Is a Laugh the opposite of a Scream?
Darwin wrote in The Expression of the Emotions in Man and Animals that he would expect the sound of laughter to be as opposite as possible from the cries of distress.
Indeed, in many ways they are. A scream is sustained, whereas a laugh is broken up. A scream maintains a constant pitch, whereas the pitch of a laugh will fluctuate. A scream has indistinct onsets and offsets. It slips out of the air and then fades off. Laughter is much more textured. It comes as a collection of colorful rhythmic bursts, punctuated by jagged staccato edges.
Scream Waveform. Notice the lack of variation, and the gradual onset and offset. image from Freesound.org
Laugh Waveform. Notice the obvious segmentation, variation, and distinct onsets and offsets. image from Freesound.org
The straightforward, streamlined structure of a scream likely helps it convey a sense of urgency more efficiently without risking giving up too much information, while the ornate, fanciful nature of a laugh allows it to carry a wealth of information about who and where it’s coming from.
You could probably invent dozens of new ways of laughing by switching up the pacing, or the number and length of intervals, or how quickly the tones flutter. Each of these different ways of laughing would paint a different portrait of a person. But if you wanted to improvise new screams, your options would be much more limited.
If you closed your eyes and heard a laugh in the other room, you’d immediately be able to tell all sorts of things about the person laughing. You’d be able to pick out little subtleties about their status or their mood or their health. But if you’d heard a scream in the other room, you’d know very little about the person except that they’re in distress.
The structural complexity of a vocal call can play a significant role in its function. In 1955, Ethologist Peter Marler, a man who spent much of his life studying birdsong, suggested that one way the structure of a call influenced its function was by affecting how easy it is for an animal to tell where the call is coming from.
Peter Marler. Courtesy of Rockefeller Archive Center.
Marler built off the work of Lord Raleigh, a celebrated British polymath who among his many other scientific achievements discovered how we localize sounds. Raleigh proposed that our brains figure out where a sound is coming from by comparing how it is received in each ear. If a sound is a little louder in one ear, or if it is heard a little sooner in one ear, the brain can measure this small difference and determine the distance and direction to the sound source. This process requires the comparison of two streams of information, one from each ear, which explains why people who lose hearing in one ear lose much of their ability to determine where a sound is coming from.
This process of sound localization is easier when there are distinct, identifiable points in the sound that make it so that the differences between the two streams of information are more obvious. In the same way that distinguishing features like tattoos and scars make it easier for police to determine if two witnesses are describing the same person, distinct, idiosyncratic signatures in a sound can act like fingerprints that make it easier for the brain to confirm that both ears are providing reliable information about the same sound.
When an alarm signal bombards the ears with the same tone for a sustained period of time, it hampers the brain’s ability to determine which ear heard the signal first. image from hookeaudio.com
Marler proposed that an animal alarm signal could minimize these identifying features by bombarding the ears with a pure tone for a sustained period of time. He noted that by stripping away all its adornments, an alarm signal would make it very difficult for the brain to pick out subtle differences between how the sound is perceived in each ear, thus making the sound hard to localize.
Marler suggested that if an animal could minimize the localizability of a call, he could alert his peers of danger without putting himself at any additional risk.
This observation gave insight to earlier theorists who were puzzled by the idea of an alarm signal, which seemed on its face completely altruistic. Natural selection favors those who are best able to look after their own self interest, so after a while you would expect all the selfless organisms to die off.
Instead, Marler characterized an alarm call as less of an act of selfless altruism, but rather a mutually beneficial transaction. The individual who sounds the alarm provides valuable information to the group, and if he is successful at convincing his peers that there is danger in the vicinity, they might scatter, creating a distraction for the predator that might allow for an escape.
On the other end of the spectrum, because laughter exaggerates all of its identifying features, it’s especially easy to locate. This means that instead of being particularly suited to convince others to scatter, its naturally adept at convincing people to gather.
Although laughter is associated with a wide variety of behaviors that don’t have a whole lot to do with each other, like child play, courtship, parent-offspring bonding, and more aggressive activities like mobbing and group harassment, what all of these activities have in common is that they occur when people cluster into groups.
There is of course a risk that comes with having your location made public; but in a strange way this additional risk might serve to put others at ease. By advertising your lack of concern about dangers in the vicinity you could provide some assurance about the safety of your environment. Making yourself more vulnerable could be a way of saying “I’m so confident about the safety of this situation that I’m willing to make myself the most prominent target of the group.”
In this case, there might still be something of a transactional element to the communication. The laugher takes on a little bit of additional risk by drawing attention to himself. But if he is able to convince his peers of safety, he might improve his ability to attain mating partners and social status.