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.

But here’s the trick: both halves of the block actually had the same weight. Before the experiment, the scientists had secretly carved out the inside of the block and filled the cavity with lead to create an even weight distribution. The scientists wanted to know whether people would be tricked by the material-weight illusion and report that the lighter-looking side of the block felt heavier. The results took the scientists by surprise: When handling these two-material blocks, people said that the heavier-looking side felt heavier. That is the opposite of what the scientists expected based on what they knew about the material-weight illusion for uniform objects.

So what causes people to perceive weight differently in some situations?

Fleming’s team says it’s all about context. In traditional experiments with objects made of one material, the brain makes a guess based on its prior knowledge and compares its guess with the actual weight that the body feels when lifting each object. If the body’s experience contradicts the brain’s guess, the brain cannot reconcile its prediction with the actual weight, so it throws the body’s observation out. The brain assumes that particular Styrofoam block must just be especially heavy—so heavy, in fact, that the brain thinks it’s heavier than a stone block, even if the blocks actually have the same weight.

But Fleming and his colleagues think that when people focus on the weight distribution of an object made of two materials, the brain calculates weight in another way. Instead of throwing the body’s observation out, the brain concludes that each side’s weight lies between the initial guess and the actual weight. If the brain expects stone to feel heavier than Styrofoam but the two actually have equal weights, the brain combines these two pieces of information and decides that the stone side still feels heavier, although not as heavy as originally thought.

The scientists reason that the brain does this because it disagrees with the body differently in the two situations. When a person lifts a deceptively heavy object, the brain and body are in conflict because the brain is not equally good at using what the body feels to estimate weight and weight distribution. This variation in accuracy between the uniform-weight and weight-distribution tasks causes people to perceive weight differently in the two situations.

Knowing how the brain estimates weight isn’t just an interesting experiment—it can actually help scientists develop smarter technologies that we use every day. Now that we know more about how context changes the brain’s decisions, programmers might be able to update technologies such as Netflix to imitate the brain more accurately and provide more fine-tuned recommendations for users. Netflix already recommends new shows on the basis of what users have watched previously, but now developers might be able to tell the algorithm to weigh several situational factors when deciding which show to recommend next.

For example, Netflix could consider factors such as the time of year and how much time has passed since your last viewing session to assess your viewing habits more reliably. This means less time browsing and more time enjoying a new show that fits your preferences.1