Expressive crowds are groups of people who come together primarily to share a common emotional experience or express feelings. Unlike acting crowds, which are focused on a specific goal or activity, expressive crowds prioritize the emotional release and collective participation in events like celebrations or mourning.

Urban legends are a form of modern folklore consisting of stories that are circulated as true, despite being unverified. They often involve mysterious or cautionary events and are passed down through social networks, reflecting the anxieties and values of the culture in which they circulate.

Sociologists classify crowds based on their purpose and level of organization. Casual crowds are loose collections of people, conventional crowds gather for scheduled events, and expressive crowds assemble to share emotions or experiences. All these types are standard classifications in collective behavior studies.

Mass hysteria, or collective obsessional behavior, refers to the rapid spread of illness-like symptoms or irrational beliefs through a population, often in the absence of a clear physical cause. Historically, these episodes are frequently attributed by participants to mysterious, supernatural, or external forces, which helps the group rationalize the shared, intense anxiety and physical manifestations experienced during the event.

Rumors are unverified accounts or explanations of events that circulate rapidly through informal social networks. They often emerge in situations of ambiguity or uncertainty where official information is lacking or distrusted, and they are characterized by their tendency to change as they are passed from person to person.

A casual crowd consists of people who are in the same place at the same time but have little to no interaction with one another. They are essentially a loose collection of individuals who happen to be in proximity, such as people waiting at a bus stop or walking through a public park, lacking a common goal.

The emergent norm perspective challenges the traditional view that crowds are inherently irrational, impulsive, or uncontrolled. Instead, it suggests that crowd behavior is guided by new norms that emerge within the specific situation, meaning participants act in ways they perceive as appropriate or logical given the context of the event.

A moral panic occurs when the mass media amplify and distort reports regarding specific groups or behaviors, labeling them as 'folk devils.' This process creates a disproportionate social reaction of hostility and fear within the public, often exceeding the actual threat posed by the group. Stanley Cohen's 1972 study of mods and rockers remains the foundational example of how media narratives construct these societal anxieties.

In classical crowd psychology, crowds are often characterized by deindividualization (loss of self-awareness), a sense of invulnerability, and high suggestibility. 'Transformation' is not a standard characteristic used to describe the psychological state of a crowd. These traits explain why individuals in a crowd might act differently than they would when acting alone, often leading to impulsive or irrational collective behavior.

Contagion theory, notably associated with Gustave Le Bon, posited that individuals in a crowd lose their sense of self and are swept up by a collective 'mental contagion,' leading to irrational and impulsive behaviors that they would not exhibit alone.