We humans can be a cocky species - so much so that a realistic self-image can be seen as a symptom of trouble.

Take the reaction to a recent survey in which about 52 percent of college students rated their emotional health as below average. About half of them are, after all, going to be below average. But the UCLA researchers who did the survey say it indicates a deeper problem. In past surveys, at least 64 percent of the respondents said they were above average.

What's going on here? Are we truly living in Lake Wobegon, where all the children are above average?

Several scientists blame evolution for our ego-inflating tendencies - call it survival of the self-promoters.

We naturally tend to puff ourselves up and kid ourselves, says Rutgers University evolutionary biologist Robert Trivers. That's because evolution has shaped many organisms into natural-born liars. In his new book, The Folly of Fools, Trivers lays out a case that we humans are such good liars we even lie to ourselves.

People tend to overrate themselves on looks, smarts, and leadership ability, he says. Academics, he notes, are particularly deluded - in one survey, 94 percent thought they were in the top half of their profession.

Wouldn't a clear-eyed view of reality give us all the best chance for survival? Not necessarily, says Trivers, since much of our success in life and mating hinges on deception. And what better way to improve our powers of deception than to believe our own lies? It's survival of the deluded.

Trivers achieved scientific prominence in the 1970s, when he revolutionized the scientific understanding of altruistic behavior, showing how it can pay off as long as good deeds are reciprocated. He also pioneered the idea that evolution works at the level of individual genes - a concept Richard Dawkins popularized in his breakout best seller The Selfish Gene.

Trivers starts his book with a discussion about ordinary deception, which happens throughout the natural world. Male sunfish fight over territory to get a mate, but small male sunfish can avoid all that by imitating females. That way, the little male gets to share the favors of a female with a clueless dominant male. Butterflies take on the appearance of toxic species to avoid being eaten without expending the energy of making toxins.

Among primates, the bigger the brain, the greater the tendency to deceive, says Trivers. We're the dishonest apes. Over the eons, it has been to our ancestors' advantage to convince the world they were nicer, prettier, and smarter than they really were.

Believing your own lies makes you more convincing, as long as you don't go overboard to the point that people laugh at you behind your back. Among children, he says, those who score highest on intelligence tests are most likely to lie to themselves and others.

Humans can also work together to magnify self-deception. In his chapter on religion, he notes the obvious problem with all religions that claim to be the one true system of belief about the one true God. They can't all be right.

From his part-time home in Jamaica, Trivers said he sees a blizzard of deception and self-deception in America today. One antidote, he said, is humor, of the type doled out by his favorite television personalities - Jon Stewart and Stephen Colbert.

University of Pennsylvania evolutionary psychologist Rob Kurzban agrees there can be an advantage to self-inflation. He lays out more examples in his book, Why Everyone Else Is a Hypocrite.

People who have had more than 100 sex partners say they are less likely than the average person to get a sexually transmitted disease, for example, and those who have been hospitalized following serious car crashes rate themselves as good or excellent drivers.

It's ironic, says Kurzban, but we expect people to be systematically wrong when it comes to evaluating themselves.

He, too, blames evolution for clouding our vision. "The social world is competitive," he said. "I want people to think I'm good."

But Kurzban offers a very different explanation. For him, there's no such thing as self-deception because there is no true single self to deceive.

He lays out a picture of the human mind as a collection of different parts, or modules, all working partially independently. Two of those parts can disagree, but neither part is the true self - both are simply components of a whole. Some modules of our brains act as press secretaries, and they tend to believe we're a little better than we really are.

The modules are like apps on an iPhone, he says. They don't necessarily share information, and that can lead to hypocrisy.

The module that judges other people might not be in complete agreement with the part that controls our behavior. So a person might make a snide remark about a friend's weight gain, attributing it to lack of willpower, and the next night eat a pint of ice cream and not notice the inconsistency.

The idea that we're not aware of most of the workings of our own minds goes back at least to Freud, with his emphasis on the drives that are unconscious or subconscious. Kurzban says the idea of a subconscious mind has given way to the more contemporary modular view.

He adds that it may be hard to distinguish self-deception from being wrong. Some people don't hide the truth from themselves - they just don't know it, and they don't know they don't know it.

Trivers says the idea of the mind as a collection of modules has some use, but it has been taken beyond the realm of reason when people start saying there is no self.

Neither Kurzban nor Trivers could explain integrity or how it fits into their different frameworks for understanding the mind. Too bad. That should be the subject of another book.

In The Folly of Fools, Trivers applies the science of self-deception to science itself, and finds plenty of examples in which people or whole fields were led astray temporarily.

However fallible individual scientists may be, he says, science is a self-correcting process. "Over the long haul . . . falsehood has no chance, which is why over time science tends to outstrip competing enterprises."

Contact staff writer Faye Flam at 215-854-4977, fflam@phillynews.com, on her blog at www.philly.com/evolution, or @fayeflam on Twitter.