Read about all the tactile illusions in our special feature
Here鈥檚 one to try out on your family and friends. Get hold of two cardboard boxes of different sizes and put a brick in each one. Check they weigh the same, then get somebody to lift them and tell you which is the heavier. The vast majority of people will say that the smaller box is heavier, even though it isn鈥檛, and will continue to maintain that it is even after looking inside both boxes and lifting them several times.
This 鈥減erceptual size-weight illusion鈥� is very robust. So much so that it works even if the smaller box is slightly lighter (). Even labelling two identical boxes 鈥渉eavy鈥� and 鈥渓ight鈥� can pull the same trick.
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The exact reason for these illusions remains a mystery. Curiously, experiments show that even though people initially use greater force to lift the larger box than the smaller one, on subsequent lifts they unconsciously equalise the amount of force they use to lift them. Despite their bodies apparently 鈥渒nowing鈥� that the boxes weigh the same, their minds still perceive the smaller box as being heavier.
Last year, J. Randall Flanagan of Queen鈥檚 University in Kingston, Ontario, Canada, complicated matters further when he showed that we can unlearn the size-weight illusion. He got volunteers to spend several days manipulating boxes that became lighter the larger they were. At the end of the process he found that their size-weight illusion was reversed. They consistently judged the larger of two objects to be heavier even though they weighed the same (). This is good evidence that the illusion arises out of experience of the world, where larger objects tend to weigh more than smaller objects of the same kind.
