In 1884, William James argued that we feel sorry because we cry, angry because we strike and afraid because we tremble, not the other way round. The James-Lange theory said that emotions are feelings made from perceptions of changes in physiological conditions. These relate to autonomic and motor functions. Scientists think emotions are made up of several parts that work together, involving both the brain and the body and how they interact. Anger and fear can both make a person shaky and make the heart beat faster, so different emotions can feel alike in the body. The experiential part of an emotion, what we feel in our bodies, is the only part we really notice. The other parts happen so fast that we do not realize they are happening.

Charles Darwin wrote a book about how people and animals show emotions, first published in 1872. You may have heard that Darwin said every facial expression is the same in every culture. In fact, he claimed only that a specific set of expressions he had observed and studied is universal. In the late 1960s, two researchers separately showed photographs of facial expressions to people in various cultures. They found strong cross-cultural agreement in how the expressions were labeled. People show their feelings with their faces and bodies, and a person who feels happy often smiles.

Appraisal theories say that emotions differ from one another because of how a person evaluates the significance of a situation. Two people can be in the same situation and feel different emotions because they have different thoughts about it. Feelings are often caused by what a person thinks about a situation. Thinking a situation is dangerous tends to bring fear, and thinking it is good tends to bring joy or happiness. Reappraisal means trying to change your thoughts about a situation, which can change how you feel.

The amygdala is a tiny part of the brain, the shape and size of an almond, that handles both positive and negative information and is especially important in fear. Scientists see a strong amygdala response to emotional photographs even when the pictures are shown too quickly for a person to see them. One path from the senses leads quickly to the amygdala, so the brain can make a rapid emotional assessment before detailed reasoning occurs. When the brain perceives a threat, the amygdala activates the sympathetic nervous system, and stress hormones such as adrenaline and cortisol are released. They raise heart rate and speed breathing.

Brain imaging lets scientists test what happens when people name a feeling. The UCLA labeling study scanned 30 people, aged 18 to 36, while they viewed pictures of faces making different emotional expressions. In 2007 the study found that the amygdala was less active when people labeled the emotion of a face. It also found that labeling a face with an emotion word such as "angry" reduced the amygdala response, but labeling it with a name such as "Harry" did not. Some people find it harder than others to work out what emotion they are feeling, and people who struggle to understand their emotions also find it harder to make themselves feel better.

Tears are another puzzle. One scientist proposed that crying flushes out chemicals present during strong feelings, but not all scientists agree. Some scientists doubt that crying flushes out enough hormones to explain relief. Even a long bout of sobbing produces only a thimble full of hormone-laden tears. Some studies suggest we cry not because we are upset but because we are trying to get over being upset, so crying may reset the body toward calm. There are three kinds of tears. Basal tears bathe the eyes with each blink, reflex tears well up when onion fumes irritate the eyes, and emotional tears have a makeup all their own.

Scientists have also tested what helps the body calm down. In a Stanford study, one minute of guided slow breathing lowered the heart rates of children about seven years old. Heart rate changed only for children who watched the deep-breathing video. The control group, the children who did not watch it, showed no change. Breathing in through the nose and out through the mouth is a way to calm down, and one exercise is to let the breath flow deep into the belly without forcing it. The parasympathetic nervous system, sometimes called the rest-and-digest system, slows heart rate, supports digestion and restores the body to balance. After danger has passed, it helps the body settle back to calm.

What are emotions for? Some researchers say emotions have adaptive value because they quickly mobilize the resources needed for life tasks and communicate socially useful information through bodily expressions. Researchers have argued that sadness may serve useful purposes, including encouraging reflection and signaling the need for social support. Neuroscience has shown that emotion is not an obstacle to reasoning but a necessary part of effective decision making, providing information for judging risk and predicting consequences. Moderate emotional arousal tends to improve memory formation, while extremely high levels of stress may impair it. Some scientists think emotions make us act without thinking, while others, like Aristotle and Hume, think emotions are an important part of thinking and making good decisions.

Words to know

• universal: the same everywhere
• appraisal: sizing up
• reappraisal: rethinking a situation
• amygdala: the brain's alarm
• arousal: body activation