Kissing on the lips is a physical expression of affection or love between two people in which the sensations of touch, taste, and smell are involved. According to the psychologist Menachem Brayer, although many “mammals, birds, and insects exchange caresses” which appear to be kisses of affection, they are not kisses in the human sense. The psychologist William Cane notes that kissing in Western society is most often a romantic act and describes a few of its attributes.

The first romantic kiss on screen was in American silent films in 1896, beginning with the film The Kiss. The kiss lasted 30 seconds and caused many to rail against decadence in the new medium of silent film. Writer Louis Black writes that “it was the United States that brought kissing out of the Dark Ages.” However, it met with severe disapproval by defenders of public morality, especially in New York. One critic proclaimed that “it is absolutely disgusting.

Young moviegoers began emulating romantic stars on the screen, such as Ronald Colman and Rudolph Valentino, the latter known for ending his passionate scenes with a kiss. Valentino also began his romantic scenes with women by kissing her hand, traveling up her arm, and then kissing her on the back of her neck. Female actress were often turned into stars based on their screen portrayals of passion.

Kissing is a complex behavior that requires significant muscular coordination involving a total of 34 facial muscles and 112 postural muscles. The most important muscle involved is the orbicularis oris muscle, which is used to pucker the lips and informally known as the kissing muscle. In the case of the French kiss, the tongue is also an important component. Lips have many nerve endings which make them sensitive to touch and bite.

Kissing can also cause the adrenal glands to release epinephrine and norepinephrine (adrenaline and noradrenaline) into the blood, thereby causing an adrenaline rush, which has a beneficial impact on the cardiovascular system because the heart pumps faster. In an experiment by Dr. Alexander DeWees, a passionate kiss generally burns up to 2–3 calories per minute.