The New York Times defines it as, "...self-delusion, the result of what’s called cognitive dissonance..." "...Psychologists have suggested we hone our skills of rationalization in order to impress others, reaffirm our “moral integrity” and protect our “self-concept” and feeling of “global self-worth.” In other words, it would seem to be ego-focused. The question then is at what level does this take place?
“If little children and primates show pretty much the same pattern you see in adults, it calls into question just how deliberate these rationalization processes are,” he says. “We tend to think people have an explicit agenda to rewrite history to make themselves look right, but that’s an outsider’s perspective. This experiment shows that there isn’t always much conscious thought going on.”
The article goes on to provide a rational for this approach that could have evolutionary consequences. "Once a decision has been made, second-guessing may just interfere with more important business. A fox who pines for abandoned grapes or a monkey who keeps agonizing over food choices could be wasting energy better expended obtaining the next meal."
There are, however, other explanations for this psychological occurrence. From John Tierney's TierneyLab Blog for the New York Times.
Other experts suggested that the monkeys’ behavior tended to support “self-perception” theory — which (bear with me) is not the same as “self-concept” theory. It’s the theory that once you perceive yourself making a choice — say, an electric sandwich press over a toaster, as in the classic experiment I described in my column — then you conclude that the toaster must be unappealing to you simply because you rejected it, not because you’re trying to banish the dissonant thought that you made a mistake.
Social psychologist Daniel Gilbert, the author of “Stumbling On Happiness” has the following take on the study.
"The data in the monkey study are extremely interesting. They could be interpreted in terms of cognitive dissonance theory, but there are other interpretations as well. For example, monkeys may be wired not to waste time making the same evaluation twice. So once they reject something, they remember that they rejected it and reject it again in the future. The fact that monkeys derogate unchosen items is novel and important, but the “Why?” question is still unanswered."
John Tierney sums it up nicely,