defense mechanism, in psychoanalysis, any of a variety of unconscious personality reactions which the ego uses to protect the conscious mind from threatening feelings and perceptions. Sigmund Freud first used defense as a psychoanalytic term (1894), but he did not break the notion into categories, viewing it as a singular phenomenon of repression. His daughter, Anna Freud, expanded on his theories in the 1930s, distinguishing some of the major defense mechanisms recognized today. Primary defense mechanisms include repression and denial, which serve to prevent unacceptable ideas or impulses from entering the conscience. Secondary defense mechanisms—generally appearing as an outgrowth of the primary defense mechanisms—include projection, reaction formation, displacement, sublimation, and isolation.
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