Motivation is a need or desire that energizes and directs behavior.

Drive-reduction theory: We feel motivated when pushed by a physiological need to reduce a drive (such as thirst), or when pulled by an incentive in our environment (an ice-cold drink). Drive-reduction’s goal is homeostasis, maintaining a steady internal state.

Arousal theory: We also feel motivated to behave in ways that maintain arousal (for example, curiosity-driven behaviors). The Yerkes-Dodson law describes the relationship between arousal and performance.

Maslow’s hierarchy of needs: Our levels of motivation form a pyramid of human needs, from basic needs such as hunger and thirst up to higher-level needs such as self-actualization and self-transcendence.

Hunger’s pangs correspond to the stomach’s contractions, but hunger also has other causes. Neural areas in the brain, some within the hypothalamus, monitor blood chemistry (including level of glucose) and incoming information about the body’s state.

Appetite hormones include ghrelin (secreted by an empty stomach); insulin (controls blood glucose); leptin (secreted by fat cells); orexin (secreted by the hypothalamus); and PYY (secreted by the digestive tract).

Basal metabolic rate is the body’s resting rate of energy output. The body may have a set point (a biologically fixed tendency to maintain an optimum weight) or a looser settling point (which is also influenced by the environment).

Hunger also reflects our memory of when we last ate and our expectation of when we should eat again.

Humans as a species prefer certain tastes (such as sweet and salty), but our individual preferences are also influenced by learning, culture, and situation. Some taste preferences, such as the avoidance of new foods, or of foods that have made us ill, have survival value.

Genes and environment interact to produce obesity.

Twin and adoption studies indicate that body weight is genetically influenced. Environmental influences include too little sleep and exercise, an abundance of high-calorie food, and social influence.

Those wishing to lose weight are advised to make a lifelong change in habits: Begin only if you feel motivated and self-disciplined; exercise and get enough sleep; minimize exposure to tempting food cues; limit variety and eat healthy foods; reduce portion sizes; space meals throughout the day; beware of the binge; plan eating to help monitor yourself during social events; forgive the occasional lapse; and connect to a support group.

Social bonds are adaptive and help us to be healthier and happier. Feeling loved activates brain regions associated with rewards and satisfaction.

Ostracism is the deliberate exclusion of individuals or groups. Social isolation can put us at risk mentally and physically.

We connect with others through social networking, strengthening our relationships with those we already know. When networking, people tend toward increased self-disclosure. People with high narcissism are especially active on social networking sites.

Working out strategies for self-control and disciplined usage can help people maintain a healthy balance between their real-world and online time.

Emotions are psychological responses of the whole organism involving bodily arousal, expressive behaviors, and conscious experience.

James-Lange theory: Emotional feelings follow our body’s response to the emotion-arousing stimuli. (We observe our heart pounding and feel fear.)

Cannon-Bard theory: Our body responds to emotion at the same time that we experience that emotion. (Neither causes the other.)

Schachter-Singer two-factor theory: Emotions have two ingredients, physical arousal and a cognitive label, and the cognitive labels we put on our states of arousal are an essential ingredient of emotion.

Richard Lazarus agreed that many important emotions arise from our interpretations or inferences. But Robert Zajonc and Joseph LeDoux have contended that some simple emotional responses occur instantly, not only outside our conscious awareness, but before any cognitive processing occurs. This interplay between emotion and cognition illustrates our two-track mind.

Carroll Izard’s basic emotions are joy, interest-excitement, surprise, sadness, anger, disgust, contempt, fear, shame, and guilt.

The arousal component of emotion is regulated by the autonomic nervous system’s sympathetic (arousing) and parasympathetic (calming) divisions.

In a crisis, the fight-or-flight response automatically mobilizes your body for action.

The large-scale body changes that accompany sexual arousal, fear, and anger are very similar (increased perspiration, breathing, and heart rate), though they feel different. Emotions may be similarly arousing, but some subtle physiological responses (such as facial muscle movements) distinguish them.

Emotions use different circuits in the brain. For example, greater activity in the left frontal lobe signals positive rather than negative moods.

Polygraphs (lie detectors) attempt to measure physical evidence of emotions; they are not accurate enough to justify widespread use in business and law enforcement.

The use of guilty knowledge questions and new forms of technology may produce better indications of lying.

We are good at detecting emotions from body movements, facial expressions, and voice tones. Even seconds-long video clips of behavior can reveal feelings.

Women tend to read emotional cues more easily and to be more empathic. Their faces also express more emotion.

The meaning of gestures varies by culture.

Facial expressions, such as those of happiness and sadness, are roughly similar all over the world.

Research on the facial feedback effect shows that our facial expressions can trigger emotional feelings and signal our body to respond accordingly. We also mimic others’ expressions, which helps us empathize.