Estimated reading time: 9 minutes
People lie. Some people lie a lot and proficiently. Given this unsettling (and somewhat unsurprising) fact, an objective means by which we could know when a person is telling a lie could serve us all well. Although there have been claims that looking in a particular direction (for example looking towards the right) is a tell-tale sign of a person lying, these assertions have been debunked (and also here). So, are there any reliable signs indicating that someone is lying?
The scientific literature suggests that in fact the eyes can give away a lie but it is pupil size, not gaze direction, that is most revealing. Research has shown that people tend to have larger pupils when deceiving compared to telling the truth in a range of scenarios. The differences are quite subtle, and scientists generally use sensitive eye-tracking equipment to measure pupil size, so it is unlikely that a casual observer would be able to detect any difference. In addition, most experimental research is a comparison between groups, and what is true for a group as a whole may not necessarily be true for any individual in that group.
In the sections below, we take a look at some of the research that has explored the relationship between pupil size and lying.
Pupil Dilation and Lying
Mock Crime Studies
In one early mock crime study, published in 1943, “guilty” and “innocent” participants found themselves in a university lecture hall. The guilty participants were told to take some money left under a seat (rarely more than a dime) and then to lie to all subsequent questions connecting them to this crime. Meanwhile, the innocent participants were simply told to exit the lecture hall and answer all questions truthfully. The critical questions relating to the mock crime were the following:
- Were you in the lecture hall a moment ago?
- Did you take the money?
- Have you been telling me the truth?
The descriptive results suggested that the guilty participants exhibited a dilation of the pupil followed by a very quick constriction when making deceitful responses. In contrast, this pupillary response was only evident for a small number of truthful respondents.
A more recent mock crime study that compared the pupil size between guilty and innocent participants also showed a similar pattern, with a larger pupillary diameter when answering deceptively as opposed to truthfully. Other researchers have shown that this relative increase in pupil size can be observed with both computer-administered questions as well as when viewing pictures related to the crime.
Simple Lie Studies
Unlike mock crime studies, some studies focus on people intentionally providing information that is either truthful or deceptive, without the necessity of a crime. For example, in one very simple experiment, participants verbally answered questions pertaining to their age, height, weight, place of birth, and physical disabilities. Immediately after answering the five questions, the experimenter informed participants that the purpose of the study was to discover a relationship between pupil size and deceptive responses, and the participants then rated their answers as completely true, partially deceptive, or completely deceptive. The author of the study, Ira Heilveil, used a self-rating procedure because he believed:
… that in order for the deception to occur, the deceiver must be cognizant of the deception at the time that it is taking place.
– Ira Heilveil, 1976
The results showed that the mean pupil diameter was statistically larger while answering the interviewer’s question deceptively (5.74 mm) versus truthfully (5.15 mm). Other more recent studies have shown a similar increase in pupil diameter when lying as compared to being truthful about general facts or non-personal episodes, on a questionnaire assessing a teacher, in an economic game to a co-player, and about a witnessed crime – whether interrogated by a human or robot.
In an interesting recent addition to deception and pupillometry research, a group of researchers described a robot with the capability to detect lies using pupil information. Although it is not simple for people to tell when someone is lying, “machine vision” can quickly use current, finely detailed information to make judgments based on small differences.
In one study examining the robot’s human interaction with truthful and deceptive humans, each participant wore an eye tracker, resulting in a continuous feed of pupil information from the participant to the robot. For the task, participants described the pictures on a series of six cards either accurately (for five cards) or deceptively (for one card). The photo below shows the robot and five cards from the deck, one is in a person’s hand and four are on the right side.
For the first part, the robot guessed which one of the six cards a person described deceptively. The robot successfully detected the card described deceptively with an accuracy of 88% by computing the mean pupil dilation during each card description and then choosing the card with the highest dilation.
For the second part, after each card description, the robot classified the description as truthful or false and then asked for confirmation. The robot had a 71% classification accuracy by comparing the mean pupil diameter to information from the first part. However, accuracy was improved to 78% if the robot updated its information after each interaction.
Pupil Dilation and Cognitive Load
Why do our pupils dilate when telling a lie? There are a number of theories, but the one getting the most play over the last few decades is cognitive load theory (see our comprehensive blog on this topic).
Cognitive load refers to the amount of resources that tasks such as lying demand from working memory – a scratchpad that holds information temporarily, controls attention, plans, and interfaces with sensory and long-term memory. Because working memory is severely limited in terms of how much information it can hold, as well as for how long, a high cognitive load on working memory will have cascading effects on other systems. Pupil diameter is a well-established physiological marker of cognitive load. As cognitive load increases so does pupil diameter.
According to cognitive load theory, to create a lie, a cognitive load is imposed on working memory as it organizes and holds information about the lie and searches long-term memory for information to construct it. For most people, telling a spontaneous on-the-spot lie is harder than telling a planned lie, and telling the truth is easier than lying altogether. Because telling a lie generally imposes a higher cognitive load and because a higher cognitive load is associated with increased pupil diameter, pupils tend to dilate more when a person is lying compared to telling the truth.
It is worth pointing out that while telling a lie may be more cognitively demanding than telling the truth for most people, this is not necessarily the case for everyone. Highly practiced and efficient liars may not necessarily have increased pupil sizes when actively deceiving, so we are still some way off from having a true lie detector.
If you would like us to feature your EyeLink research, have ideas for posts, or have any questions about our hardware and software, please contact us. We are always happy to help. You can call us (+1-613-271-8686) or click the button below to email: