OPINION19 May 2015

Bias in the spotlight: change blindness


Change blindness is the failure to notice a major visual change in our field of view if we are briefly distracted.


Normally, any change to a scene grabs our attention, but if that change is hidden by any sort of disruption, such as if we look momentarily away; blink; or cut from one view to another and back in a movie, we can and do miss large changes. 

For example, we often fail to notice major differences introduced into an image while it flickers off and on again. See if you can spot it in this image. Once you’ve seen it it’s impossible to un-see, but be warned it takes a lot of people a long time!

Change blindness is related to, but distinct from, inattentional blindness where, if people focus on a particular task that demands their attention, they will often fail to notice a fully visible, but unexpected object or event that occurs in full view. The key difference between the two concepts is where our attention is focused.

Change blindness identifies how we miss change that takes place when we momentarily look away from a scene and then look back again. Inattentional blindness identifies how we can be so focused on carrying out a task – looking at a scene continuously in this case, we fail to notice something unexpected occurring in that scene.

The classic illustration of change blindness was the door study run by Daniel Simons and Daniel Levin in 1998. Fewer than half of the unwitting participants in the study, who thought they were simply being asked for directions, noticed that halfway through the interaction the person who asked them for directions was swapped for a completely different person while their attention was buried in a map.

A similar experiment found that people – many of them aware of the concept of change blindness – were susceptible to change blindness while registering at a recent Harvard behavioural economics seminar. They failed to notice the person they’d just been speaking to had been swapped for someone else.

How perceptive to change are you? Most of us are overconfident about our ability to notice change. Test yourself with this video. Designed as an old-fashioned ‘Whodunnit?’ murder investigation, this online test was developed as part of a cycling safety campaign to illustrate how much we fail to notice even when it’s right in front of our eyes, and how easy it is to miss something – such as cyclists – drivers might not be looking for. 

For example, a driver who blinks or looks away from the road momentarily may fail to notice an important change in the road markings or the appearance of a cyclist on the road. In 2009, the CTC, the UK’s national cycling organisation launched the ‘Stop SMIDSY’ campaign – where SMIDSY stands for the typical driver response to a near miss “Sorry mate, I didn’t see you” – to tackle the judicial system’s often lax response to this plea.

Like inattentional blindness, change blindness is also relevant to areas such as eyewitness testimony. For example, in momentarily looking away from the scene an eyewitness may misidentify an innocent bystander as the criminal if they happen to be standing in the same place or wearing similar clothing. One study which showed participants a video of a crime found that people failed to notice significant differences in height and build when a burglar and bystander were wearing similar clothing.

A reminder that our senses can – and do – deceive us!

Crawford Hollingworth is founder of The Behavioural Architects