In this weekly series, LiveScience examines the psychology and sociology of opposite human behavior and personality types.

Lying — like it or not — is a part of everyday life. Most of us will bend the truth every now and then, with even the most honest person telling the occasional "white lie" to avoid hurting someone else's feelings.

Yet some people, called pathological liars, utter untruths constantly and for no clear reason. Their behavior confounds scientists and oftentimes themselves.

"Pathological liars have a pattern of frequent, repeated and excessive lies or lying behavior for which there is no apparent benefit or gain for the liar," said Charles Dike, clinical professor of psychiatry at Yale University and medical director of the Whiting Forensic Division of Connecticut Valley Hospital.

On the opposite end of the spectrum are those rare individuals who might be described as "pathological truth-tellers." These people forego socially convenient and appropriate fibs to speak the unvarnished, upsetting truth.

Intriguingly, this "lying handicap" is a common feature of the developmental disorder high-functioning autism and Asperger's Syndrome.

"People with Asperger's have a tendency to be very blunt and direct — they can be honest to a fault," said Tony Attwood, professor of psychology at Minds & Hearts, an Asperger's and autism clinic in Brisbane, Australia

Psychology and neuroscience have provided clues as to why some people lie up a storm while others have difficulty dissembling or detecting it in others. These contrasting extremes can help us learn about the default human mode of lying on a daily basis to avoid insult, get out of trouble or exploit others.

"If you define lying as 'statements intended to deceive,' then yes we all do lie, every day," said Dike.

Liar, liar

In psychiatric circles, pathological lying goes by the fancy name pseudologia fantastica, though it is not yet recognized as a distinct disorder.

What puzzles most about a pathological liar's behavior, Dike said, is that it is counterproductive. Dropping flagrant whoppers can cause trouble in jobs, relationships and even with the law through self-incriminations. [10 Most Destructive Human Behaviors]

Stranger still, the lies can be blatantly see-through in their bogusness. "Not only is there no benefit to the lies, but the lies most of the time are easily disprovable," said Dike.

Dike offers an example of a coworker declaring he has a flight later that day. The co-worker is not trying to shirk a meeting, however. As the day goes on and the coworker remains in the office, he adds to the lie by announcing that the flight was cancelled. The pattern of falsities then continues. "The next day, there is some new story," said Dike.

Anecdotally, many of us will recognize this sort of behavior, though at present there are no good statistics for the prevalence of pathological lying. "What's clear," Dike said, "is that it's not uncommon."

A mind for facts

While acting in this manner makes no sense to most of us, it is essentially impossible for people with Asperger's. Patients have expressed to Attwood puzzlement at why ordinary people lie with such frequency.

To boot, people with Asperger's have trouble detecting falsity in words and actions. "They often think other people are as honest as they are, which leaves them vulnerable and gullible," said Attwoord.

Asperger's is characterized by impairment in social interactions and restricted interests. (A well-known television character who manifests much of the behavioral profile of someone with Asperger's is Dr. Sheldon Cooper on "The Big Bang Theory.") Attwood noted that these individuals have an "allegiance to the truth, rather than people's feelings."

Key to proper socialization and its subtleties is "theory of mind," the ability to attribute mental states to other individuals. "Theory of mind is determining what others are thinking, feeling or believe," said Attwood.

Asperger's patients tend to have a poorly developed theory of mind, which presents them with great difficulty in empathizing with others. More positively, this trait makes it tough to construct deceitful ruses, and those with Asperger's who do learn how to lie often do so badly, said Attwood.

Brain scans using functional magnetic resonance imaging (fMRI) have revealed a basis for this deficit. In Asperger's patients and autistics, there is less activity in parts of the "social brain," such as the prefrontal cortex. "In Asperger's,that area is dysfunctional," said Attwood. "Areas of the prefrontal cortex that should light up don’t in fMRI."

Natural born liars

To an extent, it would seem then that humans are wired to trick their fellows. In our closest primate relatives, who also have sophisticated social structures in which they live, deception is rife. Chimpanzees for example will purposefully mislead troop members away from a tasty food source and then return later to gobble it solo.

Researchers have discovered that the more conniving a primate species, the bigger its brain. (It therefore makes sense that with our giant brains, humans are veritable founts of hogwash.) The faculties of memory and abstraction needed to mince language and appearance so as to deceive require a lot of brainpower, researchers have learned. [10 Things You Didn't Know About the Brain]

Interestingly, brain scans have revealed that the prefrontal cortexes in frequent liars are built differently from those in a typical brain. A 2005 study showed that liars had 22 percent more "white matter" than average, as well as about 14 percent less "gray matter." The former acts like wiring in the brain, while gray matter cells in this region play a role in impulse control.

"If you have more white matter, you are more able to manipulate information and words," said Dike. "You can weave thoughts in ways others probably can't."

Dike, who was not involved in the 2005 study, pointed out that it was conducted on criminals and people with antisocial behaviors who lie with purpose, unlike pathological liars. Indeed, the motivation behind pathological liars' duplicity remains another big mystery.

Researchers speculate that pathological liars experience some sort of psychological excitement from fooling others. "There has to be some sort of internal satisfaction that makes them go on with this behavior, but no one knows for sure," said Dike.

For lying, as many of us will attest, is never its own reward.

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