For this experiment, you will need:

• Two cups of different sizes and similar weights, such as:
  (1) a large plastic party cup, and (2) a small paper or Styrofoam cup
• Waterproof marker, such as a Sharpie
• Source of water, such as a sink or a water-filled pitcher
• Kitchen scale (optional)

Preparing the experiment: On the inside of the smaller cup, about 1/4 of the way down from the top, draw a small horizontal line with the water-proof marker. Pour water into the small cup up to the marked line. Then pour the water from the smaller cup into the larger cup. Next, fill the smaller cup to the marked line once again, so that both cups contain the same amount of water.

For a more precise experiment, use a kitchen scale to weigh both water-filled cups. Slowly add water to the lighter cup until it exactly matches the weight of the other cup.

This first experiment can be done by yourself, though learning about it with a friend might be more fun! The instructions are straightforward:

Pick up both cups at the same time, one in each hand. While looking at the cups, answer just one question: Which cup feels heavier?

Extra #1: Try switching which hand each cup is in. With the cups switched, now which feels heavier?

Extra #2: Try looking away from the cups while you judge their relative weights, or perhaps close your eyes while deciding. Now which cup feels heavier? Is it as obvious of an answer to you as it was when you were looking at the cups?

Did you say that the smaller cup felt heavier? If so, you agree with 98% of people who do this or similar experiments.¹  If you said the larger cup was heavier, or that they felt the same, or that you couldn’t quite tell, that is okay too! You have your own perception of the world, and that is part of what makes you a unique individual. However, the fact, which you already know from preparing the experiment, is that the cups weigh the same amount (or very close to it). Many people experience the illusion that the smaller cup is heavier, even when they already know that the cups are the same weight. This illusion usually remains even when the cups are switched around, so hand-dominance does not seem to explain the illusion. But when you looked away from the cups or closed your eyes, you may have found that the illusion diminished or disappeared. What is going on here?

This illusion, called the size-weight illusion,^1,2 was first experimentally reported by Augustin Charpentier in the late 19th century.^3,4 Since then, scientists have come up with a variety of hypotheses as to why the illusion occurs. One hypothesis is that we anticipate that the larger cup will be heavier than the smaller one without even realizing that we are doing so. Because of this, the hypothesis continues, we might unconsciously use more muscle strength to lift that cup, resulting in a feeling of lightness when the larger cup is not as heavy as expected. You can test this hypothesis yourself with Experiment #2! This one is best done with the help of another person.

Set your hands on a table with your palms facing upward. Have the second person lift the same two cups from Experiment #1 and set one on each of your palms. While looking at the cups, answer: Which cup feels heavier? If you look away, or close your eyes, is your answer the same?

Many people report still feeling the size-weight illusion even when they are not actively lifting the cups. This experience is often referred to as the passive size-weight illusion. Because the illusion is still experienced even when your arm muscles are not being used to lift the cups, we may reject the hypothesis proposed above about using one arm’s muscles more than the other arm’s muscles. But, if a difference in muscle exertion is not what causes the illusion, then what is the cause?

This question is still a very active area of research among physiologists, psychologists, and neuroscientists. One of the most experimentally supported theories is that the illusion is strongly connected to our perceptions of our surroundings, and how our brains create these perceptions. In this explanation, our brains combine sensory information from holding the objects with expectations of the weights of the objects, based on our memories of larger objects being heavier than smaller ones. When our sensory results contradict our expectations, that surprise leads to an incorrect perception.⁵

We know the cups are the same weight. And it makes sense that we might expect the larger cup to be heavier. Yet, we perceive that the larger object is lighter than the smaller object. Interestingly, you can probably repeat the experiment several times with the same result, even once you know about the illusion! Thus, the expectation component of your perception is likely not a prediction you consciously make at the start of the experiment, but a subconscious result of previous experiences deeply ingrained into your mind.

What if you had to relearn everything about a task each time you had to perform it? Think about how long it takes for a baby to learn how to use their senses to walk, talk, and perform other complex tasks. It would probably be very difficult for us to go about our daily lives if we had no previous experiences helping us rapidly respond to the environment around us. However, as in this illusion, sometimes our natural reliance on expectations fails us. For example, have you ever picked up an object you expected to be heavy but was not, such as an empty suitcase? Perhaps you accidentally flung it up into the air because you used too much force to lift it. When such incidents do occur, however, we are usually able to quickly adjust using our sensory feedback.

How we perceive the weight of objects is critical to many activities, such as carrying groceries, doing yardwork, or throwing a football. You can imagine that those affected with disorders and diseases which impair their senses, memories, or integration of these information channels might have difficulty performing such everyday tasks. Deepening our scientific understanding of perception could lead to new therapies and treatments for such conditions, thereby improving the quality of life for many people.

Uncovering more about the perception of weight may also inform our understanding of other senses. Contrast effects, which occur across the five senses, are used by our brain to rescale our sensory ranges based on our current environment. For example, if you have been listening to something quiet (such as a gentle rain shower), and suddenly hear something loud (such as a thunderclap), you may find the loud noise very startling! In contrast, if you are in a loud room trying to listen to music with headphones, you may have to turn the volume up quite loud to hear the lyrics and instrumentals clearly. These contrast effects likely also play a role in how we discriminate the weight of different objects. Therefore, learning about these effects may deepen understanding of our perceptions in a variety of contexts.