A rumor (American English) or rumour (British English; see spelling differences) is "a tall tale of explanations of events

circulating from person to person and pertaining to an object, event, or issue in public concern."
In the social sciences, a rumor involves some kind of a statement whose veracity is not quickly or ever confirmed. In addition, some scholars have identified rumor as a subset of propaganda. Sociology, psychology, and communication studies have widely varying definitions of rumor.
Rumors are also often discussed with regard to "misinformation" and "disinformation" (the former often seen as simply false and the latter seen as deliberately false, though usually from a government source given to the media or a foreign government). Rumors thus have often been viewed as particular forms of other communication concepts.
French and German social science research on rumor locates the modern scholarly definition of it to the pioneering work of the German William Stern in 1902 Stern experimented on rumor involving a "chain of subjects" who passed a story from "mouth to ear" without the right to repeat or explain it. He found that the story was shortened and changed by the time it reached the end of the chain. His student was another pioneer in the field The experiment is similar to the children's game Chinese whispers.
"A Psychology of Rumor" was published by Robert Knapp in 1944, in which he reports on his analysis of over one thousand rumors during World War II that were printed in the Boston Herald's "Rumor Clinic" Column. He defines rumor as
a proposition for belief of topical reference disseminated without official verification. So formidably defined, rumor is but a special case of informal social communications, including myth, legend, and current humor. From myth and legend it is distinguished by its emphasis on the topical. Where humor is designed to provoke laughter, rumor begs for belief.
Knapp identified three basic characteristics that apply to rumor: 1. they're transmitted by word of mouth; 2. they provide "information" about a "person, happening, or condition"; and 3. they express and gratify "the emotional needs of the community." Crucial to this definition and its characteristics is the emphasis on transmission (word of mouth, which then was heard and reported in the newspaper); on content ("topical" means that it can somehow be distinguished from trivial and private subjects—its domain is public issues); and on reception ("emotional needs of the community" suggests that though it is received by an individual from an individual, it is not comprehended in individual but community or social terms).
Based on his study of the newspaper column, Knapp divided those rumors into three types:
1. Pipe dream rumors: reflect public desires and wished-for outcomes.(e.g. Japan's oil reserves were low and thus World War II would soon end.)
2. Bogie or fear rumors reflect feared outcomes.(e.g. An enemy surprise attack is imminent).
3. Wedge-driving rumors intend to undermine group loyalty or interpersonal relations (e.g. American Catholics were seeking to avoid the draft; German-Americans, Italian-Americans, Japanese-Americans were not loyal to the American side).
Knapp also found that negative rumors were more likely to be disseminated than positive rumors. These types also differentiate between positive (pipe dream) and negative (bogie and wedge-driving) rumors.
In the 1947 study, Psychology of Rumor, Gordon Allport and Joseph Postman concluded that, "as rumor travels it grows shorter, more concise, more easily grasped and told." This conclusion was based on a test of message diffusion between persons, which found that about 70% of details in a message were lost in the first 5-6 mouth-to-mouth transmissions.
In the experiment, a test subject was shown an illustration and given time to look it over. They were then asked to describe the scene from memory to a second test subject. This second test subject was then asked to describe the scene to a third, and so forth and so on. Each person’s reproduction was recorded. This process was repeated with different illustrations with very different settings and contents.
Allport and Postman used three terms to describe the movement of rumor. They are: leveling, sharpening, and assimilation. Leveling refers to the loss of detail during the transmission process; sharpening to the selection of certain details of which to transmit; and assimilation to a distortion in the transmission of information as a result of subconscious motivations.