One of the most interesting and persistent illusions in psychology is the so-called “size-weight illusion”, first described by Augustin Charpentier way back in the late nineteenth century. We’ve probably all experienced this at one time or another…
If we have two objects that way exactly the same, but are different physical sizes, the object that is smaller always feels heavier than the larger one even though it’s not. Psychologists have been trying to explain the illusion for years, without complete success.
One popular explanation if that we expect the larger object to be heavier, so we use more strength when picking it up and it therefore seems lighter than the smaller object simply because we are trying harder. I’ve never quite bought that explanation… Seems to me it only holds true once. If we’ve already picked up both objects and figured out they are the same weight, we shouldn’t be caught out the second time we pick them up – but we are, the smaller one still feels heavier.
New research by Gavin Buckingham in Experimental Brain Research takes us at least a step closer to understanding what might be going on here, It seems the expected weight differences might be a result of learned expectations about weight encoded within the brain. In an innovative series of experiments, Buckingham found it was possible to completely reverse the SWI effect by having participants repeatedly lift supposedly identical weights that had been rigged such that the larger object really was lighter. After a period of time living in this back-to-front world, the participants began to always experience smaller objects as being lighter than larger ones, even though they were the same weight.
What’s going on here? Well, I think Buckingham’s work supports the “effort put in” explanation. After repeatedly experiencing the smaller objects as being heavier, the brain has learned this general rule and applying it; in other words, it has come to expect smaller things to be heavier, so extra effort is put in to lifting them and, as a result, they begin to feel lighter.