People do not have access to or control over the recipe of their impressions, feelings, and attitudes. There exists a vast, adaptive unconscious that does much of the work of sizing up and evaluating the world (Nisbett & Wilson, 1977b; Wilson, 2001); people often know the final product with no awareness of exactly how it was formed. Consider Ms. Green, a partner in a prestigious law firm, who is interviewing candidates for the position of associate in her firm. When she interviews Mr. Jones, a young African-American attorney, she has an immediate negative impression, finding him to be arrogant and lacking the kind of brilliance she looks for in new associates. Ms. Green decides that her impression of Mr. Jones was accurate and at a meeting of the partners, argues against hiring him. She wonders, however, whether her negative evaluation was influenced by Mr. Jones' race.

Ms. Green may have suffered from mental contamination, which Wilson and Brekke (1994) defined as unconscious or uncontrollable mental processing that results in unwanted judgments, emotions, or behavior. Her dilemma illustrates the difficulty of knowing whether one's responses are contaminated, and if so, how to correct them. In this chapter, we review and update the arguments made by Wilson and Brekke (1994) concerning mental contamination, and also address several questions about the ways in which people try to protect their minds from unwanted influences.