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Don't lie to my eyes (I.)

“Why do almost all people tell the truth in ordinary everyday life? —Certainly not because a god has forbidden them to lie. The reason is, firstly because it is easier; for lying demands invention, dissimulation and a good memory.”

– Friedrich Nietzsche, Human, All Too Human, II.54, 1878/1996

I have started this blog as my log to my master thesis. I was aiming to do some work on artificial intelligence. As it worked out, I will be working on method which should detect if people are lying when filling out online questionnaires.

But I'm still going to use this blog. The reason is that this topic is closely related to evolution and even to artificial intelligence.

The topic is still close to the idea of this blog. God-like properties, like knowing what you think, can be obtain via science and technology.

If you want to tell if someone is lying, you have to look at his physiological responses. These physiological responses have evolved over millennia and are often really hard or impossible to control by our conscious mind.

Today I am going to write about eyes. More precisely about pupil of the eye. Because your eyes can tell me if you are lying.

When you want to use pupil diameter as indicator if people are lying, you are basically trying to measure their cognitive load and emotional response.

There is still no complex theory of how our mind work, so there is lot of guess work and space for error, but if you are careful and know what you are looking for, you can get right now up to 80 percent accuracy in telling if people are lying.

Cognitive load

There are three hypothesis when it comes to cognitive load and lying. First assumes, that lying is cognitive less demanding process and so the responses to questions will be quicker and physiological measurements more subtle.

Second hypothesis assumes exact opposite. According to it lying is cognitively more demanding than telling the truth and therefore you can detect stronger physiological responses and the answers to questions will take more time.

Third hypothesis assumes, that it depends on the context. If the questions are about your personality and you want to look socially desirable, you will respond quick and will simply semantically evaluate answers and choose the most appealing.

However if you are asked different questions or you are ask the same questions in different conditions, it will be more cognitively demanding than telling the truth.

Based on the literature I have read, I will go with the third option.

Pupil dilation and what can screw your measurement

Pupil dilation is change in pupil diameter. Dilation occurs around 2-7 seconds after emotional stimuli is presented and is faster for stronger stimuli. If you want to use it in your study, you have to take into account several factors.

First you have to eliminate light disturbance. If you cannot prevent light change by controlling the environment, you can use data filters to separate the data. The pupil response to light is either rapid constriction or slow dilation, while cognitive processing triggers small but rapid increases in pupil size.

However that is not enough. Pupil dilation can be triggered by visual stimuli and even noises and that can disturb your measurement. So you have to ensure that your subjects are separated as much as possible from other stimuli than the task you have given them.

Pupil dilation can also be triggered by cognitive load, stress or even temperature. When you are using your short term memory, pupil dilate. It is dilated during the whole process of solving math problems or other cognitively demanding tasks and dilation stops when the problem is solved.

Even sound can trigger pupil dilation. Larger pupil dilation is shown when you listen to affect sounds compared to neutral sounds. The same can be told about visual stimuli. The more emotional the stimuli is, the more your pupil dilates.

Your pupil also dilates if you are waiting for an answer to even trivia question, it is enough that you are interested in the answer.

It does not matter if the emotion is positive or negative, response of the pupil is very similar.

Your pupil dilates when you are in pain, and it dilates more if you are in more pain and it also dilates if you are aroused.

Pupil dilation and deception

First study about effect of pupil dilation on deception detection comes from 1943 [2]. Since than, technology for pupil measurement has improved rapidly, our knowledge how to use it not so much.

Studies shown that pupil dilates when you are telling lies, even if you are sending deceptive messages. And the more deceptive you are trying to be, the more your pupil dilates.

Most studies shown that lying is cognitively more demanding than telling the truth. Some studies even shown, that if you tell the people that you can detect if they are lying, you are more likely to observe the signals that can tell you if they are lying.

Other however shown that lying is less cognitively demanding. Therefore I go with the third hypothesis of cognitive load.


Pupil dilation can be good indicator for cognitive load or emotional triggers, but on its own cannot provide all necessary information.

It is triggered by too much stimuli and therefore you have to create your experiment very carefully and eliminate all possible external stimuli outside of your task.

Also you shuld use other meassurement like tracking the gaze of the eyes or galvanic skin response. About that in future posts.


[1] Beatty, J. (1982). Phasic not tonic pupillary responses vary with auditory vigilance performance. Psychophysiology, 19(2), 167–172. doi:10.1037/0033-2909.91.2.276

[2] Berrien, F. K. & Huntington, G. H. (1943). An exploratory study of pupillary responses during deception. Journal of Experimental Psychology, 32(5), 443-449.

[3] Bradley, M. T., & Janisse, M. P. (1981). Accuracy demonstrations, threat, and the detection of deception: cardiovascular, electrodermal, and pupillary measures. Psychophysiology, 18(1), 307–315. doi:10.1111/j.1469-8986.1981.tb03040.x

[4] Einhäuser, W., Koch, C., & Carter, O. L. (2010). Pupil dilation betrays the timing of decisions. Frontiers in Human Neuroscience, 4(February), 18. doi:10.3389/fnhum.2010.00018

[5] Krafčíková, M. (2014). Úmyselné skresľkovanie odpovedí pri osobnostných dotaznákoch - možnosti využitia sledovania očí. Univerzita Komenského v Bratislava.

[6] Lubow, R. E., & Fein, O. (1996). Pupillary size in response to a visual guilty knowledge test: New technique for the detection of deception. Journal of Experimental Psychology: Applied, 2(2), 164–177. doi:10.1037/1076-898X.2.2.164

[7] Palinko, O., Kun, A. L., Shyrokov, A., & Heeman, P. (2010). Estimating cognitive load using remote eye tracking in a driving simulator. Proceedings of the 2010 Symposium on Eye-Tracking Research & Applications - ETRA ’10, 141. doi:10.1145/1743666.1743701

[8] Van Hooft, E. a. J., & Born, M. P. (2012). Intentional response distortion on personality tests: Using eye-tracking to understand response processes when faking. Journal of Applied Psychology, 97(2), 301–316. doi:10.1037/a0025711

[9] Wang, J. T., Spezio, M., & Camerer, C. F. (2010). Pinocchio ’ s Pupil : Using Eyetracking and Pupil Dilation To Understand Truth-telling and Deception in Games. The American Economic Review, 3, 984–1007. doi:10.1257/aer.100.3.984

[10] Webb, A. K., Honts, C. R., Kircher, J. C., Bernhardt, P., & Cook, A. E. (2009). Effectiveness of pupil diameter in a probable-lie comparison question test for deception. Legal and Criminological Psychology, 14, 279–292. doi:10.1348/135532508X398602


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