(Pincha aquí para leer una entrada relacionada con esta en español)
In Freudian psychoanalytic theory, defense mechanisms are unconscious strategies brought into play to cope with reality and to maintain self-image and self-esteem. Healthy persons normally use different defenses throughout life. A defense mechanism becomes pathological only when its persistent use leads to maladaptive behavior and the individual is adversely affected.
The purpose of the Defense Mechanisms is to protect the mind and the self from anxiety, social sanctions or to provide a refuge from a situation with which one cannot currently cope.
In Freudian psychoanalytic theory, defense mechanisms are unconscious strategies brought into play to cope with reality and to maintain self-image and self-esteem. Healthy persons normally use different defenses throughout life. A defense mechanism becomes pathological only when its persistent use leads to maladaptive behavior and the individual is adversely affected.
The purpose of the Defense Mechanisms is to protect the mind and the self from anxiety, social sanctions or to provide a refuge from a situation with which one cannot currently cope.

Why does this happen? Why are some people consumed with intense, irrational suspicions that their lovers are unfaithful?
Social psychology has a way to explain this, a theory that would also explain why gay people are vilified and attacked by many heterosexuals, or why people would form stereotypes of groups whose members they scarcely know, such Mexicans in the US or South Americans in Spain.

Here is a personal example to see how projection works.