Here's an abstract of research which suggests that the difficulty when we lie is not about our brain trying to suppress the truth, but is about our brain trying to keep other people's mental states in mind when we lie. This would mean that the reason it's more difficult to lie than to tell the truth is because we have to keep track of how our lies are working on others.
Human social cognition critically relies on the ability to deceive others. However, the cognitive and neural underpinnings of deception are still poorly understood. Why does lying place increased demands on cognitive control? The present study investigated whether cognitive control processes during deception are recruited due to the need to inhibit a tendency to state the truth, or reflect deceptive intent more generally. We engaged participants in a face-to-face interaction game and examined event-related brain potentials (ERPs) while participants lied and told the truth with or without deceptive intention. The same medial frontal negative deflection (N450) occurred when participants lied and when they told the truth with deceptive intent. This suggests that the main challenge of lying is not to inhibit a tendency to state the truth. Rather, the challenge is to handle the cognitive conflict resulting from the need to keep others' mental states in mind while deceiving them.Cognition. 2010 Jan;114(1):105-10.
A truth that's told with bad intent: an ERP study of deception.
Carrión RE, Keenan JP, Sebanz N.
Department of Psychiatry, Zucker Hillside Hospital, Long Island, NY 11004, United States. RCarrion@NSHS.edu
PubMed (the full paper is only available if you pay)>>