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Do Your Eyes Give You Away?

Do Your Eyes Give You Away?Can your eyes give away how much you’re thinking — even when you’re not consciously aware of your effort? According to new research, the answer is yes.

Previous research has shown that people spend more physical effort in a demanding physical task when they could gain a high-value monetary reward, than when they could gain a low-value reward. But the intriguing finding from this research was that this behavior occurred even when the monetary reward was presented subliminally, below the threshold of our conscious awareness. In other words, a person would work harder for more money, even if they weren’t consciously aware that more money was the reward. Other research into subliminal processing suggests people can perceive emotional messages subliminally too.

Dutch researcher Erik Bijleveld and colleagues recruited 15 participants to take part in an experiment measuring cognitive effort on a task for various amounts of money. They measured the brain effort required to complete the task through eye pupil dilation. Why would you care about pupil dilation, an automatic nervous system response? Well, it appears that you can tell a lot by the size of your pupils, because it expands with sympathetic activity, making it an unobtrusive measure of how much mental effort you’ve invested in a particular task.

The study was task was simple number recall of either three or five-digit numbers for various amounts of money. Sometimes the amount of money earned was show subliminally and sometimes it was shown explicitly. Participants completed 48 trials of the task, earning money as they went. Pupil dilation was measured by a scientific instrument designed for such measurements.

In keeping with the previous researcher, the current study’s researchers found that more valuable rewards led to recruitment of more mental resources, even when the participants were not consciously aware that the task would result in a larger monetary reward. But they also found that people didn’t just call on more mental resources arbitrarily for all high-reward tasks — they only did so when the task was difficult and the participant required more mental effort to complete the task. A person’s pupils dilated more when they were thinking harder on harder tasks, even when they weren’t consciously aware of the higher reward associated with that task.

The researchers summarized their findings succinctly: “More generally, whereas analyses of costs (required effort) and benefits (value of rewards) are usually thought to require consciousness, our findings suggest that such strategic processes can occur outside of awareness — and these processes show in the eyes.”

While the researchers studied pupil dilation with scientific instruments, their findings into these microexpressions may be able to be generalized to other kinds of interactions with others. For instance, imagine an interrogation by a police detective investigating a crime. The valuable reward in such an example is not monetary, but rather freedom versus being jailed. A suspect who’s pupils are dilating while describing their alibi may be calling upon more mental resources when they making it up (since a lie requires more mental resources for most people than simply telling the truth).

This is a tiny experiment, however, so some caution must be used in over-reaching in interpreting these results. Further study will be needed to confirm these results on a larger and more diverse population. But the data from this small study suggest that it may be possible that your eyes may indeed tell others how much you’re thinking, even when you’re not consciously aware of your own mental efforts.


Bijleveld, E., Custers, R. & Aarts, H. (2009). The Unconscious Eye Opener: Pupil Dilation Reveals Strategic Recruitment of Resources Upon Presentation of Subliminal Reward Cues. Psychological Science. DOI: 10.1111/j.1467-9280.2009.02443.x.

Do Your Eyes Give You Away?

John M. Grohol, Psy.D.

Dr. John Grohol is the founder of Psych Central. He is a psychologist, author, researcher, and expert in mental health online, and has been writing about online behavior, mental health and psychology issues since 1995. Dr. Grohol has a Master's degree and doctorate in clinical psychology from Nova Southeastern University. Dr. Grohol sits on the editorial board of the journal Computers in Human Behavior and is a founding board member of the Society for Participatory Medicine. You can learn more about Dr. John Grohol here.

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APA Reference
Grohol, J. (2018). Do Your Eyes Give You Away?. Psych Central. Retrieved on December 2, 2020, from
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Last updated: 8 Jul 2018 (Originally: 20 Oct 2009)
Last reviewed: By a member of our scientific advisory board on 8 Jul 2018
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