modern-day philosophers and psychologists has been the “incongruity theory” of humor (Morreall, 2009). According to this theory, humor requires something that is “incongruous with” (i.e., something that violates) our expectations or our normal mental patterns: for something to be humorous it must be somehow unexpected or atypical. Darwin seemed to follow this theory, proposing that humor requires “something unexpected—a novel or incongruous idea which breaks through a habitual train of thought” (1872, p. 200).
However, recent humor research has modified this received wisdom about humor: For something to be humorous, it is not enough for it to simply violate our expectations or our normal mental patterns; the violation in question must have a negative valence (Warren et al., 2020). Why, for instance, is someone slipping on a banana peel stereotypically considered humorous while someone winning the lottery is not, despite both scenarios being both incongruously surprising and atypical? Well, that is because only the former involves a violation in the sense of something wrong, bad, or threatening. For something to be humorous, then, it must be incongruous with (i.e., violate) not our expectations of how things will be or our sense of how things usually are but rather our normative sense of how they “ought” to be.
This mirrors the physical violations of play, like tickling. You cannot tickle yourself because for a tickle to constitute a mock threat it must come from outside of yourself (Claxton, 1975). But it makes no difference whether you expect the tickle. In fact, both human babies and chimpanzees will laugh merely at the expectation of being tickled (Provine, 2001). Experiments have shown, time and again, that humor does not require surprise either, only a violation. For instance, Warren and McGraw (2015) showed people videos of scenarios like a skier attempting a ski jump, pausing the videos midway through (with the skier mid-air), and asking their research participants to guess what happened next. Regardless of their expectations, they found it funny to see skiers fail and they did not find it funny to see them succeed.
Slapstick scenarios and mishaps like slipping on a banana peel or failing a ski jump bear the greatest resemblance to the physical violations of animal play, but the violations that humans find humorous come in many forms. For instance, puns and wordplay rely on violations of the linguistic norms that govern our everyday conversations, like the norm against ambiguity (Attardo, 1994). Communicative norms like these were a prerequisite for language to evolve in the first place (Tomasello, 2010). Normally, we can assume that people’s words have one meaning and not more, which is necessary to make sense of each other’s conversational contributions (Grice, 1975). With puns, we violate this norm by deliberately saying at least two things at the same time, e.g., “Everyone thinks my runny nose is funny, but it’s snot.”
Humor can also be garnered from social violations. Cringe comedies like the British The Office or its American adaptation derive their humor from depicting characters committing embarrassing violations of social norms (Hye-Knudsen, 2018). The faux pas of protagonists David Brent and Michael Scott are funny. Similarly, a lot of dark humor is generated from moral norm violations: Saying or doing the wrong thing. Entire stand-up routines are built around this premise, such as Louis C.K.’s “Of course… But maybe” routine from his 2013 special Oh My God, wherein he simply states out loud his own morally impermissible “bad thoughts” after prefacing them with the morally correct opinion: “Of course, children who have nut allergies need to be protected… But maybe, maybe if touching a nut kills you, you’re supposed to die.”
Pleasure Through Pain
For a violation to elicit humor as opposed to purely negative emotions, it ultimately has to be appraised as benignly non-worrisome. This is why a smaller violation like slipping on a banana peel is considered humorous while a more serious violation like being diagnosed with cancer is not. It is funny to watch characters on television suffer the embarrassment of committing a social violation, but we rarely find acute embarrassment funny when we experience it ourselves directly in our own lives. Similarly, Louis C.K.'s routine relies on us, as his audience, understanding that he is not actually advocating death to children who suffer from a peanut allergy. He prefaces his joke with the correct moral opinion on the matter, and he expects us to know that he is, in fact, only kidding when he says the opposite.
For something to be humorous, two appraisals must thus hold simultaneously: a violation appraisal and a benign appraisal. The things we find funny are those things that are bad yet okay, wrong yet right, threatening yet harmless. This is called the “benign violation theory” of humor, originally proposed by the linguist Thomas Veatch (1998) but then significantly expanded upon by the psychologists Peter McGraw and Caleb Warren (2010). McGraw, Warren, and other researchers have repeatedly shown that the benign violation theory is superior to other contending theories of humor at predicting what people will actually find funny in an experimental setting (Warren et al., 2020). As an elaboration of the incongruity theory, it is the most empirically well-supported theory of what makes something funny.
Reasoning backward from the characteristics of tickling, Darwin intuited the necessity of benignity for humor. For the physical violation of tickling to excite rather than irritate, he noted that “[t]he touch must be light” (1872, p. 199). So too, it seemed to him, with humor: “an idea or event, to be ludicrous, must not be of grave import… the mind must be in a pleasurable condition” (p. 199). His German colleague Ewald Hecker proposed that humor, like the physical violation of tickling, relies on a careful balance of good and bad, pleasure and pain. Both humor and tickling, he suggested, consist in a “competition of feelings,” a “swaying back and forth between pleasure and displeasure” (1878, p. 82). He approvingly quotes another writer's phrase for it: “pleasure through displeasure” (Vischer, 1846, p. 477).
During the course of human evolution, “proto-humor” was likely gradually expanded from only being a response to benign physical violations like tickling to all kinds of benign violations, like linguistic, social, and moral ones (Gervais & Wilson, 2005). We can get a sense of what this would have looked like by observing the play behavior of chimpanzees and other apes, which sometimes veers into territory that we call humor when committed by humans. While most play in chimpanzees consists in simple physical violations, they are also capable of committing more complicated playful violations like teasing (Eckert et al., 2020). One chimpanzee may offer another an object and then pull it away as they reach for it, or they may playfully disrupt others’ activities by snatching an object they are using away from them.
Just as play among chimpanzees serves both exploratory and social purposes, so too with humor. In the face of a benign violation, humorous amusement likely serves to motivate us to explore and play with its implications while our laughter in turn serves as a play signal, reassuring others of its benignity and inviting them to join us (Gervais & Wilson, 2005). In this way, humans are able to exploit benign violations of all kinds for exploratory, peer-bonding play. As the psychologist Matthew Gervais puts it: “What the humor is indexing and the laughter is signaling is, ‘This is an opportunity for learning.’ It signals this is a non-serious novelty, and recruits others to play with and explore cognitively, emotionally, and socially the implications of this novelty” (in Warner & McGraw, 2014, p. 78).
The Psychology of Comic Distance
In many cases, it is plain to see the benignity of the violations people find humorous: playful teasing is ultimately harmless and so is someone slipping on a banana peel or violating the linguistic norm against ambiguity with a pun. But sometimes people’s minds are tickled by darker things, much more severe violations. Cross-culturally, a lot of humor is generated from pain and misfortune (Hye-Knudsen, 2023). Consider a film like Stanley Kubrick’s Dr. Strangelove or: How I Learned to Stop Worrying and Love the Bomb (1964), arguably the most iconic example of the genre of black comedy. This film came out at the height of the Cold War’s nuclear anxiety and depicted what was many people’s worst fear at the time: total nuclear annihilation. Yet, it made people laugh rather than shudder at the prospect.
Under the right circumstances, even a violation as severe as the idea of one’s own demise through a nuclear strike can be rendered benignly humorous. How did Stanley Kubrick manage to transform one of his time’s most pressing causes of psychological distress into a source of humorous amusement? The answer can be found in the useful construct of psychological distance. Psychological distance describes how far removed we feel from an event. It can be split into four dimensions: temporal distance (how long ago or how far into the future is the event in question?), hypothetical distance (is it real or imagined?), social distance (is it happening to myself or someone else, someone like myself or someone different, someone I like or dislike?), and spatial distance (literally, how far away is it?).
Psychological distance modulates our emotional responses (Soderbergh et al., 2015). All else being equal, we have a stronger response to something that is happening now as opposed to in the past or future (temporal distance), that is actually happening as opposed to being imagined (hypothetical distance), that is happening to ourselves as opposed to someone else (social distance), and that is happening where we are as opposed to somewhere else (spatial distance). McGraw, Warren, and colleagues have shown experimentally that manipulating these dimensions of distance can make an otherwise distressing violation benignly humorous (McGraw et al., 2012). For instance, subjects in one experiment rated a physical abnormality more humorous when seen from further away as opposed to up close (spatial distance).
Filmmakers like Stanley Kubrick similarly manipulate psychological distance through their narrative and stylistic choices to reap their desired response from audiences (Hye-Knudsen, 2022). When Dr. Strangelove came out, the threat of nuclear annihilation was far from temporally distant, but Kubrick made his depiction benign through other dimensions of distance. All fictions are hypothetically distant in the sense of being unreal, but they differ in how much they attempt to hide their artifice and feign authenticity. Dr. Strangelove is marked by comic unrealism, from its broad comic acting to its farcical plot twists. When animals play, they will often exaggerate movements to signal their benignity to partners. The markers of hypothetical distance that characterize comic fictions are a mirror of this (Grodal, 2014).
Dr. Stangelove also employs social distance by having unlikable, cartoonish characters instead of sympathetic ones, another common characteristic of comic fictions, which has been noted by critics since Aristotle (1895). The extent to which film characters earn our emotional allegiance is indeed key to our emotional responses to their sufferings (Smith, 1995). When the nuclear apocalypse does come upon the film’s cast, they are largely deserving of it, being weak, selfish, warmongering fools the lot of them. Finally, the film employs spatial distance by shying away from close-ups in favor of more distant framings. Seeing someone in a close-up makes it hard not to feel for them, whereas more distant shot scales make it easier for us to stand back and refrain from identifying with what they are emoting (Bálint et al., 2020).
All of these things influence audiences’ responses, guiding them towards finding the depicted scenario benignly humorous instead of tragic. In the hands of a gifted director like Kubrick, even a violation as severe as the end of the world can thus be reduced to the benignity of a tickle.
A Darwinian Approach
When wielded properly, the Darwinian approach to humor in this way can act as an analytical tool for understanding comedic culture. As with many other products of imaginative culture, an evolutionary perspective offers unique insights. Even a film like Dr. Strangelove and its comic depiction of the end of the world can ultimately be understood as extensions of humor’s animal origins, giving way to new analytical insights into how comedians—across different mediums, cultures, and historical periods—get their laughs.
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