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 every 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