I am slightly embarrassed to tell you what is in these suitcases… but I will. Books. More than five or ten. More than a dozen but not dozens. Twenty-one to be exact. Some people have security blankets. I have books. I am very uncomfortable being without certain books for even a short period of time. Rather than confront my discomfort, I tote books around with me.

Books are heavy. In an effort to not exceed the magical 50-pound mark, I split the books between two suitcases. The result was that both suitcases were heavy. To be fair, the little suitcase also had four unpolished Petoskey stones that my cousin gave me. Petoskey stones are a whole other megillah aka rabbit hole. I will leave it at showing you how beautiful these pieces of fossilized corral become when polished (in this case by my talented and patient cousin):

My hope and dream is that my four to-be-polished Petoskey stones will one day look as beautiful as the two above.

[Correction on August 16, 2022: My cousin corrected me that the stone on the left is a Charlevoix stone and the one on the right is indeed a Petoskey stone. Both stones are fossilized coral. The type of coral fossilized in the Petoskey stone has a central fossilized tentacle within each polyp. The Charlevoix stone does not and thus has a smaller, tighter pattern. See the Charlevoix tourist page for more detail. You can hunt for both types of stones on the shores of Lake Michigan.]

To return to the story at hand, the small black suitcase clocked in at 29.0 pounds. The large flowery patterned suitcase weighed 35.5 pounds. I believe these weights are accurate. Taking a page from Auden’s inner scientist, I measured the weight of each suitcase multiple times on multiple instruments. Two airport scales and a bathroom scale. A total of four times (twice on one of the airport scales).

“All the instruments agree the day of his death was a dark cold day”

– In Memory of WB Yeats by WH Auden

I belabor my measurement methods and results to convey my confidence, and urge you to share that confidence, that the larger suitcase is heavier than the smaller one. By 6.5 pounds to be exact.

A Lyft ride with two suitcases

Now I take you to the scene at 6:15 am yesterday morning. With my backpack on my back, I drag behind me the two suitcases pictured at top. Down the elevator, out the door and to the curb. As promised by Lyft, James pulls up in a black Audi. Seeing my large suitcases, he pops the trunk, and comes out to load my bags. I let him.

Picking up the black suitcase, Jaye (as I subsequently learn he prefers to be called) says “This is heavy.”

“Yes, I have an embarrassing number of books in there.”

After laying the black suitcase down, he picks up the larger flowered suitcase and places it next to the black one. I say, “That one is even heavier.”

“Oh, no.” Jaye lifts each one up in turn a few times. “I know this [little] one is heavier. I can feel it.”

I try one more time. “I weighed them. That little one is 29 pounds and the big one is 35 pounds.”

Jaye feels each one twice more, switching back and forth. “No, I can tell. This little one is heavier.” He shakes his head, “I am sure. Trust me, the little one is heavier.”

Neurobiology to the explanatory rescue

Jaye is right that his brain interprets the small 29-pound suitcase as heavier than the large 35.5-pound one. But he is wrong that the small suitcase weighs more than the big one.

Why did Jaye and why would virtually everyone else perceive the small suitcase to be heavier than the large one? Because of our learned experience of density. The size-weight illusion, known for 150 years, refers to our consistent perception that the smaller of two objects of the same mass is heavier than the larger object. Here is the logic:

• We see two objects
• We unconsciously assume they have the same density
• Since the two objects have the same mass, the smaller one has a greater density than the larger one
• The one with greater density is interpreted as heavier.

The assumption that two objects have the same mass likely worked well through evolutionary time. Imagine someone picking up two chickens, two children, two stones, two pieces of wood, two watermelons. Of course they have the same density. The iso-density (iso=same) assumption still works most of the time. Under normal circumstances, two suitcases packed with what people typically pack – clothes – have the same density.

The same-density assumption failed in my case for two reasons. The big suitcase is as flimsy as they come and weighs very little when empty whereas the small suitcase is from the ’50s or ’60s and is super sturdy, quite hefty all on its own. The second reason is that, in recognition of the far sturdier makeup of the small suitcase, I loaded it up with the Petoskey stones and with a majority of the books. Consequently the density was quite a bit higher.

Even though we make the perceptual error of believing that the lighter-but-higher-density-object is heavier than the heavier-and-lower-density-object, we do not make the same error in our motor actions. In other words, as Jaye lifted the two suitcases, he used the right amount of force for the mass (not the density) of each suitcase. I know this because neither suitcase fell or hit hard due to an error in his grip. More formally, Grandy and Westwood (2006) showed that people used stronger forces to lift larger, heavier objects than smaller, lighter objects despite concurrently perceiving the larger, heavier objects to be lighter than the smaller, lighter objects. The authors write, “This conclusion flies in the face of the common intuition that one’s actions are guided by one’s perception of the target object’s features.”

The take-home message is that we are not scales any more than we are cameras or tape-recorders. Our perceptions have worked well enough for our ancestors to make it through evolutionary time to today. However, our perceptions do not recapitulate external reality. In contrast our motor systems functions in recognition of what is actually out there, not what we perceive to be out there. Action over perception for accuracy every day, all day.

Deep but not innate illusion

I wasn’t surprised at Jaye’s reaction because even though I fully knew (and accepted) the weights of the suitcases, I also experienced the smaller lighter one as heavier. Knowledge does not alter the illusion. This is true of most good illusions.

While we may not be scales, we evidently can calculate density, the physicist within us. But this ability is unlikely to be innate. In fact, I would predict that someone who has no experience with weight would not have a concept (conscious or unconscious) of density and would not show the size-weight illusion. My friend Kim is just one such person. People who lack a functional PIEZO2 gene do not feel touch or vibration and may also be unable to develop a concept of density. So except for one Kim and maybe two dozen people without the PIEZO2 gene, the size-weight illusion is a perceptual reality for the billions of the rest of us.