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Do you remember the last time you laughed and just couldn’t stop? Maybe there was a situation where you shouldn’t smile at all.
This happened to one of us while accompanying a friend to a family court in the middle of a nasty divorce case. It led to a quick dash to the bathroom while desperately trying (and failing) to regain control – not quite the kind of emotional support this friend was looking for!
Another classic example is cricket commentators Brian Johnston and Jonathan Agnew, who laughed so much when describing the day’s play in 1991 that they couldn’t speak at all. But how and why does this happen?
In fact, although we typically associate laughter with amusement, most of it happens for other social reasons. For example, you are 30 times more likely to smile when you are with someone else than when you are alone. And you will smile more when you know and love your companion.
Most laughter occurs during conversations, when people can smile by contagion – catching someone else’s smile. Studies show that when we see or hear someone smile, this can almost automatically trigger laughter in us, promoting social connection.
We often smile to show agreement and understanding, and to cement friendship – for example, over shared memories. We also laugh to reduce stress.
These common conversational laughs occur about six times per ten minutes of conversation, on average. They are short, lower in pitch, and tend to start and stop quickly.
These types of laughter, which we call voluntary, differ from helpless laughter, which is involuntary and which we cannot prevent from happening. Helpless or involuntary laughs are longer and higher pitched, with strange cries and whistles.
In our latest research, we investigated whether these two different types of laughter have their roots in very different brain systems.
Things get difficult
When trying to understand the brain networks that underlie the production of smiles, things can quickly become difficult.
For a start, people move a lot when they smile. This can cause problems for brain imaging techniques such as fMRI (functional magnetic resonance imaging). It can also be extremely difficult to make people smile naturally under laboratory conditions.
Another issue is that different regions of the brain all seem to be linked to laughter. One of us (Fausto) works with the Epilepsy Surgery Unit at the Niguarda Hospital in Milan, where patients with drug-resistant epilepsy undergo electrical brain stimulation for clinical purposes. Electrical stimulation is highly precise and unaffected by movement.
Normally, if you stimulate the motor system, the patient will move. If you stimulate the visual system, they may experience hallucinations. And when you stimulate the auditory system, they hear sounds. But there are five or six areas of the brain where stimulation elicits loud laughter in the patient. So, why is that?
We recently reviewed all the brain stimulation studies of people laughing, and how they felt when they laughed. These studies provide evidence that the areas of the brain where stimulation evokes laughter form two distinct smile networks.
In one, which includes the anterior cingulate cortex, the temporal pole and some subcortical regions, the stimulation led to laughter, accompanied by pleasant sensations and happy feelings – that is the more helpless, high-pitched smile. When there was stimulation to the other network, which is in the motor and premotor regions of the brain, laughter led to speech impairments and was not associated with an emotional state.
Evolutionary differences
The areas that make up the first, more joyful smile network, mapped onto a network for vocalizing that humans share with all other mammals. This is potentially also responsible for other spontaneous emotional vocalizations such as
The activation of this network during spontaneous laughter triggers a neurochemical cascade that, among other things, taps into the opioid system. This helps explain the sense of pleasure and reward that comes from this joyous, uncontrollable laughter. The serotonin system, which regulates our sensitivity to pain, is also affected, which may explain why this smile can temporarily mask physical pain.
From an evolutionary perspective, this network is older. It arises in the spontaneous emotional responses and rough-and-tumble play systems of animals, and is therefore largely unchanged between humans and other animals.
The other, voluntary laughter, which is mapped to a brain network associated with voluntary movements in humans, such as speaking out loud. These have a primary role in strategic communication and social negotiation, so that the particular use of laughter in conversation can be explained as a type of spoken punctuation.
This neural system is mainly found in humans, and is associated with complex vocal acts such as speaking, singing and beatboxing.
A 2022 study looking at the shapes our vocal cords make when we smile in this conversational way found that they resemble the shapes we make when we speak. In contrast, the forms of our vocal tract, when we laugh helplessly, look very different from speech, and their motor control is simpler, consistent with their ancient origins.
This research shows that voluntary laughter requires more complex and sophisticated motor control than spontaneous laughter, and occupies a middle ground between spontaneous laughter and spoken language – not only functionally, but also anatomically.
If a person has a stroke that damages this voluntary network, they can develop severe problems with speech, while still laughing, crying and swearing. We would argue that this is because the older network remains intact.
The science of laughter may seem like a rather frivolous topic – and neuroscientists have certainly devoted more effort to studying negative emotions such as fear, anxiety, anger and disgust.
However, research on human laughter is turning out to be something of a Rosetta Stone for understanding a surprising range of phenomena—including the origins of the social brain in mammals, the mechanisms of bonding and empathy, pain modulation, language in conversation, and emotional contagion.
