(Last revised: December 2024. See changelog at the bottom.)

1. tl;dr

There should be parallel explanations for laughter at two levels.

At the brain level, there should be some mechanism / algorithm that produces laughter, and it should fit the data of when people laugh in practice.
At the evolution level, there should be some explanation for why this mechanism exists in the first place. Why was it adaptive in our ancestors? And where did it come from—are there homologues in other animals?

I’ll summarize my proposals for both of these, in the opposite order:

1.1 First half of the tl;dr: Laughter in terms of evolution

I endorse the popular theory that laughter is an indicator of “play”, homologous to the play-related vocalizations and body language in other animals (e.g. the dog’s “play bow”).

The evolutionary purpose of play is “practice for future high-stakes situations”. For example, a wolf pup that engages in play-fighting and play-chasing would presumably be more skilled in its future real-life fights and chases.
- “High-stakes situation” usually means “immediate physical danger”. But situations can also be evolutionarily high-stakes for other reasons—think of behaviors related to reproduction (asking your crush on a date), or kin selection (threats to one’s children), or long-term access to resources (threats to one’s social status), etc.
The evolutionary purpose of innate communicative play signals, like laughter in humans and play-bows in dogs, is to reduce the probability of accidental escalation from practice to serious. For example, if a play-fight between two wolf-pups escalates into a real fight between the pups, that’s dangerous for both pups. If the pups are emitting and responding to communicative play signals, then that kind of escalation is much less likely to happen. It’s kinda the same idea as “safewords” in fight-related sports (among other places).

1.2 Second half of the tl;dr: Laughter in terms of brain algorithms

My (oversimplified) pseudocode brain “business logic” for laughter is something like:

PROPOSED BRAIN PSEUDOCODE FOR LAUGHTER:

(A) IF my hypothalamus & brainstem are getting some evidence that I’m in a high-stakes (e.g. dangerous) situation
- (the “evidence” here would presumably be some of the same signals that, by themselves, would tend to cause physiological arousal / increase my heart rate / activate my sympathetic nervous system)
(B) AND my hypothalamus & brainstem are simultaneously getting stronger evidence that I’m in a low-stakes (e.g. safe) situation
- (the “evidence” here would presumably be some of the same signals that, by themselves, would tend to activate my parasympathetic nervous system)
THEN I will emit innate play signals (e.g. laughter in humans), and also I will feel more energetic (on the margin), and more safe, less worried, etc.

Indeed, as I’ll discuss in Section 5, I expect that there is some genetically-specified neuron group in the hypothalamus or brainstem (or more generally, what I call the Steering Subsystem), and that when future scientists look at its various connections and their functional properties, it will be straightforwardly obvious that this neuron group and its connections are implementing the pseudocode above.

(Side note: These scientists will also find that this neuron group has various other inputs that make laughing more or less likely on the margin—inputs related to mood etc.—which I omitted from the box for simplicity.)

Note that nothing in this box is particularly tied to humans. If we’re talking about 50 kHz rat laughter instead of human laughter, I wouldn’t change a single word in the box above. However, in Section 4 below, I will talk about human laughter in particular, including humor, and I’ll argue that this pseudocode box is a plausible match to the circumstances in which people laugh.

Also, the path by which I initially came to guess this pseudocode box (namely, introspection) was independent of how I came to believe the evolutionary story (namely, I read it in a book and it seemed obviously right). But I claim that the two stories match up beautifully—that the pseudocode box above is the natural, straightforward way to implement the “spec” associated with the evolution story, given background constraints about how I think brain algorithms work in general (see Section 3.3.2 below). That reassures me that I’m on the right track.

OK, that was the tl;dr. The rest of the article will elaborate on that picture, why I currently believe it, and broader implications.

1.3 Table of contents with section summaries

Section 2 will explain the evolution story in more detail.
Section 3 will explain the brain algorithms story in more detail—in particular, what exactly do I mean by that pseudocode above? And then I’ll go through three main reasons that I think it's right: (1) the pseudocode matches the evolutionary “spec”; (2) the pseudocode is highly plausible on the neuroscience side; and (3) the pseudocode seems to match the situations in which humans laugh (and in which animals emit analogous play-signals).
Section 4 elaborates on the latter by fleshing out how to reconcile the pseudocode with everyday experience, in three domains:
- Section 4.1 asks: How does this pseudocode shed light on laughter in physical play—tickling, chasing, peek-a-boo, water balloon fights, etc.? For example, why can’t you tickle yourself?
- Section 4.2 asks: How does this pseudocode shed light on conversational laughter? For example, how is it that laughter can communicate so many different things in different contexts (e.g. friendliness versus aggression, or sincerity versus insincerity)?
- Section 4.3 asks: How does this pseudocode shed light on humor and jokes?
Section 5 asks: How exactly is this pseudocode implemented in the brain? I hypothesize that there are innate connections between neuron groups in the hypothalamus and/or brainstem that directly correspond to the pseudocode above. Alas, I cannot tell you exactly which neuron group it is—I think it’s a neuron group that nobody has studied yet. But I think I know what we should be looking for and where, and I think that a neuroscience lab could figure this out in the near future using standard experimental methods.
Section 6 is the conclusion, in which I will discuss how this post relates to my job as an Artificial General Intelligence safety / alignment researcher.

2. The evolutionary story

2.1 What is play, and why do animals have an innate play drive?

A central example of play in my mind is two young animals play-fighting or play-chasing each other.

Why are the young animals doing this? There seems to be an obvious primary explanation:

If a squirrel has spent many hours play-fighting and play-chasing other squirrels as a pup, it will presumably do better at actual-fighting and actual-chasing other squirrels as an adult.
And likewise, if a squirrel has spent many hours running away from its sibling during play-chase, it will presumably also do better when running away from a predator.

In other words, play is practice for dealing with future high-stakes situations.

And what is the evolutionary advantage of having an innate play drive? My answer is: Obviously, squirrel pups do not have the foresight and knowledge to deduce from first principles that it’s a good idea to spend some of one’s free time practicing for future high-stakes situations. So instead, play is an innate drive.

In other words: Evolutionarily, play is a means to an end, but from the squirrel’s within-lifetime perspective, play is its own reward—it’s just intrinsically enjoyable.

2.2 What is an innate communicative play signal, and why do animals have them?

Play signals are any vocalizations or body language that happen primarily or exclusively when the animal is playing.

One example is the “play bow” in dogs (which often comes with a high-pitched “play bark”):

Another is rat laughter, a 50 kHz (ultrasonic) chirp emitted by rats under various circumstances including play-fighting among young rat pups. Jaak Panksepp and his then-student Jeffrey Burgdorf first hypothesized that these chirps are the rat version of laughter in the late 1990s; see for example their paper Panksepp and Burgdorf (2000), which involved manually tickling rats.

(I know what you’re thinking—but don’t worry! Today’s scientists no longer have to suffer the indignity of tickling rats by hand. Instead they can use an “automated protocol for tickling in which … rats are forced to move and interact with a constantly rotating rod”.)

There has been a good deal of follow-up work investigating rat-laughter, some of which I will be referencing below.

So anyway, other animals have play signals. Why should humans be different? And more specifically, our chimp cousins emit laughter-like panting sounds, and “laugh most when tickled, during rough-and-tumble play, and during chasing games (the chimp being chased laughs most)” (that quote and much more in chapter 5 of Laughter: A Scientific Investigation by Robert Provine).

(See: YouTube of a young chimp getting tickled.)

So we arrive at the theory that laughter is a play-signal for humans, homologous to the play signals in other animals. This is an old theory—for example, SEP mentions early development by Max Eastman in 1936. For my part, I originally heard this theory from a chapter in the book Elephant In The Brain by Kevin Simler and Robin Hanson. (I found that book chapter very enlightening, although I don’t agree with everything they said.^[1])

Why do animals have innate communicative play signals? Imagine two young squirrels play-fighting. This is a mutually-beneficial activity, for reasons mentioned above. However, it is also possible for two young squirrels to actually fight, an activity which is very dangerous for both of the squirrels, and sometimes also for kin bystanders.

And now we see the problem: there is an obvious resemblance between play-fighting and actual-fighting—such that a play-fight could accidentally escalate to an actual fight. Therefore, each squirrel benefits from communicating to the other that it is play-fighting.

Thus, we expect animals to have evolved innate mechanisms to emit play signals, along with corresponding innate mechanisms to notice those signals and react to them.^[2]

3. The brain story

3.1 The pseudocode

I’ll re-copy the box at the top for easy reference—I think it’s something like this:

PROPOSED BRAIN PSEUDOCODE FOR LAUGHTER:

(A) IF my hypothalamus & brainstem are getting some evidence that I’m in a high-stakes (e.g. dangerous) situation
- (the “evidence” here would presumably be some of the same signals that, by themselves, would tend to cause physiological arousal / increase my heart rate / activate my sympathetic nervous system)
(B) AND my hypothalamus & brainstem are simultaneously getting stronger evidence that I’m in a low-stakes (e.g. safe) situation
- (the “evidence” here would presumably be some of the same signals that, by themselves, would tend to activate my parasympathetic nervous system)
THEN I will emit innate play signals (e.g. laughter in humans), and also I will feel more energetic (on the margin), and more safe, less worried, etc.

There are presumably various other things that modulate this circuit (a.k.a. shift the thresholds)—person-to-person variation, and variation with age (and certainly with species), and dependence on other innate signals (e.g. angry people tend to laugh less), including some degree of voluntary control (see Sections 4.2.4 and 5.3 below). But I think the box above is the main story.

3.2 What exactly do I mean by “evidence that I’m in a high-stakes situation” etc.? How are these things operationalized?

To be clear, I am not talking about consciously believing that I’m in danger, or safe, etc. For example, my fight-or-flight reaction can easily activate even when I consciously believe I have nothing to worry about, like during a scary movie.

Instead I am talking about innate signals in the hypothalamus and/or brainstem—what I call the “Steering Subsystem”. These signals (I claim) have particular innate “meanings” / algorithmic purposes with legible relation to ecological / homeostatic requirements. For example, see my discussion here of a particular group of neurons in the hypothalamus that is activated by a physiological need for food, and causes various downstream effects (like energy conservation and hunger sensations) that are appropriate to that state. These kinds of innate signals can exist even if we have no conscious (interoceptive) access to them, and no common English-language concept that perfectly aligns with them. (See my recent post contrasting my views with Lisa Feldman Barrett’s.)

Anyway, I suggested in the box above that “high-stakes” might be operationalized via some of the same innate signals that activate the sympathetic nervous system (directly or indirectly), and “low-stakes” might be operationalized via some of the same innate signals that activate the parasympathetic nervous system (directly or indirectly). Which signals exactly? I don’t know.

3.3 Three reasons I like this proposal

3.3.1 This pseudocode matches the evolutionary “spec” of Section 2

I think the correspondence here is strong and straightforward, and I see this as key evidence that I’m on the right track. Going through the items above:

(A)—some evidence that I’m in a high-stakes (e.g. dangerous) situation. If I’m in a situation that is triggering some fight-or-flight bodily reactions—regardless of the exact reason—then I’m almost definitely in a situation that resembles high-stakes situations that I might get into in the future. And therefore, being in this situation almost definitely constitutes good “practice”.
(B)—stronger evidence that I’m in a low-stakes (e.g. safe) situation. Without this ingredient, it’s not “practice”—it’s the real thing! For example, if the situation is dangerous, then I should be getting out of it, not rushing into it! I should find it aversive, not fun.

3.3.2 This kind of pseudocode is highly plausible on the neuroscience side

Unfortunately, despite some effort, I cannot tell you the exact neurons that implement the pseudocode in the box above. See Section 5 below for more details.

However, the pseudocode above is fully compatible with everything I think I know about the brain. And that counts as strong evidence for me, because “the things I think I know about the brain” are highly constraining! (For some of those constraints, see my post “‘Learning from Scratch’ in the Brain”. And see my post on social instincts for an example of how those constraints rule out lots of possibilities in practice.)

In particular, I know that the right kind of signals are in fact present in the hypothalamus & brainstem (see Section 3.2 above), and I know that the genome is easily capable of building a little cluster of neurons in (probably) the hypothalamus that perform the requisite logical operations. I have seen lots of little clusters of neurons in the hypothalamus doing broadly this kind of genetically-specified business logic.

3.3.3 This pseudocode seems to fit the data of when humans laugh (and when animals emit analogous play-signals)

As far as I can tell, the pseudocode is compatible with all our experience of laughter in humans and corresponding play-signals in other animals, including all three of the categories elaborated in Section 4 below—physical play in Section 4.1, non-“humor” conversational laughing in Section 4.2, and humor in Section 4.3.

You can read Section 4 to better understand where I’m coming from. But then you should take a long hard look at the pseudocode box in Section 3.1 above, and then scroll down to the comments section, and complain about some situation where the predictions of that pseudocode box are wrong!

…And then I’ll respond with things like:

“OK sure, but that’s because the pseudocode box of Section 3.1 is oversimplified, like I left out how the laughter-reaction is suppressed by other innate signals correlated with anger and mating and some other things like that.”
“OK sure, but I was using words like ‘high-stakes’ and ‘low-stakes’ when obviously those are just the closest English-language words I can think of, not perfect descriptions, and they come apart from the actual innate signals in various ways, which by the way I cannot specify in detail”.

…And then you will roll your eyes at me and accuse me of special pleading and unfalsifiability.

Totally fair!

But I still currently think I’m getting much more out of this pseudocode box than I’m putting in—and that’s even without knowing exactly where the corresponding neurons are and what they’re connected to.

(But please do leave those kinds of skeptical comments! They will be very appreciated!)

4. Relating the pseudocode to everyday experience

There isn’t a sharp line between them, but I’ll separately discuss three categories: laughter in physical play, laughter in non-“humor” conversation, and humor. In that order:

4.1 Laughter in physical play (e.g. tickling, chasing, peek-a-boo, etc.)

(This is the most straightforward case, and also the case where humans are most similar to other animals.)

Examples of kids laughing during physical play—getting tossed in the air (left) or sprayed with a water gun (right). Image sources: 1,2

4.1.1 What are the sources of “Ingredient (A)” (i.e., evidence of danger / cause for physiological arousal) in physical-play laughter?

In physical-play laughter, I think there are many possible sources of Ingredient (A), and they’re mostly pretty obvious. In particular, we (like almost all animals) have a diverse suite of innate defensive reactions, including in our case:

Startle and orienting responses to unexpected sensory inputs
Various types of defensive flinches in anticipation of physical trauma
Some innate reaction to the sense that you’re falling down
Running away from threats (“escape response”)
Etc.

All of these innate reactions trigger not only certain muscle behaviors, but also physiological arousal—hence Ingredient (A).

And all of these are situations where you’ll see little kids laughing uproariously during physical play.

4.1.2 More discussion of tickling in particular

It strikes me as extremely obvious that tickling is part of play-fighting. For example, the most ticklish parts of your body seem to coincide with the parts that are most vulnerable to serious injury, like the front of the neck.^[3] So the evolutionary story behind tickling is straightforward. What about the brain-level story?

I claim that the pseudocode of Section 3.1 above is perfectly adequate to explain everything about tickling at the brain-level. To flesh that out, here are some additional details and discussion:

The source of Ingredient (A) in tickling is basically the normal startle and orienting responses to unexpected sensory inputs, as mentioned above. But, for obvious evolutionary reasons, those innate reactions are presumably especially strong for touch sensations on vulnerable parts of your body. Think of the circumstances when those reactions would naturally trigger—maybe your enemy has successfully gotten his hands around your neck during a fight.

You can’t tickle yourself for the same reason that you do not routinely have an involuntary startle reaction from the sound of your own voice when you start talking, and the same reason that you do not routinely have an involuntary orienting reaction when you wave your own hand in front of your own eyes. (Unless of course you have schizophrenia—see here—or if you are an infant surprising yourself by making a funny noise for the first time, etc.)

The source of Ingredient (B) in tickling is the knowledge that you’re safe among trusted friends. If you’re getting “tickled” in a situation where you’re genuinely terrified for your own safety, my impression is that you’re going to be screaming rather than laughing.

Related to this, even if someone enjoys getting tickled in general, they will still try to push your hand away from the most ticklish areas. The evolution-level reason for this behavior is obvious: that’s part of how play-fighting works. It would hardly be good defense practice otherwise! But what about the brain-level reason? I propose that when their sensitive areas are successfully getting tickled, Ingredient (A) gets so strong that it cuts off Ingredient (B), tipping the experience from fun into aversion.

4.2 Laughter in non-“humor” conversation

4.2.1 Background: Most conversational laughter is not “humor”

Even if we ignore physical play, the connection between “humor”^[4] and laughter is less tight than you might think. In Provine’s book he talks about his “ecological” studies of modern USA people hanging out in public, and says “only about 10%-20% of [comments immediately preceding somebody laughing] were estimated by my assistants to be even remotely humorous”. Then follows a helpful table of typical laugh-eliciting comments, including such zingers as “I’ll see you guys later!” and “Can I join you?”

Pay close attention next time you’re in (or overhearing) a normal face-to-face group conversation, and you’ll probably notice something similar.

Other cultures seem to be similar to the USA in that respect—for example this article talks about hunter-gatherers laughing in response to gentle teasing and such—not “why did the chicken cross the road”.

4.2.2 What are the sources of “Ingredient (A)” (i.e., evidence of danger / cause for physiological arousal) in conversational laughter?

Section 4.1.1 listed a bunch of obvious ways that arousal gets invoked in physical play, examples like “you think you’re alone but someone suddenly jumps out at you from hiding while screaming at you”. But what about conversation—can mere words invoke physiological arousal too? Yes, obviously! Wandering into a stressful conversation topic can get your heart rate up just as surely as can wandering into a swarm of angry bees.

To be more specific, it seems to me that there are a great many natural sources of Ingredient (A) in conversations, including:

Confusion
Embarrassment / guilt / shame (possibly vicarious)
Disgust (possibly vicarious)
Surprise (possibly vicarious)
Threats (both bodily threats and status threats) (possibly vicarious)
Taboo-breaking

These are not mutually-exclusive, and there are others too.

4.2.3 Laughter communicates a pretty universal message about my transient “internal” state, but then the listener has to infer why I feel that way, and the latter inference is complicated, contextual, and widely-varying.

Remember from Section 2 above, I’m claiming that “avoiding accidental escalation during practice for future high-stakes (e.g. dangerous) situations” is the evolutionary explanation of why the brain mechanism for laughter is there in the first place. But given the existence of that brain mechanism, it will be active in lots of situations, many of which may have nothing to do with “avoiding accidental escalation during practice for future high-stakes situations”. (See “Adaptation-executors, not Fitness-maximizers”.) And I think this is much more true in humans than in other species, who seem to mostly just laugh during physical play.

In all situations, I think the universally-shared “meaning” of laughter is related to certain signals in the head of whoever is laughing, as specified by the pseudocode of Section 3.1. But then the listener needs to infer from context why those signals are in the laugher’s head. And here things get very complicated and contingent. Some examples:

Scenario 1: You are walking by me in middle school, when I just dropped my contact lens. I say “please help me” with a desperate tone, and you laugh as you say “sorry, I don’t help losers”.
How I might take that: I know that you feel safe (ingredient (B)), and I might guess that it’s because I’m in your outgroup and that you regard my suffering as no threat to your own well-being. I also know that you feel a bit of ingredient (A), and maybe I’ll guess that you feel like it’s cringe that I even dared ask you for help in the first place. In short, your laughter communicated to me that you feel unsympathetic and superior.
Scenario 2: You are my spouse. I say “Man, my company is really dysfunctional”, while slightly laughing, and in response you laugh as you say “yeah”.
How I might take that: Well, for my part, I was laughing mainly because I felt kinda annoyed about the dysfunction (Ingredient (A)), but also not overly worried for my own sake (Ingredient (B)). Then when you laugh in your response, I might infer that you are invested in my well-being, and empathetically mirroring my feelings on both counts. In short, your laughter communicated to me that you feel empathy and camaraderie.

I could go on. I think that, in different contexts, laughter can signal friendship, or animosity, or superiority, or inferiority, or sincerity, or insincerity, etc. etc. There is no simple theory, because we can feel a certain way for many unrelated reasons.

4.2.4 …And given that laughter is able to communicate stuff, people skillfully wield laughter as part of their communicative toolkit

In the previous section I was implicitly treating laughter as an incidental side-effect of the emotions that someone feels in the course of a conversation. But once humans have learned (consciously or unconsciously) that laughter communicates things, they will start wielding laughter to skillfully advance their communicative intentions. For example, in Scenario 2 just above, maybe you laughed in part because you wanted to communicate empathy and camaraderie.

This doesn’t have to be a conscious explicit decision or desire, and in fact it probably usually isn’t. It’s probably more often an unconscious habit—in lots of previous conversations, you’ve laughed or not-laughed in a certain way in a certain context, and it led to good results, so you unconsciously learned to repeat that behavior next time you’re in a similar situation.

While purposeful control of laughter is not directly part of the “business logic” I wrote down in Section 3.1, we obviously can in fact voluntarily laugh. Mechanistically, I think it typically (though not always^[5]) happens indirectly—if we want to laugh, we steer ourselves into a transient emotional state that has Ingredients (A) & (B), and if we want to not laugh, we steer ourselves into a transient emotional state that doesn’t have both ingredients. How do we do that? Well, we have some control over our transient emotional state because we can attend to some aspects of our situation rather than others, choose which frames / analogies to mentally invoke, etc.

4.3 Humor

As mentioned in 4.2.1 above, humor (as I define it^[4]) does not necessarily cause people to laugh, and conversely most laughter occurs in the absence of humor.

That said, humor can obviously lead to laughter, so I ought to say something about how. I don’t have a grand theory of humor, nor do I think there is one, beyond what I’ve already said in this post. I’ll just mention a few considerations that I find helpful to keep in mind when thinking about humor and jokes.

4.3.1 What are the sources of “Ingredient (A)” (i.e., evidence of danger / cause for physiological arousal) in humor?

I think the list is pretty similar to Section 4.2.2 above for conversational laughter above. I won’t re-copy it—you can scroll up. Again, that list is not exhaustive, nor mutually exclusive.

4.3.2 There’s an inverted-U dynamic for “Ingredient (A)”

According to the pseudocode box above, if there is too little of Ingredient (A), there’s no laughter (e.g. a boring conversation), and if there’s too much (A), then there’s also no laughter, because it undermines Ingredient (B) (e.g. it might just feel stressful, painful, scary, confusing, etc.) Somewhere in between is optimal for laughter.

So I think we wind up with inverted-U dynamics like this:

My theory predicts an inverted-U dependence between Ingredient (A) (in the pseudocode box above) versus laughing—more (A) leads to more laughing, until it’s so much that it cuts off Ingredient (B) and tips to aversion. (See prior discussion of lots of proposed inverted-U’s in humor in Martin & Ford 2018.)

(I was implicitly talking about this same inverted-U in Section 4.1.2 above, when discussing why someone might enjoy getting tickled a little bit, but find direct tickling of their most ticklish spots to be too much, and thus aggressively push the tickler away.)

4.3.3 The (somewhat arbitrary and drifting) cultural expectations / tropes / rituals surrounding “humor” can be a key component of the explanation of why something is funny.

In particular, if a listener has a general cultural expectation that “humor” should involve X (e.g. a punchline), then a joke-teller can either:

Increase the amount of Ingredient (B) by conspicuously including X (e.g. an obvious punchline), OR
Increase the amount of Ingredient (A) by conspicuously excluding X (e.g. punchline-free absurdist anti-humor)

Either of these can be helpful for the joke, depending on how much (A) and (B) are present from other sources.

And if the latter (exclusion of X) happens a lot, then we collectively stop expecting X in the first place, causing gradual (anti-inductive^[6]) drifts in what people find funny, or schisms between humor-subcultures.

4.3.4 In humor, Ingredients (A) & (B) generally come from two different conscious thoughts in rapid succession

Let’s pause for some more general background:

The “pseudocode box” of Section 3.1 talks about signals in the hypothalamus and brainstem. By contrast, I claim “conscious thoughts” are (more or less) activation patterns in the cortex, the 75%ish of the human brain that understands the world, processes language, and much else.

Conscious thoughts can of course lead to signals in the hypothalamus and brainstem. For example, suppose you say “hey there’s a spider on your back”. My cortex transforms this meaningless audio waveform into a meaningful conscious thought. Then, this conscious thought leads (via the amygdala and other pathways) to physiological arousal and other reactions in my hypothalamus and brainstem.

Anyway, my guess is that it’s rare—maybe even impossible—for one conscious thought to trigger Ingredients (A) and (B) simultaneously.

If so, then there would be two main ways to get laughter.

First, in the context of tickling and other physical play (Section 4.1 above), I’d guess that Ingredient (B) often comes from a conscious thought, whereas Ingredient (A) often doesn’t involve conscious thought at all, but rather comes from innate triggers in brainstem sensory processing systems (see §3.2.1 here).

Second, in the context of humor and other conversational laughter, maybe the only way to get both (A) & (B) signals to overlap, and hence to get laughter, is by the rapid succession of two thoughts. Let me explain that next.

We can flip from one conscious thought to another quite different one very fast—multiple times per second. By contrast, I think things like physiological arousal tend to fade in and out more slowly—perhaps a second or two.

Thus, I propose that we can get laughter when there are two different conscious thoughts following each other in rapid succession, one creating Ingredient (A), the other Ingredient (B). The resulting hypothalamus and brainstem signals would be present simultaneously, and thus we’d get laughter. The laughter could last longer if we flip back and forth between those thoughts a few times.

I should give a concrete example, shouldn’t I. Sigh. OK fine. I don’t want to cherry-pick, so here’s “the funniest joke in the world” according to a survey:

Two hunters are out in the woods when one of them collapses. He doesn't seem to be breathing and his eyes are glazed. The other guy whips out his phone and calls the emergency services. He gasps: “My friend is dead! What can I do?” The operator says: “Calm down, I can help. First, let's make sure he's dead.” There is a silence, then a shot is heard. Back on the phone, the guy says: “OK, now what?”

I claim that this joke is forcing the listener to think about the situation in terms of (at least) two “frames” (in the sense of “frame semantics” in linguistics or “frame languages” in GOFAI).

FRAME 1 is the “someone is gravely injured, and someone else with them is calling the emergency services” frame. In this frame, “make sure X is dead” is understood to mean “check whether X is dead or not”.
FRAME 2 is the “people are out hunting in the woods” frame. In this frame, “make sure X is dead” is understood to mean “ensure that X is dead”. (X would normally be the animal being hunted.)

Umm, I don’t personally find this joke to be very funny. But insofar as I maybe chuckled a bit, I think it’s because, when I first heard the joke, I experienced a period of transient confusion, during which my brain flipped back and forth a few times between these two frames. And during this transient period, whenever FRAME 1 was active, that thought was evoking Ingredient (A), and whenever FRAME 2 was active, that thought was evoking Ingredient (B). As above, the flipping back and forth would cause Ingredients (A) & (B) to be simultaneously present in my hypothalamus, triggering the laughter circuit.

That’s just one of infinite examples. Ingredient (A) can be based on the feeling of confusion, and (B) by its resolution. Or Ingredients (A) and (B) can come from different ways to view the same situation, one of which is normal / safe and the other embarrassing / dangerous / etc. In still other cases, I wonder whether we’re all just following a cultural telling-a-joke script, and as a listener, Ingredient (A) is my concern that I don’t “get the joke” and will be embarrassed to admit it, and Ingredient (B) is my relief when I do.^[7] It can also be several of these things simultaneously, and more.

4.3.5 I’m not generally impressed by “theories of humor” beyond their overlap with the discussion above.

For example, the Psychology of Humor textbook (Martin & Ford 2018) describes three “classic theories of humor”. I’ll comment on those next, followed by two other interesting takes in the literature:

• “Relief theory”: In the joke above, you’re switching quickly from a frame that evokes Ingredient (A) to a frame that evokes Ingredient (B). You could call that experience “relief”. I think that’s basically the kernel of truth underlying “relief theory”. (On my account, you’re also just as likely to laugh upon switching frames in the opposite, “anti-relief” direction, as far as I can tell. So it’s not a perfect match.)

• “Superiority theory”: In many instances of humor, Ingredient (A) is vicarious—from imagining someone in danger or duress—while Ingredient (B) comes from my comfort in the knowledge that I, the listener, am not that person, and am superior to that person, and have therefore nothing to fear for myself. In my opinion, this formula is just one of many ways to evoke humor, as opposed to a universal theory.

• “Incongruity theory”: There’s a sense in which a situation X “should” evoke Ingredient (A), or Ingredient (B), but not both. Like, intuitively, there should be a “right answer”—either physiological arousal is appropriate to situation X, or it isn’t!!

So there’s something odd about a situation that calls for high physiological arousal when you think about it one way, but calls for low physiological arousal when you think about it in another way. It’s like an error in your brain. In fact, I think it’s literally an error, in the sense that some supervised learning algorithm(s) in your brain are getting an error signal while this is happening. Maybe our world-model is failing to capture something, maybe the situation is out-of-distribution for some reason—I think there are a lot of possible causes here. But I think the word “incongruous” is pointing roughly towards this type of situation.

(Incongruity is not sufficient for laughter, on my models. As above, it’s not enough to have two different incongruous frames for thinking about the situation; we also need one to trigger Ingredient (A) and the other to trigger Ingredient (B). However, even advocates of incongruity theory agree that incongruity is necessary but not sufficient for laughter—at least according to the Martin & Ford textbook.)

• Marvin Minsky’s “Jokes and the Logic of the Cognitive Unconscious” came up in the comments section. If I understand the article correctly (a big “if”!), Minsky was trying to relate humor to learning metacognitive rules of the type “Mental Operation X seems to be locally a good idea, but actually it isn’t”. He calls these learned rules “censors”.

I think that, on my models, Minsky is pointing to a real pattern that I can explain. As above, humor involves a situation that can be viewed in either of two frames, one triggering high arousal and the other not. When you’re flipping back and forth between them, your brain is simultaneously learning (or strengthening) a metacognitive pattern: If I see a situation that pattern-matches to this one, and I’m viewing it in FRAME 1, then I should flip it into FRAME 2, and vice-versa. Mechanistically, there’s nothing mysterious here—it’s just sequence learning, which your brain tends to do automatically. (Think of learning physical habits, or song lyrics.) So your brain can and will apply this new metacognitive pattern going forward.

OK, that’s my argument that “humor events” involve learning or strengthening what Minsky calls “censors”. It’s somewhat of an incidental side-effect on my account, as opposed to the main story, but it’s there nonetheless.

What about the converse—does learning or strengthening “censors” inevitably lead to laughter? I don’t think so. Maybe Minsky wasn’t claiming that anyway, but if he was, then I disagree. For example, I think deliberate practice in any domain involves learning lots of “censors”, but it’s not funny (more discussion in the comments).

• “Inside Jokes” book by Matthew Hurley, Daniel Dennett, Reginald Adams Jr.: In my model, if I’m laughing, then I’m learning, in the sense that learning algorithm updates are happening in my brain. See the previous two bullet points. If I understand correctly (again, a big “if”!), Hurley et al. take note of that fact, and put it center-stage: epistemology, they propose, is the evolutionary purpose of laughter. Well, I like my presentation better: laughter is associated with play, and play evolved as practice for future high-stakes situations. “Practice” always involves learning, by definition, but I think lumping it in with epistemology is a bit odd, when it centrally includes things like honing your skills at hand-to-hand combat. For example, a person getting tickled is directly practicing the critical life skill of keeping another person’s hands off their neck—but Hurley’s lengthy discussion of tickling (Chapter 11.C) doesn’t even mention that fact!

5. More detailed discussion of the neuroscience

5.1 Overview

The kind of “business logic” pseudocode of Section 3.1 is to be found, I claim, in what I call the “Steering Subsystem” (hypothalamus and brainstem—see definition and discussion here). My guess is more specifically as follows:

Speculative diagram of some of the key hypothalamus & brainstem (“Steering Subsystem”) pathways involved in laughter. This diagram is mostly based on an old review of squirrel monkey data (Jürgens 1998), combined with my general preconceptions about how these things ought to work (e.g. see Fig. 9 of Swanson 2000), plus Gloveli *et al.* (2023) which recently nailed down the “motor pattern initiator(s)” box in the upper-left. See text for more details and caveats.

5.2 Where exactly in the brain is the “laugh behavior controller” (top box in that diagram) where I can read out the alleged pseudocode of Section 3.1?

Sadly, I have failed to deduce from existing literature where in (probably) the hypothalamus we would find the core “business logic” pseudocode of Section 3.1. (It could also be split among a couple places.)

Focusing on rats (although I expect the answer to be similar in humans), one candidate mentioned in the literature is the so-called “parvafox nucleus” of the lateral hypothalamus. Check out Alvarez-Bolado & Celio (2016) for an argument along those lines. As far as I can tell, the strongest evidence in favor of this hypothesis is that bilateral destruction of the parvafox nucleus dramatically (factor of >10) reduces rodent laughter, according to Roccaro-Waldmeyer et al. (2016). Relatedly, there is some evidence (including from laughter-inducing “gelastic seizures”) that “stimulations of [the] tuberal portion [of the lateral hypothalamus] provoke bursts of laughter” (Alvarez-Bolado & Celio (2016)), and that’s in the general vicinity of parvafox.

However, I think the current balance of evidence is that parvafox is not the controller for laughter, but rather is related to defense behavior—based on both direct stimulation of those cells (e.g. Cola et al. (2023)), and looking at where in the brain they project to (e.g. Celio et al. (2013), Bilella et al. (2016)). We still need to explain the Roccaro-Waldmeyer results from the previous paragraph, but there are a couple possibilities for that, including (1) that parvafox is essential for “Ingredient (A)” of the pseudocode of Section 3.1 (and thus upstream of laughter), or (2) that parvafox is physically proximate to the “real” laugh behavior controller and that Roccaro-Waldmeyer destroyed the latter accidentally in their experiments. (Lesion experiments are notorious for “collateral damage” of nearby neurons and fibers, if I understand correctly, but be warned that I’m not an expert.)

Well, if it’s not parvafox, then what is it?

I don’t know. If someone wanted to make progress on this question experimentally—to actually find that pseudocode implemented via innate brain signaling pathways—I think an obvious immediate next step would be a retrograde neural tracing experiment starting from the (lateral) part of periaqueductal gray (PAG) associated with laughter (as pinpointed in the very recent article Gloveli et al. (2023)), followed by further characterization of whatever upstream neuron-groups show up. Maybe such data already exists, and I missed it. (Parvafox does not seem to project to the correct part of PAG to match up with the Gloveli et al. neurons, as far as I can tell, although I’m not super-confident.)

Areas that I see as especially suspicious include:

neurons nearby but not part of the parvafox nucleus, in the tuberal region of lateral hypothalamus (for reasons stated above);
neurons somewhere in or around the medial preoptic area of the hypothalamus—for which stimulation can cause rat laughter (Wintick & Brudzynski (2001)), and which is known to be the home of the main behavior controllers for at least two other positive social behaviors that I know of, namely the behavior studied by Liu et al. (2023) mentioned above, plus aspects of mating as discussed in Bayless et al. (2023);
more generally, the rest of the hypothalamus too.

5.3 What about the “Learning Subsystem” (cortex, striatum, amygdala, hippocampus, etc.)?

As I described here, I claim that the brain should be divided into two subsystems, the “Learning Subsystem” which runs randomly-initialized learning algorithms, and the “Steering Subsystem” which runs “business logic”. Above I was only talking about the “Steering Subsystem”; but in fact laughter also interacts with the “Learning Subsystem”—i.e., the cortex, striatum, amygdala, hippocampus, thalamus, cerebellum, etc.

I claim that the laughter-triggering signals coming from the Learning Subsystem are in the following two categories:

(A) Outputs trained by reinforcement learning to maximize some signal from the Steering Subsystem (hypothalamus & brainstem) that acts as a “reward”, or
(B) “Short-term predictors” of some signal from the Steering Subsystem.

For example:

An example of (A) would be voluntary / learned control of laughter (Section 4.2.4).
An example of (B) would be a “self-fulfilling prophecy”, where you laugh because you’re in a situation that pattern-matches to other situations that have caused you to laugh in the past.

I think there are probably other examples too, but that’s outside the scope of this post.

Direct evidence that the cortex is not necessary for play is provided by “decorticate” rats (= rats whose cortex has been surgically removed), whose play is “much the same [as] controls” (Whishaw 1990, Panksepp et al. 1994). I can’t immediately find direct measurements of the presence of the normal 50 kHz “laughing” vocalizations in decorticate rats while they play, but if their play is similar in other respects, I would certainly guess that they are also vocalizing in a roughly normal way.

6. Conclusion

I have long believed that the hypothalamus and brainstem are full of hundreds-to-low-thousands of specific neuron groups with specific connections that are all written directly into the genome, and which correspond to “business logic” that makes evolutionary sense—things like “if you’re malnourished, reduce your sex drive”. I think that understanding this tangle of “business logic” is annoying but possible, and that doing so seems possibly helpful for Artificial General Intelligence (AGI) safety and alignment, for reasons spelled out here.

I consider laughter an “easy” example of such business logic, as compared to something like the human innate status drive (assuming that there is a human innate status drive, which I currently believe but am not 100% sure). I think the human innate status drive needs substantially more convoluted pseudocode than the box in Section 3.1. (Update: it took a year or so after initially publishing this post, but I think I’ve made real progress towards that more convoluted pseudocode! See Neuroscience of human social instincts: a sketch.)

So this post on laughter was mainly just a warm-up. In particular, I don’t think this post is directly important for AGI safety—if we iron out the details of the pseudocode of Section 3.1, and put it into the source code of future AGIs, and then we find that those AGIs are laughing in a vaguely-psychopathic-human-like way while they mercilessly murder me and my family and every other human … then that doesn’t really make me feel much better!

Still, even if understanding laughter is just a “warm-up” for more safety- and alignment-relevant things like compassion and friendship, I am still very interested in getting this right! I spent much longer on this blog post than usual (admittedly a low bar), including trying to be reasonably comprehensive in reading relevant sources, etc., and I am very eager for feedback.

(Thanks Seth Herd, Linda Linsefors, Justis Mills, and Miguel De Guzman for critical comments on earlier drafts. Thanks to commenters, especially Unnamed and Carl Feynman, for further ideas and updates after the initial publication.)

Changelog

December 2024: Since initially posting:

• I merged the updates from A Theory of Laughter—Follow-Up into the original post. This included:

A better discussion of rapid consecutive thoughts (now in Section 4.3.4 and 4.3.5), and how that relates to other theories of humor. The original discussion was not qualitatively different from what you see now, but the explanations weren’t as crisp, and there were fewer examples.
Replacing the word “dangerous” and “safe” with “high-stakes” and “low-stakes” respectively in many places throughout the post. As an example of how they come apart, asking your crush on a date feels “high-stakes”, but it’s probably not “dangerous”.
There was a missing figure caption for a few days after I initially posted it.

• I deleted my suggestion that there’s a 3rd necessary ingredient for laughter in the pseudocode box, namely “(C) AND my hypothalamus & brainstem have evidence that I’m in a social situation”. I originally had two justifications for putting this in. (1) Provine found that laughter was 30× less frequent when people were alone; (2) Evolutionarily, there’s no point in emitting communicative signals when there’s no one around to hear them.

What changed my mind?

The main thing was, one of my test-readers has autism, and had noted that this inclusion was puzzling and incompatible with their own experience. The internet offers more support—see e.g. here. That had bothered me, but I was sufficiently confused about social instincts that I put it aside.

…Then over the next year, I figured out much more about social instincts (see Neuroscience of human social instincts: a sketch), and how those relate to autism, and found that autism poses an even worse problem for my theory than I suggested above. Yes I still think there’s such a thing as “my hypothalamus & brainstem have evidence that I’m in a social situation”, but I think such evidence is especially likely to be absent in autistic people—see discussion in §6.2 of that post. Therefore, my original pseudocode would seem to predict that autistic people should never laugh, which is obviously false.

OK, so the former ingredient (C) is out. But then, what about (1) and (2) above, i.e. my original two reasons for believing that (C) should be there?

Well, (2) was never that strong an argument. I figure, there isn’t any benefit to emitting play signals that no one will hear … but there isn’t much harm either! OK sure, maybe a predator could hear it, but then again, so could a potential playmate. And maybe non-human animals rarely get (A) and (B) simultaneously when they’re alone, so it doesn’t come up much.

As for (1), the original version already had the following caveat:

I think Provine’s claim that there’s 30× more laughter in social situations is less clear-cut than it sounds. In fact, it’s hard to do an apples-to-apples comparison of laughter in social versus nonsocial situations, because (1) social situations are drawn from a different distribution from nonsocial situations in many respects, (2) the presence of other people can change Ingredients (A) and (B) too, and (3) social situations can also affect laughter via voluntary control (Section 4.2.4). I still think (C) is probably a real ingredient in the algorithm, because Provine’s factor of 30 is so extreme that it seems difficult to fully explain by any of those indirect mechanisms.

I think I’m actually now willing to say that these considerations are enough to account for the full factor of 30. For example, voluntary control of laughter doesn’t just have to look like forcing a laugh when you don’t feel anything. It could also look like adding juice to an already-present sincere laugh, to make it louder, for social reasons. Maybe purely involuntary loud laughter is just much rarer than anyone realizes—maybe 90% of the time that a neurotypical person emits loud laughter, they feel only an involuntary mild chuckle, and they’re cranking that feeling up via voluntary control, because the laughter is also serving some social purpose.

• Other changes: I made some minor wording improvements, and updated some links. Also, in the list of other theories of humor (Section 4.3.5), I added another bullet-point entry for the book Inside Jokes by Matthew Hurley, Daniel Dennett, and Reginald Adams Jr., which I recently skimmed.

By the way, I’m happy to share access to archived earlier versions of this post, if anyone wants that.

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Since the works of Robin Hanson are popular on this forum, I will say a bit more about where I differ from Elephant In The Brain. My biggest complaint is the part where they say:
As we mentioned earlier, people are profoundly ignorant about laughter’s meaning and purpose (at least in our default state, before learning the science). But where does this ignorance come from? Why does introspection fail us so spectacularly here?
It’s not simply because laughter is involuntary, outside our conscious control. Flinching, for example, is also involuntary, and yet we understand perfectly well why we do it: to protect ourselves from getting hit. Thus our ignorance about laughter needs further explanation.
I disagree that it “needs further explanation”. I think we start out ignorant of literally everything, until we learn it / figure it out. And I think that figuring out the evolutionary purpose of laughter is just inherently much harder than figuring out the evolutionary purpose of flinching. It’s less obvious / salient, for various reasons that I claim are pretty obvious if you think about it. I don’t think there’s any more to it than that.
I also don’t think there can be more to it than that. To explain what I mean by that, imagine if I said: “Here’s the source code for training an image-classifier ConvNet from random initialization using uncontrolled external training data. Can you please edit this source code so that the trained model winds up confused about the shape of Toyota Camry tires specifically?” The answer is: “Nope. Sorry. There is no possible edit I can make to this PyTorch source code such that that will happen.” By the same token, even if, as that book argues, there is a strong evolutionary pressure to make humans specifically confused about the evolutionary purpose of laughter, I don’t think there is any possible genetic change that would make that happen. Related discussion here.
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Whenever there’s a claim about communicative signals, one can ask a follow-up question of whether these signals are game-theoretically stable against “lies”. Like, what if Squirrel A laugh-squeaks to signal play, then Squirrel B lets down its guard, then Squirrel A attacks for real? If this happened a lot, wouldn’t squirrels eventually evolve to ignore play signals? And if that happened, then wouldn’t squirrels further evolve to stop emitting play signals in the first place? Good question! It’s worth thinking about those kinds of things. But I do think a good answer exists, even if I don’t know it in full detail. I think there are various ways that signals can be hard or costly to fake. And I also think that animals treat a play-signal by itself as insufficient reason to let down one’s guard, which reduces the benefit of lying. Animals are reacting to other cues too, like whether there is an existing trusting relationship. For example, if I’m a prisoner, and the cruel guard is pointing at me and laughing, that sure wouldn’t make me feel more relaxed.
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I’m extremely far from an expert on what parts of the body are vulnerable in combat today, let alone 100,000 years ago (How often were people punching each other, versus slashing with sharp rocks, versus getting bitten by wolves or spiders? What were the lived consequences of different types of injuries? Beats me!). But I still think this claim is probably true. For example, this article claims that ticklish areas correlate with areas protected by involuntary defensive reflexes.
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I am defining the word “humor” narrowly—something like “humor = the kind of stuff you find in the ‘humor’ section of a bookstore”, e.g. recognizable jokes and so on. On this definition, if someone walks up to my lunch table and says “can I join you?” and then laughs a bit, that’s conversational laughter but not “humor”. I believe Provine takes this definitional approach.
Alternatively, one could define the word “humor” very broadly, as something like “humor = whatever makes someone spontaneously laugh”. On this definition, it’s perfectly possible that someone walking up to my lunch table and saying “can I join you?” is an instance of “humor”, when taken with its full context. I believe Hurley et al. take this definitional approach.
Anyway, this is just semantics. “Humor” is just a word, and within limits, we can define it however we want. I find the narrow definition to be helpful for the purposes of this post, so that’s how I’m using it.
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Transient manipulation of our own emotional state is not the only method of learned / deliberate manipulation of laughter. Alternatively, you can laugh by pure voluntary motor control—just move your larynx, lungs, etc. and make laugh sounds, the same way you might voluntarily move your arm. I think such laughter can come across as fake sometimes (like maybe it can sound slightly different, and the eyes don’t squint in quite the same way, see Duchenne smile)—although people still do it plenty.
Incidentally, I think the “summoning transient emotions” method of laughing and the “pure voluntary motor control” method of laughing in this footnote are not mutually exclusive, and indeed that we’re almost always using both strategies at once.
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If you haven’t heard the term “anti-inductive”: "Inductive" reasoning is where you assume that if you’ve seen something a bunch of times in the past, then it’s likely to happen again in the future. “Anti-inductive” reasoning is the opposite—the more past evidence you have for something, the more likely it is to be false next time.
As the old joke goes: “I recommend anti-inductive reasoning to anyone. After all, using anti-inductive reasoning has been a catastrophically bad idea every time I’ve tried it in the past. So it’s definitely a good idea going forward.”
Here’s an example. If you’re up against a master at rocks-paper-scissors, you might want to use anti-inductive reasoning about the future: The more evidence you have for a pattern in your opponent’s behavior, the likelier it is that your opponent wants you to notice that pattern, and therefore the more you should expect that pattern to reverse on the next throw. (But this is happening at every level of abstraction simultaneously—which is kinda weird to think about.)
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I suspect that kids will laugh a comparable amount in practice when solving a riddle / brainteaser posed by a friend as when “getting” a pun told by the same friend, other things equal. Other things are not always equal though; puns often get an extra emotional “kick” from some other source, like from being sexual, or from the friend’s mood, or just from the very fact that it’s a pun, both because there’s a cultural trope that making puns is a shameful activity, and because puns are “supposed” to be funny and the expectation of laughter can be a self-fulfilling prophecy, see Section 5.3 below.