The new issue of Scientific American Mind has an interesting article about why we lie to each other, and to ourselves:
We lie by omission and through the subtleties of spin. We engage in myriad forms of nonverbal deception, too: we use makeup, hairpieces, cosmetic surgery, clothing and other forms of adornment to disguise our true appearance, and we apply artificial fragrances to misrepresent our body odors. We cry crocodile tears, fake orgasms and flash phony "have a nice day" smiles. Out-and-out verbal lies are just a small part of the vast tapestry of human deceit.
The obvious question raised by all of this accounting is: Why do we lie so readily? The answer: because it works. The Homo sapiens who are best able to lie have an edge over their counterparts in a relentless struggle for the reproductive success that drives the engine of evolution. As humans, we must fit into a close-knit social system to succeed, yet our primary aim is still to look out for ourselves above all others. Lying helps. And lying to ourselves–a talent built into our brains–helps us accept our fraudulent behavior.
If this bald truth makes any one of us feel uncomfortable, we can take some solace in knowing we are not the only species to exploit the lie. Plants and animals communicate with one another by sounds, ritualistic displays, colors, airborne chemicals and other methods, and biologists once naively assumed that the sole function of these communication systems was to transmit accurate information. But the more we have learned, the more obvious it has become that nonhuman species put a lot of effort into sending inaccurate messages.