The eminent jurist was not quite justified in his satisfaction at his captive’s fate, for many of the tens of thousands kept in endless isolation in American prisons end their lives not with a whimper, but a scream. Some do fall into insanity in such places, but much as the religious right might celebrate their mental decay, they would be dismayed to learn that Moussaoui will lose his mind for Darwinian reasons. Guy the Gorilla, star of London Zoo in the 1950s, was admired for his solemn disposition. In fact, the animal was deeply depressed, kept as he was for years alone in a small cage (although unlike his human equivalents he had no opportunity for suicide). Homo sapiens is a social primate and - like gorillas or chimpanzees - descends from an ancestor with the same habits. Had our forefathers been solitary beasts like the orang-utan, which spends most of the year alone, the worst of all punishments would not be solitary confinement but an endless dinner party. The constant exchange of subtle emotional cues around the table would drive all those present to their wits’ end.

Science is often asked to explain what makes men different from chimpanzees or orangs but in many ways that is not a scientific issue at all. Such questions deal with the mind rather than the body or the brain; a topic that most competent biologists consider to be outside their expertise. Even so, as scientists compare man’s anatomy and behaviour with that of his relatives, biology does reveal a little about how humans became what they are. We are, says all the evidence, creatures that crave society. To satisfy that yearning we spend large parts of our time in silent and sometimes subliminal conversation with each other.

Rousseau had a different view of the origin of human nature. He saw man as in decline from a pure and animal state and modern society as a corruption of what the world should be. ‘Savage man, left by Nature to bare instinct alone . . . will begin with purely animal functions . . . His desires do not exceed his physical needs: the only goods he knows in the Universe are food, a female, and rest.’ The true life was near-solitude, on a remote island best of all, with a bare minimum of interaction with others. The French philosopher’s ideas were romantic, but wrong. Members of all communities, human or otherwise, must negotiate to maintain peace, to have sex and to reap the benefits of cooperation. They use signals both self-evident and subtle to test the mental state of their fellows and to advertise their own, and even the solitary orang hoots now and again to impress its neighbours. Civilisation is based on the ability to respond to another’s sentiments and to express a mood of one’s own.

In 1879, at the Derby, Darwin’s cousin Francis Galton noted that he could assess ‘the average tint of the complexion of the British upper classes’ as he observed the crowd through his opera-glass. Then the race started, and in a letter to Nature entitled ‘The Average Flush of Excitement’, he observed that it became ‘suffused with a strong pink tint, just as though a sun-set glow had fallen upon it’. A shared hue was a statement of a common passion and Galton could work out what it was even when he could not tell individuals apart. In the same way, someone exposed to an image of a group of people who bear a range of expressions from happy to miserable can sense their general state of mind far faster than he could by scanning each visage separately. Our brain, it seems, has a filter that picks up not just how many are in a crowd, but how, on average, they feel. The ability has its down-side. Mass hysteria can spread through society as shared sentiments feed on themselves; as Charles Mackay put it in his 1841 book Extraordinary Popular Delusions and the Madness of Crowds, in an account of the South Sea Bubble and other mass fantasies, men ‘go mad in herds, while they only recover their senses slowly, and one by one’.

Our own sentiments have long been compared to those of other creatures. The seventeenth-century painter Charles Le Brun, who is referred to in the Emotions book as a pioneer in the study of human feelings, urged those who tried to portray their subject’s mood to scrutinise beasts first. A few hours with swine, lascivious, gluttonous and lazy as they were, would, he was sure, help depict the inner life of a debauchee. Charles Darwin’s friend George Romanes went further. He set out a scale with fifty ranks. Worms and insects came in at step 18 as they could experience surprise and fear; dogs and apes were equal at point 28 as each had ‘indefinite morality along with the capacity to experience shame, remorse, deceit and the ludicrous’. Levels 29 to 50 were reserved for men or women of greater or lesser virtue.

Psychology is still marked by such ideas. Emotions’ central theme was, as ever, a world in which all of life’s attributes, from anatomy to anguish, emerge from shared descent. Science uses that logic on elephants, cows, apes, fruit flies and bacteria in its attempts to build a shared narrative of inner feelings. Those who transmit their sentiments expect a response from those who receive them. That two-way commerce involves a need to acknowledge, to copy and to respond to the moods of others. People gasp in sympathy at a sad tale, gaze at where another person’s eyes are directed or avoid food that someone else has rejected. Such reflections of another individual’s mental state are part of what makes us human.

Charles Darwin, a practical man, had little interest in philosophy. Even so, he realised that the biology of the mind was harder to interpret than was that of the body. He wrestled with the issue in rather the same way as modern psychologists try to come to grips with some of their own sometimes murky ideas. Can our thoughts be explained just as the ‘direct action of the excited nervous system on the body, independently of the will’ and if so, what (if anything) does that mean? Shakespeare speaks of Cardinal Wolsey when ‘Some strange commotion/ Is in his brain; he bites his lip and starts;/ Stops on a sudden, looks upon the ground . . .’ That, Darwin writes, came from the ‘undirected overflow of nerve-force’ - but is that phrase just an attempt to avoid deeper and less tractable questions? The task was made harder by his quarrel with the anti-evolutionist Charles Bell, author of the standard text on facial anatomy. Bell was convinced - and he was wrong - that humans had unique muscles divinely designed to express morality, spirituality or shame: a notion not of much help to someone anxious to understand the smile or the blush, but an early example of the preconceived truths that still plague many attempts to understand the human mind.

After a long stumble through the Freudian fog, the study of the mental universe has once again become a science, even if the many claims to have found the neural foundations of society do not yet deserve that status. Now, physicists and chemists busy themselves with questions once raised only by intellectuals. In institutes of psychiatry and neurology, cats, mice and dogs are used to dissect human habits. Even bacteria behave in a rational fashion when they settle down close to a source of food, or join hands with their colleagues to form a sticky film over teeth or wounds. Certain fruit-fly genes lead to homosexual behaviour and others to loss of memory, which might one day help in the study of illnesses such as Alzheimer’s disease. In mice and monkeys, experiments on brains once done with a scalpel are now carried out with machines of fantastic complexity. They are also used on people with brains damaged by strokes or accidents, while drugs help understand the mental universe of the normal, the reckless and the insane. Many of the questions raised in The Expression of the Emotions have a notably modern air and many remain unanswered.

Emotions is in some ways a less satisfactory work than are the plant, barnacle or earthworm books and an unusual note of apology creeps in: ‘Our present subject is very obscure . . . and it always is advisable to perceive clearly our ignorance’ (and there its author was franker than some of his successors). Charles Darwin soon found that even what looked simple - the objective description of the facial expression of a man or a dog, for example - was hard, while to represent the sentiments behind it was even harder. That problem, in spite of the wonders of electronics, still baffles students of the nervous system. He was suspicious of phrenology - the notion that particular segments of the brain are associated with, for example, obstinacy, pride or guile - even if an admirer had claimed that the naturalist’s own head had ‘the bump of reverence developed enough for ten priests’. He struggled long and hard with the question as to just where felt experiences might be seated.

Even in play the Beagle’s naturalist was serious, and he soon identified some general rules about human and animal behaviour. Intimations of happiness or grief, of welcome or rejection and of other opposed sentiments often came as mirror images. Thus, a frown is the opposite of a smile and a look of surprise the converse of a greeting. Some gestures emerged from movements that once had a function of their own. To beg with open hands is related to the posture taken when holding food and, in the same way, a person who rejects an advance closes his eyes and looks away, as if from an unpalatable meal. Animals seemed to follow similar rules and the paterfamilias of Down House saw almost the same downcast looks in his household pets as those adopted by his infant son.

From such simple observations emerged the science of comparative psychology. It began with dogs.

Pets gain their status because they seem, to their owners at least, to be almost human. Darwin was no exception and kept a dog - Sappho by name - even when he was a student. He saw no problem in describing canine sentiments in the same terms as our own. His pet when in ‘a humble and affectionate frame of mind’ acted in a way quite different from that of a hostile animal with its bristling hair and stiff gait. The ‘principle of antithesis’ was hard at work, for opposed sets of muscles were set into action to express contrasting emotions. The ‘piteous, hopeless dejection’ of his favourite hound when it discovered that it was not about to go out for a walk but instead was to sit in on an experiment in the greenhouse was manifest in a ‘hothouse face’, the ‘head drooping much, the whole body sinking a little and remaining motionless; the ears and tail falling suddenly down, the tail by no means wagged’. That was quite different from its expression when happy and excited, with the head raised, ears erect and tail aloft.

As well as such individual shifts of mood the proud pet-owner noted marked differences in personality among breeds. Descent with modification could, it appeared, change minds as easily as it could bodies. Certain kinds, such as the terrier, grinned when pleased while others did not. Spitz-dogs - huskies, elkhounds and the like - barked while the greyhound was silent. The canine universe encompasses a wide range of talents. Some varieties herd sheep and cattle (and, in the case of the Portuguese Water Dog, chivvy fish instead) while others guard, hunt, guide or annoy the general public. The various breeds when taken together show a wider range of behaviour than that found among all wild canines - wolves, foxes, coyotes and jackals - across the world. Many of the differences are innate, and The Origin tells of a cross with a greyhound which gave a family of shepherd dogs a tendency to hunt hares. So impressed was its author with the animals’ divergence in habits that he suggested some of the household types had descended from distinct wild ancestors (and there he was wrong).

His favourite pet is back at the centre of the emotional stage. The world has four hundred million dogs and the efforts of their owners and the wonders of science have transformed the creatures into a gigantic experiment on the biology of sentiment. Even in the brief period since modern breeds began to emerge in Victorian times dogs have undergone large - and inherited - changes in temperament.

Men long ago began to use dogs in the hunt. They soon learned to choose those with their own special abilities - to track, to run, to squat into a ‘point’ position when prey is spotted, or to bite and tear or recover corpses - as parents for the next generation. Such remnants of the chase live on in the behaviour of Bloodhounds, Pointers, Setters, Retrievers and Bull Terriers. Herding dogs such as Border Collies stalk a sheep and do not bite it, but those used to control larger animals - like the Corgis once used with cattle - go further through the sequence and snap at their charges. Pit Bulls complete the job and are vicious creatures that will hold a bull by the nose and as a side-effect sometimes kill their owners. Guard dogs such as Pyrenean Mountain Dogs, whose job is to frighten off predators, have given up the hunt sequence altogether. They play like huge puppies and show little interest in their herds, but their conduct is odd enough to persuade wolves to stay away. Such differences emerge from inherited variation in behaviour within the common ancestors of each breed, from new genetic errors as the generations succeed each other, and from the accumulation of change by human choice.

One way to assess a dog’s personality is to startle it with the appearance of a stranger. Does the animal play with the visitor, back off, sniff him or chase him out of the room? Does a sudden noise anger the beast, terrify it or leave it unmoved? Other tests include the ability to sit still, to cope with solitude without whining or panic, to run through a maze or to find hidden food.

Cocker Spaniels are calm and obey orders, while Basenjis are nervous and almost impossible to train. Crosses between the two suggest that the difference in their nature is inborn, for the offspring have a range of talents, intermediate between each parent. A survey of ten thousand German Shepherds and Rottweilers in Sweden showed, within each type, a shared inheritance of excitability, a tendency to wag the tail and a need to bark, while aggression appears to be under separate control. In an echo of Expression’s principle that antithetical emotions are expressed as mirror images, variation in all those capacities depends on just how shy or how bold a particular breed might be.

As the dog-fanciers’ tastes became more refined, more and more specialised varieties emerged. Some began to develop habits that perturbed their owners. Mating like with like exposed rare and once-hidden genes, many of which had undesirable effects on personality. Some have parallels in the mental lives of men and women. In an echo of human obsessive-compulsive disorder, Bull Terriers chase their own tails for hours until they collapse, while Springer Spaniels may savage their masters as they fall into a sudden attack of uncontrolled rage. Certain families of Bassett Hounds suffer from a delusion reminiscent of paranoid schizophrenia and cower at the slightest noise. Some Dobermans, in contrast, fall into a heavy slumber after an unexpected snack. They have narcolepsy, a distressing and sometimes dangerous condition also found in people - and the dogs respond well to the drugs used to treat human patients.

The double helix reveals why some breeds diverge so much in personality. The first complete sequence came from a Boxer. The animal had less DNA than we do, with about twenty thousand genes altogether, several thousand fewer than ourselves. The hope is to find canine matches to our own disorders, and some have already emerged. The sleep problem in Dobermans involves damage to a certain receptor protein on the surface of brain cells - and the human equivalent is due to a fault in the same gene. No doubt our companions will help track down many more of the inherited errors behind our own mental illnesses, as they already have for conditions such as blindness. Charles Darwin would be proud.

Primates, like people, reveal their feelings on their faces. Someone who has never before seen a macaque can at once identify its mood as sad, happy or enraged, when shown the appropriate photograph. Many chimpanzee expressions have been named. They include the closed-mouth smile, its bared-teeth equivalent (which descends from an ancestor shared with our own smile), the bozo smile and the play face (a relative of human laughter), together with subtler statements of mood such as the stretched pout-whimper. Bonobos have an amused expression - and noise - which is uncannily like a guffaw. A German expert has identified an Orgasmusgesicht or ‘orgasm countenance’ in that species, although its existence in humans remains to be demonstrated. Gorillas are more impassive for they grin and make bozo faces but otherwise keep their thoughts to themselves unless they are simply furious.

Apes and monkeys can interpret their fellows’ moods to a considerable degree. Electronic avatars of chimps can have their looks manipulated to simulate pout-whimpers and the rest. When real animals are presented with their artificial comrades, they pick out different expressions at once, screaming faces best of all. They also show some insight into another’s emotions. If one animal sees another grimace in fear when it hears a sound it had learned to associate with an electrical shock, the observer will flinch when the buzzer goes off, even if it has never itself experienced the shock.

Humans are even better at sensing the moods of others. We are so aware of facial features that we often see them when they are not there (which explains the sad ape-like countenance in NASA’s pictures of the mountains on Mars). Two scrunched-up newspapers look much the same although their shapes are quite different, while two faces are seen as quite distinct although their shapes are almost the same. A simple bar code, the position of six stripes of dark and light - hair, forehead, eyebrows, nose, lips and chin - stores most of the data. Most people can recognise thousands of individuals and sense dozens of emotional states. Faces are important even to infants. Darwin noted that, when they were very small, his children spent long periods gazing at their mother. Now we know that a baby responds to a human countenance - even in a photograph - within minutes of birth.

Men, like apes, speak with their faces and use more or less the same language to do so. Angry people and angry gorillas bare their teeth and a frightened chimpanzee looks rather like a frightened person. For humans, as for apes, some expressions are ambiguous. Men and apes bare their teeth when amused but do the same when filled with terror. Emotions has a picture of a Sulawesi macaque as it grins in pleasure when stroked - but in other macaques the same gesture marks submission to a threat. Not all our grimaces are shared with our relatives, for apes never signal disgust and their noses, which are more sensitive than our own, remain unwrinkled even to a repulsive smell. A wide-open mouth is a threat in many primates but conveys no more than mild surprise for humans and while elephants weep, our closest kin do not.

Monkeys and apes reflect their moods in their postures as well as their expressions, and gorillas really do slap their chests in rage. Men, orangs, chimps and gorillas share the Italianate habit of waving their hands. Bonobos flap their wrists in irritation, point at themselves when they need a hug and stick out their palms when food is on offer. In a further nod at our common heritage, they prefer to signal with the right hand.

Darwin was keen to discover whether signals such as the blush were the same in every human culture, or whether, like skin colour, they changed from place to place. He rejected the popular notion that different races had evolved from higher or lower forms of primate and that their mental lives and expressions of mood reflected this. Soon he began to accumulate a mass of anecdotes that made the case for the universal nature of facial cues. People also wrote to him about their dogs frowning in concentration or showing moral courage when their teeth were pulled. His correspondents included William Ewart Gladstone, who commented on statements about the Greek visage found in Homer, but also ‘Captain Speedy who long resided with the Abyssinians; Mr Bridges, a catechist residing with the Fuegians and Mr Archibald O. Lang of Coranderik, Victoria, a teacher at a school where aborigines, old and young, are collected from all parts of the colony’. One letter told of a Bengali boy with ‘a thoroughly canine snarl’. Its recipient fired off a series of questions to those servants of the Queen, sometimes to ludicrous effect (‘Mr B.F. Hartshorne . . . states in the most positive manner that the Weddas of Ceylon never laugh. Every conceivable incitive to laughter was used in vain. When asked whether they ever laughed, they replied: “No, what is there to laugh at?”’).

In spite of the Weddas, Darwin became certain that such signs were more or less universal across the globe: ‘The young and the old of widely different races, both with man and animals, express the same state of mind by the same movements.’ Hard as it is to believe, that observation was forgotten and for many years students of humankind assumed that expressions were determined by culture and were not coded into DNA (even if nobody found a place where people laughed in pain or screamed in welcome). Looks of anger, disgust, contempt, fear, joy, sadness and surprise all are universal. One tribe in New Guinea cannot separate expressions of fear from those of surprise - but in that society any intruder is a threat. People from different cultures do find it harder to identify each other’s guilty or shamefaced looks than they do a smile or an expression of terror, so that such subtle statements of mood may in part be learned. Even smiles are equivocal, for the beam, grin, smirk, snigger, simper and leer each convey a different message while people who smile too often come across as nervous rather than contented. Darwin, too, saw some ambiguities. The expression in a photograph of a man almost in tears was recognised by some as a ‘cunning leer’, a ‘jocund’ frame of mind or even as someone ‘looking at a distant object’.

In today’s world of fraud, terrorism and identity cards such attempts to put facts on to faces have become an industry. Remarkable claims are made about the ability to identify people and to sense their states of mind. Some enthusiasts recognise thirty indications of anger and eight of sadness, with additional criteria based on how the subject holds his head. George W. Bush’s countenance was more or less blank whatever message he tried to put across, but his Department of Homeland Security has spent millions on machines that claim to detect when a terrorist is about to attack by the look on his countenance. Nobody denies that the expression of a Scotsman with a grievance is easy to distinguish from a ray of sunshine but such claims go too far.

The face says a lot about how we feel, but - as in apes - the body adds information to the stream of emotional cues: a man with raised fists is not about to make a visitor welcome. Psychologists tend, for practical reasons, to use pictures of faces alone. That can be a mistake. An image of a man with a disgusted expression taken from a modern catalogue of facial poses is interpreted as manifesting revulsion when superimposed on to a body holding a pair of dirty underpants - but as a look of anger when added to a torso with fists raised, or of triumph when stuck on to the beefy frame of a body-builder. The same photograph shown against the background of a cemetery is interpreted in a different way than when seen against a neutral surface. For students of the emotions, the assumption of simplicity can confuse results taken from the most complicated machines.

The face is a real mirror to the soul. Even a brief glimpse reveals the presence of another person, identifies who it might be and gives a strong hint as to what its bearer will do next. Most westerners interpret a set of features with a quick triangular scan of both eyes and the mouth, each of which say a lot about identity and state of mind - but the Chinese tend to concentrate their attentions instead in a fixed look at the nose and pick up the general expression of the whole visage in the background. Scans show that when someone flashes into view, the brain first notes his or her presence, then identifies who it might be and last of all of tests their mood: this is a face, it belongs to Fred, Fred is furious. It processes a portrait twice as fast as a picture of other objects. A certain part lights up about a tenth of a second after a face is first seen, notes its identity about a fifteenth of a second later and takes even longer to interpret what humour the person might be in.

Some expressions are easier to identify than are others. The smile is coded deep within the skull and everyone has an inborn ability to assume it. As Darwin noted, babies born blind smile without difficulty and (as he did not) blind athletes raise their arms in the air in a chimp-like gesture of triumph when they win. Children find it easier to pick out expressions of good cheer than they do those of fear or disgust. Women smile more at strangers than do men, while men are worse at working out mood from a slight movement of the lips. A lopsided grin to the right is seen by most of us as more joyful than is its equivalent on the left. Even sheep, when given a choice of a smirking or a sombre shepherd from whom to take food, prefer the cheery individual. We smile or raise our arms not to reassure ourselves that we are happy or proud, but to tell others how we feel. Context is all; when Chelsea score, fans respond with roars of triumph rather than smiles of delight, but gold medal winners as they stand on the Olympic podium have wider grins than do those who have gained bronze.

Signs of delight or terror seem simple enough, but there are real differences in the ability to decode them. I have a talent that illustrates that fact, for I can waggle my eyebrows. It began in school when I was rebuked for glowering. I then tried dumb insolence with a one-brow grimace rather than the full two-brow scowl and in time it became easy to alternate. It is still my occasional habit to amuse small children with the trick - and almost always they smile back. Unfortunately, an occasional infant screams instead. The signal is clear but the response uncertain.

Both steps can go wrong. Some people cannot tell individuals apart from their faces and use clues from voice or clothes instead. In one instance, a litigant wandered into court and discussed his case with a barrister - not his own, but his opponent’s. The context was right; a lawyer, with a gown, in a courtroom. The face alone did not fit. Needless to say, he lost. Face-blindness may be caused by a stroke, but a certain form runs in families with perhaps just a single gene involved.

Other unfortunates lose the ability to broadcast their emotions. For some reason - injury, infection, cancer or brain haemorrhage - the facial nerve no longer works and the patients cannot express their feelings. They find it hard to assume looks of happiness, fear or surprise, and their wives, husbands and friends soon notice the problem. The condition might appear to be trivial, but in fact causes real distress and sometimes even suicide, most of all when an attempt to smile emerges as a grimace or a leer because the eyebrows - usually lifted at a happy moment - refuse to obey instructions. Some people have their brows surgically moved upwards (which gives them a permanent look of surprise), while others grow a long fringe that hides the offending forehead. The readiness to take such steps shows how much a signal of mood is a passport to society.

The ancient Greeks had used electric fish to treat headaches but for many years the galvanic fluid was no more than an entertainment. An entire community of monks was once connected by a mile-long iron wire and made to jump for the amusement of the King of France (castrati were tested to see if they acted as insulators, but they did not). Emotions contains several pictures of faces stimulated by shocks to give expressions that resemble the natural look of horror, rage and the like. They came from the French physician Guillaume-Benjamin-Armand Duchenne de Boulogne. Duchenne is best remembered for the muscle disease named after him but he also studied the expression of what he called the ‘passions’, using electrodes touched to different parts of a countenance to stimulate the muscles. He was the first to notice that a genuine smile involved raised eyebrows, and his machine could easily activate those ‘sweet muscles of the soul’ to simulate a happy beam. He even made the visage of a decapitated criminal assume a simulacrum of pleasure with a probe upon its cheek. Duchenne chose as his main subject an aged man of feeble intellect, for he ‘wanted to prove that, despite defects of shape and lack of plastic beauty, every human face can become spiritually beautiful through the accurate rendering of emotions’. His pictures first came to public attention when they were published in The Expression of the Emotions. They played an important part in Darwin’s attempts to give an objective account of expressions of pleasure or pain.

The machines have marched on. Where Duchenne used a battery, a metal rod and a plate-camera, scientists in search of the springs of sentiment now depend on electro-encephalograms, positron emission tomography or functional magnetic resonance imaging (FMRI). Tiny electrodes are used to activate single nerve cells, while the EEG and its relative the magneto-encephalograph pick up electrical activity within the skull. PET scanners use a sugar marked with a radioactive label which is taken up by active parts of the brain and then detect its decay products. The fMRI machine, in contrast, senses tiny changes of blood flow through the grey matter from a shift in the magnetic properties of the red pigment haemoglobin as it gains or loses oxygen.

Marvellous as such techniques are, they run into many of the problems that plagued Charles Darwin. He had found it hard to decide just where the jaw ends and the cheek begins or to identify the precise arrangement of facial muscles. Today’s arguments about the boundaries between areas of the brain as defined by electronic scans - confidently coloured and labelled as the images might be - reflect his own doubts about the anatomy of the human countenance. Some claim that particular emotions can be mapped to a definite part of that organ. Others see the brain - as he saw the face - as a connected structure, with most sections contributing to most of its functions. Any attempt to pinpoint centres of anger, joy or despair might be of its nature a mistake.

Another problem for both the nineteenth and the twenty-first centuries comes from the need to describe broad sentiments in narrow terms. Darwin was happy to talk about dogs in a ‘humble and affectionate frame’ of mind - but how is it possible to put figures on humility or affection? Objective fact soon slides into mere interpretation and Expression was itself not immune from that temptation. Its photographic plates are not originals but engravings, some touched up to make a point. A mad lady with tousled hair was given a furrowed brow by the engraver and a screaming infant was made to look even more miserable than before by copying the portrait and re-photographing the sketch (the picture sold hundreds of thousands of copies to a gullible public). The stream of lurid images of centres for pain, passion and pleasure that decorate the scientific literature and leak into the press are also in some senses fakes. Digital information is processed in a complicated and sometimes subjective way to make a picture which is often rather more than the sum of its parts.

A final difficulty for both the Victorians and their descendants was to find subjects who were willing to display their emotions to the world. Duchenne set up a theatre in which the public could be delighted by actors galvanically activated to produce an air of grief or delight. Many of Expression’s pictures are also based on members of that profession. Among them, a bearded thespian looks remarkably implausible as he strikes his attitudes. Actors still play an important part in neuroscience. Their photographs are taken as they simulate a mood and are then shown to subjects whose brains are scanned to see which bits light up. Many of the images look just as posed as do those of Darwin’s theatrical friend. The artistes overdo the job, often to a bizarre degree. People shown pictures of frightened or unhappy people taken from real life have far less of a nervous response than they do to images of those who simulate a mood as they strut and fret upon the laboratory floor. Most of us find it harder to interpret the sentiments (apart from laughter) in silent clips of Hollywood stars from Dustin Hoffman to Meryl Streep than we do the simulated joy or terror of a ham actor - and yet the hams are used as raw material for experiments of huge technical sophistication and expense.

A blank stare is a signal of terror and Shakespeare knew as much. A furious Othello says to his supposedly unfaithful (and frightened) wife Desdemona before he kills her: ‘Let me see your eyes.’ We have larger eye-whites than any other primate and take more notice of them, for the mouth is far more important than the eyes as a chimpanzee emotional signal. We process eyes quicker than any other feature and fearful, stretched-open eyes even faster - and women do the job better than men. One woman could not recognise a picture of a terrified individual because she did not look at the eyes. When instructed to do so she at once understood the subject’s frame of mind.

The brain’s main activity in response to a frightened look takes place in a pair of structures called the amygdalae. They are almond-shaped groups of nerve cells deep within the temporal lobes, the side sections of the brain, one on each side, embedded into what is sometimes seen as the organ’s most primitive parts. Each is connected to other brain centres, to the hypothalamus - that hormonal bridge between the nervous system and the bloodstream - to nerves that feed from pain receptors and from the eyes, and, in primates more than other mammals, to nerves to and from the face itself.

Animals in which the structures have been damaged find it hard to pass the classic test in which fear of an electric shock becomes associated with the sound of a bell. Experiments on monkeys in which those parts of the brain were cut out showed that the unfortunate creatures in addition lost their ability to recognise familiar objects, together with their nervousness about humans, and a mother’s affection for her infant. Human patients with damaged amygdalae have similar problems with emotionally draining tasks. The amygdala is also involved in memory. People recall where they were on 11 September 2001 with its help, but those in whom the structure is damaged remember the Twin Towers disaster no better than what they had for breakfast.

The amygdalae are busiest when a frightened gaze is directed straight at its target - which fits Darwin’s idea that a countenance stricken by terror is an immediate signal of danger. A few people have such severe brain damage that they perceive themselves as blind - but show them a scared person and the amygdala lights up. We are slower to notice the racial origin of an angry than of a happy face, so that fear has priority over familiarity. In the United States, images of black people shown to whites stir up more activity than do those of individuals of their own skin colour.

The case for the amygdala looks persuasive but, as usual when it comes to the contents of the skull, real life is not simple. Other parts of the brain are also involved in the response to a terrified countenance. The amygdala lights up in response to a whole face rather than just the eyes, and does so to some degree whether or not the subject shows signs of alarm. Its main role might be to notice new events, whatever they might be, rather than to make a specific response to a particular emotion.

The structure helps to process a nerve-transmitter called serotonin (which is also involved in temperature control, sleep, hunger, lust, response to injury, liver repair and more). Many antidepressants work because they change the way in which serotonin is broken down, or taken into cells. Variation in the ability to respond to or to make the substance might be behind individual responses to fear. Some people are terrified even by the simplest problems of society. Darwin writes of a dinner party given for a man who, in response, ‘did not utter a single word; but he acted as if he were speaking with much emphasis. His friends, perceiving how the case stood, loudly applauded the imaginary bursts of eloquence, whenever his gestures indicated a pause, and the man never discovered that he had remained the whole time completely silent.’ The unfortunate fellow could now be comforted with the information that he may have a more active amygdala than normal and that his nervousness might be treated with drugs that alter his body chemistry.

Inborn errors in the ability to synthesise serotonin make some people sad, angry or suicidal. A gene whose product helps remove the chemical from the junctions between nerve cells comes in two common forms, one better at the job than the other. The less active type is more frequent among people who are anxious, neurotic or depressed - and its bearers are less able to decode expressions of fear or sadness than are their fellows. The orang-utan - the most solitary of our primate kin - has a version of the gene that is even less busy than that of the most socially isolated human. Whether its feeble serotonin pump has much to do with its lonely life and presumed dislike of dinner parties remains to be proved.

People with severe depression often find it hard to sense the emotions of others. Drugs that affect serotonin can help the illness - and their immediate effect, sometimes within hours of the first pill, is to improve a patient’s ability to interpret their fellow citizens’ feelings from their faces. That simple talent turns the key that restores them to society.

William and his brothers and sisters were lucky for they were raised in an affectionate household. Many youngsters are less fortunate. An infant brought up in isolation or by cruel parents may never adjust to the world around it and can feel isolated for the rest of its life. The fit between childhood abuse and adult depression is well established and those taken into care because of poor parenting are at far higher than average risk of emotional problems later in life. A failure to be provided with the signals of affection that bind children to their mothers and fathers and to society as a whole is to blame.

A few unfortunates suffer from loneliness or despair for the opposite reason. What condemns them is not neglect by those who should provide the crucial emotional messages, but their own inability to receive and interpret them. Such children are often diagnosed as autistic. They may live in isolation and unhappiness, with an existence that can seem scarcely human at all, for children with severe autism cannot make or understand the cues needed to find a place among their peers. Their plight shows how central is the ability to express, and to understand, emotions in allowing every citizen to take part in society.

Autistic children are now treated with sympathy and concern, but once they were regarded almost as animals. To those curious about where the essence of humanity might come from, they were useful raw material for speculation. Rousseau wondered whether a youth brought up ‘wild, untamable and free’ would be safe from the corruption faced by those who undergo a normal education. He pondered an ‘impossible experiment’: to raise a newborn infant in isolation, but as he wrote, ‘by our very study of man, the knowledge of him is put out of our power’ - nobody would be so cruel as to do such a thing. Such a child might, he thought, show how the true signals of inner sentiment emerge in a creature that had never received them.

The eighteenth century was a vintage era for ‘wild children’, those raised - metaphorically or otherwise - by wolves, in the fashion of Romulus and Remus. The naturalist Linnaeus classified them as Homo ferus - wild men - whose nature would reveal what made thinking humans, Homo sapiens, different. Most of the supposed examples were fakes, but a few were not.

In 1797, a young boy was found alone and almost naked in the forests of the Aveyron, in south-central France. He was captured, escaped, recaptured and escaped again, but in time he emerged from the woods under his own volition. He was about twelve years old, unable to speak and savage in his behaviour. A vicious scar on his throat hinted that his parents had tried, but failed, to kill their aggravating child. The lad appeared to have been without contact with others for almost his whole life and showed no obvious signs of joy, fear or gratitude when at last he met members of his own species. Here, perhaps, was an opportunity to investigate the springs of emotion.

A young student, Jean-Marc-Gaspard Itard, heard the story and saw the chance to test Rousseau’s ideas. He took the forlorn boy to Paris and set out to try to raise him to the spiritual level of his fellow citizens.

Itard had trained as a tradesman, but took up medicine at the time of the French Revolution and later became a pioneer in the study of diseases of the ear, nose and throat. In stark contrast to Rousseau he was convinced that the essence of the human condition lay in the ability to sense the feelings of others and, armed with that talent, to build a society in which passions could be kept in check for the good of all. In his ‘Historical Account of the Discovery and Education of a Savage Man’ he set out his theory that ‘MAN can find only in the bosom of society the eminent station that was destined for him in nature . . . that moral superiority which has been said to be natural to man, is merely the result of civilization’.

The doctor took young Victor - whom he named after one of the few sounds, ‘o’ (as in the French word for water), he was able to recognise - into his household and attempted to train him to express, and respond to, emotions. He was soon disappointed. The boy was ‘insensible to every species of moral affection, his discernment was never excited but by the stimulus of gluttony; his pleasure, an agreeable sensation of the organs of taste; his intelligence, a susceptibility of producing incoherent ideas, connected with his physical wants; in a word, his whole existence was a life purely animal’.

Itard laboured for five years with both kindness and cruelty (the latter based on his charge’s fear of heights) to transform the boy from monster into Frenchman, but with little success. Victor’s behaviour stayed strange: he was obsessed with the sound of cracking walnuts but ignored gunshots close to his ears, and loved to rock water back and forth in a cup. He never learned to speak and showed no gratitude for food or shelter. The sole sign he made of any response to the sentiments of others was that, when Itard’s housekeeper was in tears after the death of her husband, Victor appeared to comfort her. Apart from that he stayed apart from his fellow men.

His protector insisted that the young man’s failure to adapt to the inner worlds of those around him and to express his own feelings arose because he had been rescued too late to pick up the skills needed, but that view was too optimistic. The lad would nowadays be diagnosed as deeply autistic; as unable to respond to, or give, the signs - the smiles or frowns or conversations - that bind people to their parents, to their friends and to the community in which they live. The dire effects of the illness show how the expression of our own emotions and our response to those of others makes us what we are.

The term ‘autism’ was invented in the 1940s to describe a condition in which children fail to interact or to smile or express sentiments apart from anger and unhappiness. They speak with difficulty or not at all and are filled with obsessions about particular foods, places or clothes. About a third suffer from epilepsy. Three out of four of those with a grave form of the illness struggle to cope with society throughout their lives. Autism shades from the severe disturbance shown by Itard’s Wild Boy himself, through Asperger’s syndrome, in which the language problems are less marked, to general problems in the development of normal conduct. Often, the problem is noticed when parents become concerned by their child’s depression or rage. Some autists, once unkindly referred to as idiots savants, have remarkable talents in drawing or in particular mathematical tasks, but their gifts do no more than disguise their deeper problems. Once, the illness was said to be rare, with one child in two thousand affected, but now the diagnosis is made far more often, with an incidence of one in a hundred in Britain.

Autists cannot understand the signals of their fellows or make the full complement of their own. All children have that difficulty in their earliest years. As Darwin wrote in the Sketch of an Infant, ‘No one can have attended to very young children without being struck at the unabashed manner in which they fixedly stare without blinking their eyes at a new face; an old person can look in this manner only at an animal or inanimate object. This, I believe, is the result of young children not thinking in the least about themselves, and therefore not being in the least shy, though they are sometimes afraid of strangers.’ For most infants such self-absorption soon passes but an autistic child is locked into that phase for life. Many, when they look at other people, ignore the eyes, the flags of sentiment. They are just as unconcerned when someone else gazes long and hard at them.

The Expression of the Emotions used the blush as a prime example of a social cue but embarrassment plays a lesser part in life today. Yawns - unacceptable in a nineteenth-century parlour - are more frequent. We do not know why we open our mouths when tired or bored (although the book discusses the gesture as a threat in baboons). Yawn and the world yawns with you and even to read about it can spark the gesture off, as about half the readers of this book can now attest. The habit begins at about the age of six. Not, however, for children with autism, for a yawn sparks off far fewer responses among them than among the general population. Such failures of empathy lie behind many of their problems.

Psychologists talk of ‘theory of mind’, the ability to infer the mental state of others from smiles, frowns, gestures and speech. People with autism have little or no insight into the inner world of their fellows and cannot express their own internal universe in a way that makes much sense to those around them. They are blind to the messages written on another’s countenance and find it hard to separate gestures of anger, fear, sadness or joy. Like chimpanzees (but unlike dogs) some autistic children cannot understand what is meant when their parent or doctor points at an object. They are denied even that simple social talent.

Autists also find it harder to tell people apart or to recognise a photograph of themselves. A certain group of brain cells is activated when monkeys or men see or copy the movements of others or observe an expression of pain, fear or disgust. They are also involved in the shared response to a yawn or a smile. These mirror neurons, as they are called, are almost silent in children with the severe form of the disease. Perhaps they are part of the system that helps us see into the souls of those around us. In their failure they condemn people with autism to a world whose other denizens act in a mysterious and unpredictable way.

Nobody knows what causes autism and the condition has no cure, even if some of its symptoms such as insomnia or depression can be treated. The illness is four times more frequent in boys than girls, but shows no fit with race, social class or parental education. Infection, immune problems, vaccines, heavy metals, drug use while pregnant, Caesarean births and defective family structure in Freudian mode (the child psychologist Bruno Bettelheim spoke of ‘refrigerator mothers’) have all been blamed but those claims do not stand up. Some say that the brain of a typical autistic child grows too fast too soon, but then slows down. The amygdalae - those detectors of fear - are overactive in some patients, but many other parts of the brain have also been implicated. Problems with serotonin, that universal alibi for disorders of emotion, may be to blame, for some autistic children synthesise the stuff less well than normal. Certain drugs used against depression can help, as a further hint of a tie between social isolation and the emotional universe.

Genes are without doubt involved in some patients, even if not more than a tenth or so of cases can be ascribed to a definite genetic cause. If an identical twin has autism its sib is at a seven-in-ten risk while the figure risk for non-identicals is far lower. The incidence increases by twenty times above average in the brothers and sisters of those with autism and some among them are tactless, aloof or silent but are not diagnosed as ill.

Such behaviour sometimes presents itself as part of a larger medical problem. Fragile-X syndrome is the commonest cause of mental disability among boys. It comes from a huge multiplication of a short segment of DNA upon the X chromosome. Some patients have symptoms quite like those of autism and some individuals diagnosed with that condition may in fact have the chromosomal abnormality. Other deletions, duplications or reversals of a segment of chromosome are behind other cases of the illness. Often, these arise anew in the children compared with their parents. Some badly affected patients have problems with a gene involved in the transmission of impulses between nerves. A few may have errors elsewhere in the DNA - and dozens of genes, with a variety of tasks, have been blamed. One candidate belongs to a group of genes that is multiplied in number in humans compared with all other mammals, is active in the brain and is damaged in at least a few autistic children. In spite of such hints the biology of autism remains obscure and there are likely to be several explanations for a condition that is not a single disease but many.

Autistic children are an experiment in emotion. Their isolation is mental rather than physical, for they are cut off by an inability to respond to the flow of information that passes between others. A world full of autists could not function, for all societies depend on a silent dialogue in which every member’s intentions are overtly or otherwise expressed. Civilisation turns on the ability to bear another’s company.

Those who break civilisation’s rules must be punished; and part of that invariably involves the manipulation of a criminal’s mental state. Prisons are, of their nature, places in which social interactions are forcibly reduced. Solitary confinement is a penalty far more severe than mere imprisonment, for it is autism imposed: a permanent denial of what it means to be human, inflicted upon someone who once experienced the full range of human emotion. The penalty is bitter indeed and is much appealed to by punitive societies, from medieval England to the modern United States. Charles Dickens visited such a penitentiary in Philadelphia and wrote that ‘I hold this slow and daily tampering with the mysteries of the brain to be immeasurably worse than any torture of the body.’ The infamous ‘Supermax’ at Marion, Illinois, a jail built to hold the most violent offenders, together with political prisoners such as Black Panthers and members of the American Indian Movement, allowed almost nobody out of their cells for twenty years, even to exercise. It closed in 2007, but some of its forty and more replacements are just as brutal. Some even feed their inmates on tasteless ‘Nutraloaf ’ further to reduce their contact with the world of the senses. Many inmates - like autists - become anxious, agitated and angry, and may end in insanity, killing themselves should the chance arise.

If Zacarias Moussaoui, sentenced to life in solitary for his supposed ties to the Twin Towers outrage, were allowed reading material in his soundproofed Colorado cell he might learn something from both Dickens and Darwin about why he feels such hatred for those who do not share his views. As books are not available, he may wish instead to spend his solitary hours in contemplation of the expression of a condemned prisoner as the electricity passes through his head, which is said - in an echo of the great naturalist’s own observations - to be of ungovernable horror.