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Everyone in this scene is witnessing the same event. A pivotal hand in a high stakes game of cards. However, their emotional experience is all over the spectrum. Emotion is a complex psychological state that involves three distinct components— a subjective experience, a physiological response, and a behavioral, or expressive, response. But what exactly is emotion?

Emotion is a primitive guidance system. It tells us, as well as any other animal that experiences it, which way to go. Whether to approach or to avoid. Whether to go left or to go right. It tells us what ends we should be seeking, and it tells us when we've found them.

One of the earliest scientists to systematically study emotions was Charles Darwin. Darwin argued that emotions reflect evolutionary adaptations to the problems of survival and reproduction. Like Darwin, today's evolutionary psychologists believe emotions help us to solve adaptive problems posed by our environment. Emotional displays serve the important function of informing other organisms about an individual's internal state. Our emotional experience and expression, as well as our ability to understand the emotions of others, are crucial to the maintenance of social relationships.

Well, Darwin did for emotion what he did for everything else in psychology, which is, he taught us that we are one animal among many. And the things we experience and things we do have reverberations in the rest of the animal kingdom.

Most emotion researchers today agree that there are a limited number of basic emotions that all humans in every culture experience. They have amassed considerable evidence that each basic emotion represents a sequence of responses that is innate, and that facial expressions for the basic emotions are hard wired into the brain. Your emotional experience is not limited to pure forms of each basic emotion. Rather, each basic emotion represents a family of related emotional states. For example, consider the many types of angry feelings which can range from mild annoyance to bitter resentment or fierce rage.

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Psychologists don't agree about every emotion, but every human culture shows six basic emotions— happiness, fear, sadness, anger, surprise, and disgust.

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Just like every city on a map can be located by its longitude and latitude, every emotion can be located in a two-dimensional space made up of arousal and valence. From active and passive to negative and positive. Human beings broadcast their emotions with extremely subtle cues— micro expressions in the face.

But the subtlety with which we transmit emotions is equalled by the skill we have in decoding them. So all I have to do is move my eye just a little bit— just a tiny bit— to tell you I'm suspicious, and you immediately pick up what I'm telling you. It appears as though these two dimensions underlying emotions are themselves universal. In every culture we've studied so far, arousal and valence seem to capture majority of the variants in our discussion of emotion.

Our shared heritage may be why all humans express the basic emotions with similar facial expressions. A sneer, for example, retains elements of an animal baring its teeth in a snarl. Emotional expressions may enhance our survival in other ways, too. Surprise raises the eyebrows and widens the eyes, enabling us to take in more information. Disgust wrinkles the nose, closing it from foul odors. Although emotions like extreme joy, fear, and grief objectively feel very different, they all involve increases in heart rate, breathing, and blood pressure.

What are the physiological mechanisms behind extreme emotions? The sympathetic branch of the autonomic nervous system triggers the fight or flight response. It directs the adrenal glands to release key hormones that cue the liver to provide energy by pouring extra sugar into your bloodstream. To help burn the sugar, your respiration increases to supply needed oxygen. Your heart rate and blood pressure increase. Your digestion slows, diverting blood from your internal organs to your muscles.

With blood sugar driven into the large muscles, running becomes easier. Your pupils dilate, letting in more light. To cool your stirred up body, you perspire. If wounded, your blood could clot more quickly. When the crisis passes, the parasympathetic division of your autonomic nervous system takes over, calming your body.

Emotion is something that we seem to share with every other mammal. So it's not surprising that it seems, by and large, to be generated by parts of our brain that we also share with other animals— those structures deep inside the brain— the subcortical structures, the amygdala, the limbic system.

Of all the emotions, the fear response has been one of the most studied. Compared with observers watching angry faces, those watching fearful faces show more activity in their amygdala, the emotional control center in the brain's limbic system.

Brain scans and EEG recordings show that the involvement of other brain areas differs depending on the emotion being read or expressed. There is some tendency for negative emotions to be associated with the right hemisphere and positive ones to the left. Disgust, for example, triggers more activity in the right prefrontal cortex than the left.

Other human beings are the primary source of rewards and punishments in our environment. And our ability to predict them— to control them— rests on our ability to know what they're thinking, what they're feeling, and what they're planning. The fact that I can read your emotions from subtle features of your face gives me a great evolutionary edge in my attempts to control you, in my attempts to use you, in my attempts to mate with you.

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