Which Weighs More, a Pound of Stone or a Pound of Styrofoam?


It’s not a trick question: your brain answers differently depending on whether they’re part of the same object or not.



For more than a century, scientists thought they knew the answer to a curious question: why does 10 pounds of a low-density substance such as Styrofoam feel heavier than 10 pounds of stone? It isn’t heavier, of course, but repeated experiments have shown that it feels that way.

Now psychologists say their initial explanation may have been incomplete, and the new explanation could have far-reaching consequences, including for the way Netflix designs the algorithms that recommend movies to its customers.

Scientists have known for decades that when asked to lift two objects that seem like they should have different weights but are actually equally heavy, people will say the lighter-looking one feels heavier. Experts believed this illusion, called the material-weight illusion, occurs when the brain’s expectations about weight are contradicted: Throughout life we learn through experience that some materials are heavy and others are light. Over time we become skilled at guessing an object’s weight from its appearance alone.

But new evidence suggests that the brain bases some guesses on how weight is distributed across an object. In a recent study scientists looked at how people perceived the weight of a block made of two materials. A team led by Roland Fleming, a psychologist at the University of Giessen, created blocks composed of two halves that appeared to be made of materials with different densities and thus could be expected to have different weights: stone, wood or Styrofoam. The team asked people to lift a block made of two of these materials (such as stone paired with Styrofoam) and rate the relative weight of each side of the block.


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