Biometrics and Experimental Consumer Psychology

Ben Ambridge

These days, biometrics are one of the most important tools in conversion optimisation; but what exactly do they tell us? And, more importantly, how can we use that information to optimise consumer behaviour and drive sales?

In the third of a five-part blog series looking at all of the major biometric techniques, we summarise the state of the art consumer psychology research using pupil dilation.

What is Pupil Dilation?

As for most biometric measures (e.g., our previous blog on EEG), pupil dilation – that’s when a customer’s pupils increase in size as they view a website – can mean more than one thing.

Most people know that your pupils grow bigger when you’re looking at something (or someone!) that you like. But this is only part of the story.

Pupil dilation is a measure of general arousal; it means your brain is working on overdrive. Perhaps, yes, because you’ve just seen the product (or the person) of your dreams.

Or perhaps – less romantically – because you are working hard to fill out an overly complicated checkout form.

Bigger isn’t always better

The two-sided nature of pupil dilation was demonstrated nicely in a now-classic 2003 study by Finnish researchers Timo Partalaa and Veikko Surakkaa1.

The researchers found that participants’ pupils dilated when they were played both enjoyable sounds (e.g., a baby laughing) and upsetting sounds (e.g., a baby crying), but not neutral sounds (e.g., office noise). So, when it comes to pupils, bigger isn’t always better; exactly what it means depends on the context.

For example, a study published by Rik Pieters2, Professor of Marketing at Tilburg University in The Netherlands, found that – to everyone’s surprise – pupil dilation in response to different parts of an advert did not change depending on the customer’s goal.

The assumption was that customers would concentrate harder, and so show more pupil dilation, when asked to evaluate the merits of particular products, rather than when asked simply to browse or learn about them.

Interestingly, though, customers did show more dilation for pictures than for branding or text, even though they spent more time on the latter (as revealed by eye tracking). Pupil dilation tells us, then, that pictures cause people to look “shorter but harder”; something that is obviously important to bear in mind for website design.

How to maximise the effect of pictures

A 2017 pupil dilation study by Henrik Hagtvedt3 and colleagues at the Carroll School of Management, Boston College, provides an insight into how best to maximise the effect of pictures.

Higher colour saturation led not only to increased pupil size, but also – at least for products where bigger is better (e.g., food products) – increased willingness to pay.

But beware, the relationship between pupil size and purchasing remains a complex one, and one that can flip depending on seemingly-trivial factors such as how the customer is sitting.

A 2017 study by Thomas Zoëga Ramsøy4 (of the rather ominous-sounding “Singularity University”) found that when customers viewed a clothing website, the effect of pupil dilation depended on their posture: when customers were leaning forward, larger pupils meant higher preference ratings for the clothes (as per the conventional wisdom).

However, when customers were leaning backwards, larger pupils meant lower preference ratings for the clothes.

The lesson is clear: to get the most out of pupil dilation (or any other biometric measure) you need not only the raw data, but expert knowledge of how to interpret it. And with over 10 years of experience in Biometric Research, we at Endless Gain are both the pioneers and the masters.


[1] Partala, T., & Surakka, V. (2003). Pupil size variation as an indication of affective processing. International Journal of Human-Computer Studies, 59(1-2), 185-198.

[2] Pieters, R., & Wedel, M. (2007). Goal control of attention to advertising: The Yarbus implication. Journal of Consumer Research, 34(2), 224-233.

[3] Hagtvedt, H., & Brasel, S. A. (2017). Color saturation increases perceived product size. Journal of Consumer Research, 44(2), 396-413.

[4] Ramsøy, T. Z., Jacobsen, C., Friis-Olivarius, M., Bagdziunaite, D., & Skov, M. (2017). Predictive value of body posture and pupil dilation in assessing consumer preference and choice. Journal of Neuroscience, Psychology, and Economics, 10(2-3), 95.