Why do only humans weep? The evolutionary puzzle of crying.

The first word that people associate with laugh is cry, and that tells us something. In word associations, the two words usually belong to the same semantic category but are set off by a salient contrast (night–day, girl–boy, dog–cat). Like laughter, tears express an emotional state by means other than the muscles of the face. They are involuntary, conspicuous to a perceiver, and unique to Homo sapiens (a conclusion flaunted in the title of the most comprehensive book on the subject, the psychologist Ad Vingerhoets’s Why Only Humans Weep). And they seem engineered to generate common knowledge. A weeping person feels the welling in his sockets and the trickle on his cheeks and sees a blurry world through his own tears, a world that contains other people seeing the same tears from the outside.

The contrast between laughing and crying is obvious: tears convey sadness, not enjoyment. The sadness comes from a loss, defeat, or humiliation, and is accompanied by a feeling of helplessness and self-pity. The obvious coordination game resolved by common knowledge of a loss is Hawk–Dove. Like the white flag of surrender or throwing a towel into a boxing ring, crying signals that a person acknowledges defeat and can no longer put up a fight, sparing both sides in a conflict from the costs of further fighting when the outcome is a foregone conclusion. Koestler, whose analysis of weeping is as insightful as his analysis of laughter, describes a prototypical instance in a contest between boys:

“A little boy is beaten up by a gang of bullies. For a while he tries to fight back, to hit, scratch, and kick, but his tormentors immobilize him, and at last he begins to cry in ‘impotent rage.’

“But the expression is misleading. Anybody who has watched children fight knows that weeping will start only after the victim has given up struggling and wriggling and accepted defeat. After a while new outbursts of rage may renew the struggle, but, each time this happens, weeping is interrupted. It is not an expression of rage (although the two may overlap) but an expression of helplessness after rage has been exhausted and a feeling of being abandoned has set in — a yearning for love, sympathy, consolation.”

In a similar way, a bickering couple can sense that one of them has “gone too far” in pressing an argument when the other starts to cry.

As Koestler observes, crying is a signal not just of surrender but of neediness, designed to elicit succor and comfort from sympathizers. This function is consistent with the developmental origin of crying in the infants’ need for nurturance, and its evolutionary origin in the separation call of juvenile mammals. A critical shift in both sequences is from the noisy demanding wail of crying to the teary pathetic whimper of weeping. No one knows why the processes of evolution recruited the lachrymal glands to convey this helplessness. Perhaps tearing originally grew out of a physiological reflex that restored moisture to the eyes and nasal passages after they had been dried out by the pressure of wincing or the hyperventilation of arousal. More likely it’s that humans both see the world through their eyes and fixate on the eyes of others (more on this soon), and a scrim of tears was the best way to capture the attention of the expresser and beholder simultaneously.

The idea that crying is a conspicuous signal of surrender in a conflict is a satisfying theory of the prototypical scenario. But as you read the explanation, I’m sure counterexamples surged into mind. Far from signaling misery, crying can be a source of pleasure, as when people pay good money to read a tearjerker or watch a three-hankie movie. People cry for joy at weddings. They cry when they are crowned Miss America, or win the NBA championship. They may cry in the presence of the sublime, like a closeup sighting of a whale, or, in the case of one of my colleagues, reading a brilliant PhD thesis in cognitive neuroscience.

And people may cry when they apprehend love and compassion in others. The closest words in English for this emotion are touched and moved. It’s the most common trigger for me, and many memories can make tears well up. The sight of a soldier in an airport returning from Iraq, locked in an embrace with his wife, neither able to let go. My parents’ toasts to each other on their 60th wedding anniversary. Ray Charles performing “Georgia on My Mind” in a joint session of the Georgia legislature in 1979 when they adopted his rendition as the official song of the state, which in his lifetime had been a bastion of segregation and oppression. My great-aunt Sabena, who lost her husband and children in the Holocaust, standing up at the end of each seder and delivering a short speech in Yiddish thanking the family for giving her a new life in Canada. A young scientist discussing her research with me over coffee at a conference, who during a lull in the conversation got a faraway look and murmured, “I miss my son.” A Peanuts strip in which Linus describes a last-second upset victory by the home team in a football game — “Thousands of people ran out onto the field laughing and screaming! The fans and the players were so happy they were rolling on the ground and hugging each other and dancing!” — and Charlie Brown replies, “How did the other team feel?”

How do we make sense of tears of joy, tears of sublimity, tears of compassion? One of the apparent counterexamples is easy to explain, the tearjerker. People take pleasure in self-administering safe, controlled doses of harmful stimuli, presumably a motive to calibrate and control their emotional reactions. And so they enjoy hot chili peppers, roller coasters, thrillers, saunas, strong cheese, water-skiing, bungee jumping, and other borderline or illusory dangers. A simulated tragedy in the comfort of a theater seat may be another example of this benign masochism.

The others are more puzzling. With weddings and victories, the joy may be mixed with a dollop of poignant sadness. Perhaps a bride walking down the aisle brings to mind a daughter lost forever to her parents, or a history of loneliness and heartbreak now put behind her. Weeping victors sometimes invoke the obstacles they had to overcome to reach their moment of triumph, or a deceased parent who would be proud. Koestler suggests that a stroke of good fortune, or a vision of the sublime, may evoke a sense of powerlessness — of being rapt, overwhelmed, enraptured, entranced — which overlaps with the helplessness of loss and defeat, and which evokes a similar surrender. All these are plausible as far as they go, but they don’t add up to a satisfying explanation. Let me suggest a different direction.

In his forgotten masterpiece Darwin explained the evolution of emotional expressions with three principles. The first is “serviceable habits.” Animals configure their faces and bodies in certain postures for practical reasons — for example, unsheathing their teeth before biting, widening their eyes for a panoramic view of danger, flattening their ears to protect them in a fight. These preparatory movements then become habits that are carried out in a weaker form even when the action is suppressed. So we bare our teeth in anger and become headlight-stricken in fear.

It’s Darwin’s second principle that’s relevant here, “antithesis.” When an animal is in a state opposite to one that triggers a certain posture, it produces the physically opposite posture. When a dog is hostile, it stiffens its body, retracts its lips, raises its head and shoulders, and holds its tail erect and rigid, all in preparation for attack. When a dog feels affection, it does not have to prepare for action, but it assumes a posture that is part-for-part the antithesis of the attack pose: it crouches, wriggles, slackens its ears and lips, and rocks a limp tail from side to side — the mystery of tail-wagging solved. In a similar way, a man who is defiant stiffens his neck, squares his shoulders, lowers his brows, and clenches his fists with his knuckles forward. What does a man do when he feels resigned or impotent? He slackens his neck, raises his shoulders and brows, and opens his hands with palms outward. We have the evolution of the shrug.

Though Koestler appeared unaware of Darwin’s work on emotional expression (one of the signs that the book had been forgotten), he made a strong case that weeping is the antithesis of laughing. The two facial expressions are literally inversions of each other — as the song says, a smile is just a frown turned upside down. When we weep, we inhale in the short, deep gasps of a sob, then exhale in a long sigh. When we laugh, we exhale in the short bursts of a ha-ha-ha, a sob in reverse, followed by a long, deep inhalation, a sigh in reverse. When we laugh, we throw our head back. When we weep, we lower our head into our hands, onto the table, or on someone’s shoulder. When we laugh, our muscles contract and our bodies flail, banging the table, slapping our knees. When we weep, our muscles go flaccid, our shoulders droop, and we slump into our chair. Even the musical reminders are opposite. The comedian uses the staccato rim shot of a drum; the cinematic tearjerker uses legato strains of violins.

Now let’s work backwards, from the diametrical outward displays to diametrical internal states. Suppose the mental triggers for weeping are the antithesis of the triggers for laughter. This is obvious enough in the contrast between the aggressive takedown in humor and the helpless surrender in tears. But the less obvious stimuli for weeping can also be seen as the diametrical opposites of the things that make a joke funny.

Humor savors an infirmity — a foible, a failing, a venality, a flaw. Weeping savors a virtue — compassion, tenderness, pity, love. Humor responds to a misfortune with sadism and schadenfreude. Weeping responds to good fortune with empathy and shared joy. Humor delights in the degraded, the debased, the sordid—puking and farting, shitting and pissing, fornicating and cuckolding, pratfalls and pies in the face. Weeping delights in the exalted, the sublime, the magnificent — a symphony, a vista, megafauna, even a brilliant dissertation.

So there is, after all, a common denominator beneath the diverse things that make us weep: They’re the diametric opposite of the things that make us laugh. Now the question becomes why we have a conspicuous bodily display for the unlaughworthy (above and beyond the core of surrender and helplessness). Darwin thought there was no good reason for displays to come in opposites: Once an organism has evolved an emotional expression, its antithesis just falls out of the mechanical pushes and pulls of the nervous system. And perhaps there is no reason we weep in response to joy or compassion or sublimity, other than that we laugh at the ridiculous, and human emotional responses come in opponent pairs.

Alternatively, perhaps we have reason to signal that we are sensitive to tenderness and magnificence and joy — and for others to know that we know that they know we are sensitive. Perhaps it commends us in the market for good communal partners, or coordinates like-feeling people in some as-yet-unanalyzed game. I’m not prepared to make this argument just yet. But I hope that it’s true, because it’s comforting to think that we are physically designed to share our appreciation of the best that life can offer.