A theory developed by William James and Carl Lange in the 19th century, this hypothesis explores the idea that emotion does NOT precede physiological changes. Instead, a person experiences an event, has an instinctual physiological response to the event, and the brain, sensing the physiological change through the nervous system, prompts an emotion.

For example: A person sees a bear. Their heart rate rises, their perspiration increases, and their mouth becomes dry. Based on these responses, the brain decides fear is an appropriate emotion for the environmental stimuli.

The reasoning behind this theory is that emotions help a person figure out the best course of action. In this example, the brain picks fear as the emotion, and the person picks "flight" as course of action in order to get away from the source of fear. 

The Schachter-Singer Theory, also known as the Two-Factor Theory, is a cognitive theory of emotion. It states that physiological arousal occurs, and then one must appraise the reason for that arousal and assign an emotion to it.

Achievement motivation is a social motivation wherein we are driven to achieve complex goals and attain knowledge or skils. This need to achieve varies and is inconsistent amongst people. 

Unlike the James-Lange theory of emotion, the Cannon-Bard theory indicates that a specific physiological state and cognitive awareness occur simultaneously.

The James-Lange theory of emotion suggests that our emotional experiences occur because we are aware of our physiological responses to emotionally-charged stimuli. For example, if we witness an emotional moment in a movie and begin to feel our eyes tear up and our cheeks get hot, our brain processes this physiological state and determines that we are feeling sad.

Selye's outline of the GAS holds that the first stage is alarm reaction, in which the body readies itself by activating the sympathetic nervous system. The second stage is resistance, in which the body remains in a state of readiness, but gradually becomes weaker. The third, and final, stage is exhaustion, in which the parasympathetic nervous system returns the body to a normal state.

Charles Darwin posited that emotions evolved because they had an adaptive value to the human species. For instance, fear allows us to act in ways that avoids a potential danger, which makes it a very useful emotion for our basic survival. This core idea is generally accepted by evolutionary psychologists who have since expanded the tenets of this claim.

Amongst evolutionary psychologists who have expanded on Charles Darwin's theory of natural selection, it is generally accepted there are seven core emotions that have evolved due to their adaptive value. These are contempt, happiness, anger, surprise, sadness, disgust, and fear. They argue that any other emotion is a blend of these core emotions, or a variation of intensity within these emotions basic emotions (for example, feeling ecstatic would not be another basic emotion, but rather a variation in the intensity of happiness).

The James-Lange theory of emotion posits that emotions reflect physiological states in the body. The James-Lange theory holds that human bodies FIRST experience physical sensations, and that humans will think, act, then feel afterwards. One of the Cannon-Bard's counter-arguments to this is that one can experience an emotion simultaneously with a physical reaction and prior to an action. For instance, I see a bear holding a knife, I feel fear (and possibly bewilderment) and my body tenses up immediately, then I run away. By the time I am running away, I have definitely already felt afraid of the knife-wielding bear.

The cognitive appraisal theory holds that one's emotional experience depends on one's interpretation of the situation. This was the first theory of emotion to consider the person's appraisal of a situation, in addition to physiology and behavior.