One of the exhibits in the Mütter Museum in Philadelphia that is guaranteed to make you scrunch up your face in disgust is a bust of the unfortunate Madame Dimanche, who, aged 76, started growing a horn from her forehead. By the time she was 82, it was 10 inches long and curved down to below her chin. The bust is both disgusting and fascinating - precisely the reason it is in a museum that specialises in anatomical and pathological specimens.
But why do we find it so repellent? And why do we get a thrill from things that disgust us? After all, there is nothing upsetting about horns, per se. As a mark of how curious this emotion can be, think of hair. A long head of hair that hangs down the back is beautiful and admired, but a long strand of hair growing out of the cheek would be weird and disgusting. In the 19th century people flocked to freak shows, and today we relish the sight of celebrities covered with flies or locked in boxes full of cockroaches.
The key to what's going on lies with some of the surprising ways that the emotion of disgust works, and in understanding what it is for. The idea that emotions have a clear and discernible purpose is a relatively new and powerful idea. Thinkers from the ancient Greeks to Freud and the behaviourists of the last century saw emotions as messy, dangerous things that needed taming and controlling to allow the higher faculties of reason and planning to function properly.
But a new approach turns that on its head. Far from being internal devils to be wrestled with, emotions are now seen as potential helpers; a sort of built-in software that is designed to improve our survival chances. Rather than blotting out the light of reason, recent research has shown that when links between the emotional and rational centres in the brain are damaged, it is almost impossible to make any decisions.
The purpose of each emotion and their individual quirks are the subject of a new book by the broadcaster Claudia Hammond. Emotional Rollercoaster: A Journey Through the Science of Feelings is an entertaining, informed guide to the responses that drive and colour our lives.
For most of us, our emotions behave like unruly beasts that can suddenly plunge us into sadness or trigger a flash of anger, but realising why can help us to control them. Hammond, in the manner of a 19th-century naturalist, tracks down these sometimes unstable companions and identifies their habits. She shows how they provide a "ready repertoire of actions", and how we can make them work in our favour.
The precise emotion of disgust, for instance, provides an instant response to rotting or bad food, and this is reflected in the area of the brain devoted to it. Unlike most other emotions, disgust has two different control centres: the "insula", which also responds to strong flavours, like salt, and the basal ganglia.
These disgust centres fire up when faced with body products - snot, vomit, faeces - especially when they might reach our mouths. This is a warning device against something just as deadly as rotting meat: the risk of infectious diseases. But it's possible to damp down the disgust response to body fluids - both sexual attraction and newborn babies moderate it.
But just when you think you've got disgust wrapped up, Hammond points out that warding off food poisoning and disease can't be the whole story, which brings us back to Madame Dimanche. Why does she provoke a shiver of disgust with no body fluids in sight?
The answer might be found in another, and rather surprising, way of defining disgust - it is a response to food and body products that are out of place. So, the rotting carcass of a mouse in the fields is not seen as disgusting, but one placed on your pillow is. Something becomes disgusting because it provokes contradictions. Vomit is disgusting because it is something that belongs inside but has come out, and the horn is disgusting because horns don't belong there. The entertainment value of disgust is like that of a rollercoaster - it allows you to experience an emotion designed to warn of something dangerous in a safe environment.
But like all emotions, disgust has a pathological side. When you can't switch it off or it's triggered too easily, it can result in an obsessive compulsive disorder (OCD), just as anxiety is the result of an over-active fear response. A rough-and-ready reaction to things being out of place is triggered inappropriately. Every day, OCD sufferers are filled with feelings of disgust that most of us experience only occasionally; feelings that drive many to wash endlessly at just the possibility that they touched something unclean. And often it's not just dirt that troubles them. Recently, scans have revealed a high level of activity in the insula region of the brain when these patients are shown pictures of "ordinary domestic untidiness", like an unmade bed.
Disgust seems to be a self-contained emotion, but it shares the vital functions of being an unmissable signal, and a ready response package, with most of the nine other emotions that Hammond explores. You know when someone has eaten something disgusting, just as there is no mistaking anger or sadness.
In the same way that disgust warns you to think twice before trying that chunk of meat, so anger sends a clear signal that you have stepped over some boundary, making it valuable for survival in a group. Tears can have a useful social role, too - they signal that you have come to the end of your resources. One theory explored by Hammond is that, among our ancestors, those failing to contribute to the group might have been thrown out had they not asked for help with their tears.
But emotional signals are no good if they can't be read, and the system is prone to error. Although OCD patients find it difficult to recognisedisgust on the faces of others, as a general rule we are better at recognising emotions that reflect how we are feeling at a particular time.
When you're depressed, you're better at spotting that look on the face of someone else, but when you're are on top of the world, you respond more quickly to a happy look. Anger is usually unmistakable, but if you interfere with the dopamine system in someone's brain, he or she will find anger very hard to recognise. Sensitivity to how other people are feeling is a vital social skill, and a better understanding of how emotional signalling works could help in treating various disorders.
Although all emotions have strong default settings - most people jump when faced with a snake or feel a buzz of excitement at the sight of an attractive person - we can learn to make them work for us. Anger, for instance, readies you for attack: no other emotion can keep you charged with energy for such long periods. But even in social circles where fighting is unacceptable, anger can be useful as a signal that something is wrong. Rather than immediately lashing out, you can start to ask what's triggering it, and use the energy to take steps to change it.
This suggests that the notion that anger is not to be bottled up is missing the point. Not only does immediately sounding off make it unlikely you will notice the underlying issues, but there's now evidence that regular angry outbursts triple your risk of heart attack by increases levels of fatty acids and cortisol in your blood. Hard as it may be to avoid, responding angrily to a child having a tantrum is particularly unhelpful. It's more likely to produce a chronically angry adult.
These new emotional insights can shape our thinking. While anger largely convinces you that you're right, joy can make you more creative and flexible. You'll see more possibilities if you bring a happy frame of mind to the negotiating table and you'd be wise to wait for a joyful moment before asking for a pay rise.
But while anger is hard to miss, happiness turns out to be surprisingly shy. Most people, when asked about their emotions during the previous week, will recall anger, anxiety or sadness, and only fleeting moments of happiness. But a study in which researchers paged people through the day and asked, "What are you feeling right now?" revealed that happiness was twice as common compared with those asked about their previous week.
Happiness is worth noting because it is closely allied to one of our most valuable emotions - hope, which is notable for having no associated facial expression. Without the feeling of hope that the future will be brighter, why do anything at all? People who believe that they are lucky score high on hope, and such a belief is a self-fulfilling prophecy. Research at Hereford University has shown that people who learn to be more hopeful also become luckier. Far from being messy add-ons to the life of the mind, emotions are essential tools for our survival.
WHY THE TEARS?
* Like most animals, we need constantly to produce tears - known as basal - to keep our eyes lubricated.
* We also produce two other sorts: reflex, which come when you peel onions, for example; and emotional. A few years ago, researchers found that emotional tears were packed with chemicals linked with depression and anxiety, apparently supporting the popular idea that crying was a valuable release.
* Since then, researchers have found that although people say they feel better afterwards, they don't behave any differently.
* Generally, people are most likely to cry emotional tears between 6pm and midnight. Women cry most between 7pm and 10pm, most probably because that's when they are with their partners.
* The female hormone prolactin may encourage women to cry on average five times more than men, whiletestosterone may prevent men from crying.
'Emotional Rollercoaster: A Journey Through the Science of Feelings', Claudia Hammond (Fourth Estate £15.99)
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