The Expression of the Emotions in Man and Animals

Darwin investigates why humans and animals express emotions through specific bodily movements and facial configurations. He argues that emotional expressions are not divinely designed nor arbitrary but are products of evolution, inherited habit, and the physiology of the nervous system.

Darwin opens by surveying prior scholarship on expression, distinguishing his project from physiognomy, the practice of reading character from permanent facial features. He credits Sir Charles Bell's Anatomy and Philosophy of Expression (1806) for demonstrating that expressive movements are linked to respiration and Dr. Duchenne's Mécanisme de la Physionomie Humaine (1862) for using electrical stimulation and photography to isolate the action of individual facial muscles. He faults most earlier writers for offering vague or fanciful explanations, including M. Moreau, an editor of Lavater's Physiognomy who contributed essays on facial-muscle movements, and the painter Le Brun, a 17th-century theorist of expression. Darwin singles out the theorist Herbert Spencer as the one prior writer who approached a correct understanding: that expressions are weaker versions of the actions that would accompany the actual experience of the thing feared or desired. He insists that because anthropoid apes possess the same facial muscles as humans, an evolutionary perspective is essential for investigating the causes of expression.

To build his evidence, Darwin draws on six sources: observing infants, who express emotions with extraordinary purity; studying individuals in psychiatric institutions, who often express emotions without restraint, drawing especially on notes from Dr. J. Crichton Browne of the Wakefield asylum; showing Duchenne's photographs to observers to test whether expressions are universally recognized; examining works of art; circulating a questionnaire to observers worldwide, receiving 36 responses covering geographically distinct populations; and observing expression in animals, which he considers the most reliable evidence because animal expressions are not conventional.

From these observations, Darwin proposes three general principles that account for most involuntary expressive movements. The first, Serviceable Associated Habits, holds that actions originally useful under certain mental states become habitual through repetition, so that even a feeble recurrence of the same state triggers the associated movement whether or not it remains useful. A dog circling before lying on a carpet replicates the grass-trampling behavior of its wild ancestors. When such habits are partially suppressed by the will, the muscles least subject to voluntary control still act, producing movements recognized as expressive.

The second principle, Antithesis, states that when a directly opposite mental state is induced, there is a strong involuntary tendency to perform movements of a directly opposite nature, even though these serve no purpose. Darwin illustrates this through the contrasting postures of dogs. A hostile dog walks stiffly upright with raised head, erect tail, bristling hair, and a fixed stare. The same dog, upon recognizing its master, instantly reverses every element: its body sinks and becomes flexuous, its tail lowers and wags, its hair lies flat, and its ears droop. None of these affectionate movements has any practical function; they are explicable only as the complete opposite of the hostile posture.

The third principle, Direct Action of the Nervous System, accounts for effects produced when the nervous system is strongly excited and nerve-force is generated in excess or interrupted, independently of will or habit. Trembling from fear, cold perspiration, the loss of hair color from extreme terror, and altered glandular secretions all fall under this principle.

Darwin applies these principles across animal species, surveying vocal sounds, the erection of hair and feathers under anger and terror, the inflation of the body, and the retraction of ears in animals that fight with their teeth. He examines expressions in dogs, cats, horses, and monkeys, demonstrating that the principles operate consistently. Monkeys display pleasure through chuckling sounds and brightened eyes, anger through uncovered teeth or protruded lips, and astonishment through motionlessness and widely opened eyes.

Turning to human expression, Darwin traces the chain of causation from infant screaming to adult weeping. Screaming infants firmly close their eyes to protect them from blood engorgement during violent exhalation. This protective contraction of the orbicular muscles, the circular muscles that close the eyelids, draws up the upper lip and, over countless generations, becomes associated with distress. The lacrymal glands, the tear-producing glands near the eyes, are stimulated reflexively by pressure on the eyeball and by distension of ocular blood vessels. Through long-continued habit, even mild distress in adults triggers tear secretion, because the lacrymal glands are among the bodily functions least subject to voluntary control.

Darwin devotes close attention to two puzzling features of grief: the oblique eyebrow and the depressed corners of the mouth. The oblique eyebrow arises when a person tries to suppress crying. The muscles that would close the eyes during screaming are partially checked by the central portion of the frontal muscle, which raises only the inner ends of the eyebrows, producing characteristic wrinkles on the forehead. Few people can produce this expression voluntarily, and it appears more frequently in children and women than in men. The depressed mouth corners result from the action of the depressores anguli oris, the muscles that pull the corners of the mouth downward, which Darwin identifies as among those least responsive to the will.

He analyzes laughter and smiling as expressions of joy, tracing a continuum from violent laughter to a gentle smile. A genuine smile requires contraction of both the zygomatic muscles, which draw the mouth corners upward and back, and the lower orbicular muscles of the eyes; without the latter, the expression appears false, as Duchenne's experiments confirmed. Darwin discusses anger and rage, noting the universal retraction of the lips to expose the teeth as a vestige of ancestral progenitors who fought with their canine teeth. He examines contempt and disgust as expressed through movements around the nose and mouth derived from the habitual exclusion of offensive odors or the rejection of unpleasant food. He explains the shrug of helplessness as a pure instance of antithesis: every element, from raised eyebrows and open palms to relaxed mouth, is the exact opposite of the indignant posture of clenched fists, squared shoulders, and lowered brow.

Darwin considers blushing the most distinctly human of all expressions. He defines it as the reddening of the face caused not by physical stimulus but solely by a mental state: the consciousness that others are attending to one's personal appearance. Blushing is involuntary, inherited, and observed across nearly all races. Darwin proposes that attention directed to any part of the body tends to interfere with the normal contraction of its small arteries, and that this tendency has been powerfully strengthened through generations of self-conscious attention to the face.

In his concluding chapter, Darwin reflects on the broader implications of his findings. The universality of expressions across all human races, he argues, provides evidence that all races descend from a single parent-stock already essentially human in structure and mind. He speculates on the evolutionary sequence: laughter and fear were acquired very early, shared with monkeys; weeping came later, since apes do not weep; the oblique eyebrow of grief appeared only after the habit of restraining screams developed; and blushing arose last, requiring an advanced degree of self-consciousness. Darwin closes by emphasizing that expressions serve as the first communication between mother and infant, reveal thoughts more truthfully than words, and intensify or soften the emotions they accompany.