In European folklore and folk-belief of the Medieval and Early Modern periods, familiar spirits (sometimes referred to simply as "familiars" or "animal guides") were believed to be supernatural entities that would assist witches and cunning folk in their practice of magic. According to the records of the time, they would appear in numerous guises, often as an animal, but also at times as a human or humanoid figure, and were described as "clearly defined, three-dimensional… forms, vivid with colour and animated with movement and sound" by those alleging to have come into contact with them, unlike later descriptions of ghosts with their "smoky, undefined form[s]".
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Yet instances of selfish behavior also abound in society. One recent study used a version of the classic Prisoner's Dilemma, which can test people's willingness to set aside selfish interests to reach a greater good. After modeling different strategies and outcomes, the researchers found that being selfish was more advantageous than cooperating. The benefit may be short-lived, however. Another study showed that players who cooperated did better in the long run.