The words gullible and credulous are commonly used as synonyms. Goepp & Kay (1984) state that while both words mean "unduly trusting or confiding", gullibility stresses being duped or made a fool of, suggesting a lack of intelligence, whereas credulity stresses uncritically forming beliefs, suggesting a lack of skepticism. Jewell (2006) states the difference is a matter of degree: the gullible are "the easiest to deceive", while the credulous are "a little too quick to believe something, but they usually aren't stupid enough to act on it."
Yamagishi, Kikuchi & Kosugi (1999) characterize a gullible person as one who is both credulous and naïve. Greenspan (2009) stresses the distinction that gullibility involves an action in addition to a belief, and there is a cause-effect relationship between the two states: "gullible outcomes typically come about through the exploitation of a victim's credulity.