Robin Lloyd in LiveScience.com:
It boils down to the shifting sands of the self and trying to look good both to ourselves and others, experts say.
“It’s tied in with self-esteem,” says University of Massachusetts psychologist Robert Feldman. “We find that as soon as people feel that their self-esteem is threatened, they immediately begin to lie at higher levels.”
Not all lies are harmful. In fact, sometimes lying is the best approach for protecting privacy and ourselves and others from malice, some researchers say. Some deception, such as boasting and lies in the name of tact and politeness, can be classified as less than serious. But bald-faced lies (whether they involve leaving out the truth or putting in something false), are harmful, as they corrode trust and intimacy—the glue of society.
Many animals engage in deception, or deliberately misleading another, but only humans are wired to deceive both themselves and others, researchers say. People are so engaged in managing how others perceive them that they are often unable to separate truth from fiction in their own minds, Feldman’s research shows.