Learning to lie: Effects of practice on the cognitive cost of lying

Cognitive theories on deception posit that lying requires more cognitive resources than telling the truth. In line with this idea, it has been demonstrated that deceptive responses are typically associated with increased response times and higher error rates compared to truthful responses. Although the cognitive cost of lying has been assumed to be resistant to practice, it has recently been shown that people who are trained to lie can reduce this cost. In the present study (n = 42), we further explored the effects of practice on one's ability to lie by manipulating the proportions of lie and truth-trials in a Sheffield lie test across three phases: Baseline (50% lie, 50% truth), Training (frequent-lie group: 75% lie, 25% truth; control group: 50% lie, 50% truth; and frequent-truth group: 25% lie, 75% truth), and Test (50% lie, 50% truth). The results showed that lying became easier while participants were trained to lie more often and that lying became more difficult while participants were trained to tell the truth more often. Furthermore, these effects did carry over to the test phase, but only for the specific items that were used for the training manipulation. Hence, our study confirms that relatively little practice is enough to alter the cognitive cost of lying, although this effect does not persist over time for non-practiced items.

A 3-group repeated measures design with 42 participants completed 924 trials across four phases using autobiographical yes-or-no questions presented in colors indicating lie or truth responses, with the training phase manipulating lie-truth proportions (75%-25%, 50%-50%, or 25%-75%).

This study investigated whether the cognitive cost of lying—measured as slower reaction times and higher errors—can be reduced through practice, and whether such effects generalize beyond trained items. Using a reaction time-based Sheffield lie test, researchers randomly assigned 42 participants to frequent-lie, control, or frequent-truth groups, each completing 924 trials over four phases. During the training phase, participants in the frequent-lie group showed significantly reduced reaction time differences between lying and truth-telling (53 ms vs. 239 ms in the frequent-truth group), contradicting earlier claims that cognitive costs of deception are resistant to practice. However, these benefits did not transfer to unpracticed items during the test phase, indicating that practice effects are item-bound rather than reflecting generalized skill acquisition.

These findings have critical forensic implications, suggesting that suspects who have rehearsed specific lies may show reduced cognitive load during polygraph testing for those particular items, potentially compromising detection accuracy. Future lie detection research and practice should account for the item-specific nature of deception training and consider that extensive practice on particular scenarios may selectively reduce detectable cognitive signatures.

Background & Context

Cognitive theories of deception hold that lying requires more cognitive resources than truth-telling, a principle supported by consistent findings that deceptive responses show increased reaction times and higher error rates compared to truthful responses. This fundamental assumption has long been considered a cornerstone of cognitive lie detection approaches.

However, the traditional view that the cognitive cost of lying is resistant to practice has been challenged by recent research demonstrating that training can reduce this cost. This 2012 study by Van Bockstaele and colleagues addresses a critical gap: understanding whether the frequency of lying versus truth-telling affects the cognitive difficulty of deception, and whether such effects persist beyond the immediate training context.

The research has important theoretical implications for understanding deception as an executive control process, as well as practical relevance for forensic contexts where suspects may have rehearsed their lies extensively before interrogation.

Research Design & Methodology

The study analyzed data from 42 participants, with 14 participants randomly assigned to each of three experimental groups. Researchers employed a modified Sheffield lie test—a reaction time-based paradigm where participants respond to questions that appear in different colors indicating whether to lie or tell the truth.

The experiment consisted of 924 trials across four phases: an acquaintance phase (24 trials), baseline phase (180 trials), training phase (540 trials), and test phase (180 trials). The task used 72 autobiographical yes-or-no questions about actions participants had or had not performed on the day of testing.

The critical manipulation occurred during the training phase through the use of "filler trials" with varying lie-truth proportions:

• Frequent-lie group: 75% lie trials, 25% truth trials
• Control group: 50% lie trials, 50% truth trials
• Frequent-truth group: 25% lie trials, 75% truth trials

The study used 3 (Group) × 3 (Phase) repeated measures ANOVAs on both reaction time and error data to assess how practice affected the "lie-effect"—the difference in performance between deceptive and truthful responses.

Results & Key Findings

During the training phase, lying became significantly easier for those who lied frequently and harder for those who told the truth frequently. The lie-effect in response latencies showed a clear linear gradient: 53 ms for the frequent-lie group, 145 ms for the control group, and 239 ms for the frequent-truth group.

The interaction between experimental phase and group showed a significant quadratic pattern, F(1, 39) = 4.73, p < 0.05, demonstrating that the training manipulation successfully altered the cognitive cost of lying during practice.

Key statistical findings included:

• No group differences at baseline: F(1, 39) = 2.88, p = 0.10, confirming equivalent starting points
• Training effects did not persist in the test phase for unpracticed items: F < 1, with all three groups showing similar lie-effects
• For filler trials (the practiced items), effects carried over to the test phase: F(1, 39) = 6.27, p < 0.05
• A significant linear effect emerged in the test phase for practiced items: F(1, 39) = 4.11, p < 0.05, f = 0.33

These effects carried over to the test phase, but only for the specific items used in the training manipulation—the effect did not persist for non-practiced items. This specificity suggests that practice effects are item-bound rather than reflecting a generalized skill improvement.

Discussion & Significance

The findings directly contradict earlier studies by Johnson et al. (2005) and Vendemia et al. (2005), which had argued that the cognitive cost of lying is resistant to practice. The current study demonstrates that the proportion of lie versus truth trials powerfully modulates deception difficulty.

The results align with two potential cognitive mechanisms: task switching costs (which would increase lie-effects in the frequent-truth group and reduce them in the frequent-lie group), and goal neglect processes. The data suggest that lying becomes cognitively less demanding when people lie frequently, and more difficult when people frequently tell the truth.

The forensic implications are notable: if suspects are required to respond truthfully to many verifiable questions during interrogation, they may experience greater difficulty when lying on crucial incriminating questions. Conversely, guilty suspects who have repeated the same lies extensively to police, lawyers, and judges may experience reduced cognitive load when lying.

Limitations & Considerations

The small sample size of only 14 participants per group may have limited statistical power to detect smaller effects. The autobiographical questions used were not emotionally salient nor crime-related, limiting ecological validity for forensic applications.

Some participants may have been uncertain about their answers, particularly concerning habitual behaviors or actions performed the day before testing. The item-specific nature of the training effects—where benefits did not generalize to unpracticed questions—substantially limits the practical applicability of the findings.

The study cannot address whether results would replicate in high-stakes contexts with actual consequences, where arousal and emotional distress are elevated, or whether countermeasures could influence the observed patterns.

Practical Applications

For polygraph examiners and interrogators, the research suggests a strategic approach: increasing the number of verifiable truth-telling questions in an examination may amplify the cognitive difficulty of lying on critical items. This "truth saturation" technique could enhance detection accuracy.

However, examiners must recognize that with repeated lying, deceptive responses may cognitively mirror truth-telling, potentially hampering lie detection. Well-rehearsed lies—particularly those repeated verbatim across multiple interrogations—may not show the typical cognitive markers of deception. This finding underscores the importance of introducing unpredictable or unanticipated questions for which suspects cannot have prepared specific responses.