When Eyes Weigh

Even our "direct" perceptions, such as of how much a box weighs, are greatly influenced by our expectations.  From a recent New Scientist:

Get hold of two cardboard boxes of different sizes and put a brick in each one. Check they weigh the same, then get somebody to lift them and tell you which is the heavier. The vast majority of people will say that the smaller box is heavier, even though it isn't, and will continue to maintain that it is even after looking inside both boxes and lifting them several times. … Curiously, experiments show that even though people initially use greater force to lift the larger box than the smaller one, on subsequent lifts they unconsciously equalise the amount of force they use to lift them. Despite their bodies apparently "knowing" that the boxes weigh the same, their minds still perceive the smaller box as being heavier. …

[Someone] showed that we can unlearn the size-weight illusion. He got volunteers to spend several days manipulating boxes that became lighter the larger they were. At the end of the process he found that their size-weight illusion was reversed. … This is good evidence that the illusion arises out of experience of the world, where larger objects tend to weigh more than smaller objects of the same kind.

This makes me more forgiving of people whose mistaken beliefs are contradicted by evidence right "before their eyes."  Our eyes don't see nearly as much as we'd like.

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  • http://profile.typepad.com/michaeljameswebster michael webster

    That is why we weigh things mechanically, because we know our senses cannot be relied upon.

    Unfortunately, we do not have access to the same type of machinery to weigh reasons when making deliberations.

  • Kenny Evitt

    That was rather poignant.

  • Grant

    Well, larger boxes are more difficult and unweildy to carry because of their size. It takes more muscle effort just to hold them, so its difficult to judge the weight relative to a smaller box. I can see people’s social minds thinking “more difficult = heavier”, but I’m not sure I’d call this a cognitive bias? It seems to me its a good heuristic when estimation is difficult and there are no rewards for being correct.

  • Venu

    Hmm.. I wonder if the left brain is to be blamed for this. May be squirting cold water into the subject’s left ear canal before they compare the weights will reduce/remove the error?

  • http://profile.typepad.com/6p00e551b9414f8833 Psy-Kosh

    Hrm… Well, there’s also other effects that may have more to do with physics and part of how we “feel” the mass of something rather than our eyes. Specifically, large box with small concentration of mass inside means effectively more torque available to us when tilting it. In other words, feels easier to tilt, feels more wobbly, etc.

    This may have something to do with it too, right?

    The bit about identical boxes labled with “heavy” and “light”, though, well, I got nothing as far as mechanical explanations.

  • http://williambswift.blogspot.com/ billswift

    Grant – the original perception was the **smaller** box was heavier.

    I noticed this a long time ago. When I need to compare the weights of two items and don’t have a device available to do it, I always close my eyes and concentrate on my kinesthetic feedback alone.

  • http://profile.typepad.com/robinhanson Robin Hanson

    So anyone notice that the last line of the quote makes no sense?

  • ducktator

    Yes, the last part of the quote does not make sense, at least for me.

  • Marshall

    I think it actually does make sense, Robin (and I didn’t notice anything untoward). It makes sense in the context of the next to last sentence and the reverse-illusion experience.
    Our real world experience says a smaller box will weigh less. We experience by comparison, that the smaller box is heavyier than expected – we have to use more effort than expected – and this becomes translated to “heavyier”.

    The reverse-experiment gives the expectation and life-experience that the larger box should be lighter – thus more musclestrength has to be used to overcompensate – giving the perception that the larger box is heavier.

    As I understand it, that is what the last sentence i saying:

    “This is good evidence that the illusion arises out of experience of the world, where larger objects tend to weigh more than smaller objects of the same kind.”

  • Zac

    I’d like to try this experiment myself. I’m rather confident that once I saw that the same brick was in both boxes (maybe I would switch the bricks, to add confidence) I would agree that the boxes weighed the same. I would be even more confident if they were placed on a working scale and shown to weigh the same. Consequently, this makes me no more forgiving of people whose mistaken beliefs are contradicted by the physical evidence.

  • http://silasx.blogspot.com Silas Barta

    Looks like the experiment is tricking people into violating the law of conservation of probability by “explaining” contradictory outcomes. Do I hear a “That’s impossible!”? 😉

  • Grant

    Oh, heh. After reading the last line of the quote I thought the study indicated the larger boxes were thought to be heavier.

    Either way, I think you need to equalize the carrying mechanism. Dumbbells might work well, since they all have equally-sized handles. Just use two dumbbells of different sizes but the same weight.

  • http://lpetr.org/blog/ Leo Petr

    Maybe it’s just that our brain confuses weight with density. The smaller box is the same weight, but it’s denser, which leads to a perceptual difference.

  • ducktator

    Possible explanation for the first part of the quote:
    The weight of the brick is distributed over a larger surface in the big box and on a smaller surface in the small box therefore the perception of pressure on the hand (which for the big box has the fingers fanned out to be able to hold it!!) is stronger for the small box => seems heavier initially.

  • http://profile.typepad.com/6p01156f67d182970b Shalmanese

    Could it just be that our intuitive notion of “heavy” is related to density and not mass?

  • http://williambswift.blogspot.com/ billswift

    Most of the commenters’ explanations don’t work. As I wrote earlier, I have sometimes had to compare items of different sizes – if it was distribution of weight over area of hand or density being read as weight, I would not be able to feel equal weights by closing my eyes. And zac, back when I was a teenager and first noticed this effect, I experimented using known weights – it does not change the “feel” that the weights are different even when you **know** the weights are the same, it’s similar to optical illusions that way.

  • jedermann

    This makes me wonder what the subjects in the Asch experiments actually saw. I always figured that the subjects knew the right answer, but (on the back of a cost/benefit calculation) went along with the group for the sake of maintaining group harmony. Presumably the conformist pressures in Asch’s experiments could be just as powerful as the size of the boxes in determining what we actually see.

    http://en.wikipedia.org/wiki/Asch_conformity_experiments

  • http://silasx.blogspot.com Silas Barta

    @billswift: And zac, back when I was a teenager and first noticed this effect, I experimented using known weights – it does not change the “feel” that the weights are different even when you **know** the weights are the same, it’s similar to optical illusions that way.

    Interesting. It’s true — I know of some optical illusions where something will not appear right to your eyes after you first see the illusion, and will *still* appear wrong even after you delete everything that’s supposedly causing the illusion. Case in point: the different-color squares illusion.

    I tried to eliminate the illusion by deleting the cylinder and the shadow, but the squres *still* looked different to me. Nothing short of touching them together would shatter that illusion.