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.

The evolutionary theory of emotion believes that emotions developed in order to help facilitate human survival, in order to allow for reproduction. An evolutionary theory looks at emotions in the context of an evolutionary framework.

The physical muscles of the face are connected to perception of emotion, and placing one's face into a "smile" by biting a pencil can lead people to perceive happiness. The key to this question was understanding the terminology used in the question. In this case, the "physical symptoms" are actually positive, they are the muscles of the face being stimulated into the shape of a smile.

The James-Lange Theory is based on the idea that physical changes elicit emotion. William James and Carl Lange developed the theory independently, but formulated a cohesive theory. The James-Lange Theory, in its initial formation, held that if physical sensation were completely removed emotional response would also be removed. Modern theorists have since revised the theory to soften this stance, and hold rather that physical sensation significantly affects emotional response, but the relationship is not total. Support for this comes from studies of spinal cord injuries in which emotions are experienced less intensely.

In Schachter's Two-Factor Theory of emotion, the two factors are physiological arousal and cognitive label. First a person will experience a physiological sensation, and will then try to place a label on the context surrounding this response, in an effort to explain it. According to Schachter both factors are needed, and are mutually reinforcing. 

Schacter-Singer's two-factor theory of emotion poses that in order for an emotion to be considered as such, a person must experience physical arousal and be able to cognitively recognize and label this.