In the video below, a boy tickles his little brother. At first, he smiles and giggles, but his peals of laughter quickly give way to howls of anguish.
[I asked] moms who came into the baby lab what kinds of things their babies laughed at, and many of them said they loved the laughter but also feared it, because it usually meant a bunch of crying was on the horizon.
If the internet is any indication, this experience is not uncommon, and spans the globe (I was able to find fickle babies in France, Korea, and Japan). But why should laughter portend crying? Are happy moods really that transient? Or is there some deeper link between laughter and crying that this phenomenon points to?
In search of answers, I turned to Darwin, who was a consummate observer—not only of the natural world, but of his own family. In an 1877 article, he describes instances that incited his son to laughter. Along with standard crowd-pleasers like peek-a-boo and cheek-pinching, Darwin tested how his four-month-old son would react to more unusual events:
I approached with my back towards him and then stood motionless; he looked very grave and much surprised, and would soon have cried, had I not turned round; then his face instantly relaxed into a smile.
On another occasion:
I had been accustomed to make close to him many strange and loud noises which were all taken as excellent jokes but... I one day made a loud snoring noise which I had never done before; he instantly looked grave and then burst out crying.
What all of these situations have in common—tickling, pinching, peek-a-boo, strange noises emitting from the caregiver—is that they are possibly dangerous, at least from a baby's perspective. The child is trying to resolve this ambiguity, and may come to alternating conclusions about what is going on. As we get older, we continue to laugh at events or ideas that pose ambiguous (and sometimes real) threats to us. According to Darwin, reports surfaced during the Franco-Prussian war that
German soldiers, after strong excitement from exposure to extreme danger, were particularly apt to burst out into loud laughter at the smallest joke.
My cat, who on a cuteness scale ranks somewhere between a baby and a German soldier, exhibits an analogous behavior. When I play with him he shows every external sign of enjoying himself, wagging his tail and nipping at my hands in a pantomime of fighting. But sometimes, out of nowhere, he will let out a cry, bite me hard, and run away with a puffy tail. While humans are among the only animals with laughter, the problem of figuring out what is real and what is harmless—which can often look quite similar—may be evolutionarily ancient.
I consult with Prof. Graham about this idea, who confirms with an observation about his own son:
He laughs the hardest when we scare the [baby poop] out of him, like looking away or pretending to sleep and then yelling "boo" in his face.
Babies, then, do not laugh when something really good happens, but when something bad could be happening. This tension between danger and safety, stress and relief, is at the heart of adult laughter, as well. Little wonder then, that the psychologist John Y. T. Greig concluded, "The laugh, so far from being expressive of pleasure, is really an expression of displeasure."
So, are laughing babies happy? Even laughing babies might not know.