|On her trolley! Janine, the mannequin|
Imagine you're at the entrance to a narrow corridor and further down, several feet away, hanging on the right-hand wall, there are three rectangular mirrors (30cm x 45cm) at head height. At what point, as you proceed down the corridor, do you think you'll be able to see your face in the mirrors?
The correct answer is that your face will only be visible in each mirror when you are passing directly opposite. At no point will your face be visible in more than one mirror.
Lawson first tested people's understanding of this idea by having them stand at one end of a corridor and say in which of four positions in the corridor a mannequin "Janine" (moved about on a trolley) would be able to see herself in each of the three mirrors. Only two of the four positions in question were actually directly opposite one of the mirrors. So, of the 12 possible position/mirror combinations, the answer "yes, Janine can see her face" should only have been given twice. In fact, the 18 Liverpool University students answered yes an average of 6.1 times, grossly overestimating how often the mannequin could see herself in the mirrors. The errors weren't randomly distributed, they tended to be made when the mannequin was located near to the mirrors, but not directly opposite them.
The same errors were made when a single, larger mirror was divided up into three using duct tape (to ensure that participants realised the mirror surfaces were all flat against the wall and not angled like a dressing-table mirror), and also when the original three mirrors were arranged on the wall vertically, rather than horizontally.
Perhaps, Lawson reasoned, the participants were performing so poorly because it was confusing assuming the perspective of a mannequin. And also, perhaps because they were asked about each position and each mirror one at a time, so that they didn't realise the full implications of what they were saying: that the mannequin could see her face in multiple mirrors from a single position, and in the same mirror from different positions (an optical impossibility in the situation as described).
"This multiple reflection error is particularly surprising," Lawson said "because it directly contradicts our everyday experience that mirrors reflect a single coherent scene."
So why do people misunderstand mirrors in this way? Lawson said there are probably multiple reasons. One participant described her naive belief that whenever you turn your eyes towards a mirror, wherever it is, you will see yourself reflected in it - "mirrors look back at you," she said. No doubt this belief was held implicitly by many of the other participants.
"Almost nobody will have a clear, thought-through and self-consistent theory of optics which they use to guide their predictions," Lawson said. "Most people probably use a set of underspecified beliefs and heuristics, some of which are incompatible, leading them to make unsophisticated, noisy and inaccurate predictions. People rarely think explicitly about optics and what determines what they can see in a mirror or a window - or indeed, what they can see directly."
Lawson, R. (2012). Mirrors, mirrors on the wall…the ubiquitous multiple reflection error. Cognition, 122 (1), 1-11 DOI: 10.1016/j.cognition.2011.07.001
Link to Mirrors and the mind (Psychologist magazine article).
Link to New York Times article on the psychology of mirrors.
Post written by Christian Jarrett for the BPS Research Digest. Thanks to Rebecca Lawson for providing images of her experiment.