It’s pretty obvious to most people that the two shoes in this picture are the same size:
But if you actually took a ruler to your computer display it would show that the image of my right shoe is nearly twice as wide as the image of my left shoe. Young children will mistakenly say that the real left shoe is smaller than the right shoe; it’s only gradually that we learn to take the context of the photo into account and conclude that the shoes are the same size: the shoes appear to be a matched set; the left shoe is a bit out of focus, the railing to the left shows that the left shoe is farther away than the right shoe; we know more distant objects tend to appear smaller, and so we determine that they are the same size. Of course, it’s also relatively easy to fool the perceptual system:
In an Ames room, the height of the walls, the pattern on the floor, and the size of the doors is varied so that our usual visual cues don’t work. We’re fooled into thinking the person on the left is a giant, while the person on the right seems tiny (in fact she’s simply farther away than she appears).
If we can be so easily fooled, it might make you wonder: What, exactly, are we learning about the sizes of distant objects as we grow older? The prevailing notion for decades has been that we gradually learn cues to the size of objects and how far away they are. By the time they are ten, most kids are about as good at judging the size of distant objects as adults — they are said to have achieved “size constancy.” But recently Carl Granrud has begun to challenge that notion. Even in normal perceptual circumstances (in an open field, say, rather than an Ames room), adults make systematic errors, overestimating the size of distant objects. Granrud believes that the perceptual system simply doesn’t give us enough information to accurately judge the size of distant objects, and so we develop a system of compensation. While young kids undercompensate, adults often overcompensate.
Granrud asked kids ranging from age 5 to 10 to judge the size of white disks mounted on sticks in a field, either 6.1 meters away or 61 meters away. The children stood next to nine reference disks, and after each disk was shown to them, they pointed to the reference disk that matched its size. As expected, all the kids were accurate judging the size of the nearby disks, and older kids were better than younger kids judging the size of the faraway disks.
But Granrud didn’t stop there. He also tested each child on her or his knowledge of basic perceptual rules for judging size and distance of objects. For example, they might be shown a photo like the picture of my shoes at the start of this post, and asked whether the shoes were the same size or different, or like this picture of Nora “holding up” the leaning tower of Pisa:
Kids who recognized that pictures like this are just tricks got credit for a correct response. The responses were divided into high- and low-scoring groups. This graph shows how high-scoring kids compared to low-scoring kids in each age group:
As you can see, when kids did better on the test of visual rules, they were also more accurate judging the size of the distant disks, regardless of their age. It appears that these rules can be learned by children as young as five. Granrud says this supports his contention that size constancy is a matter of compensating for the apparent distance of an object. When we see a far-away object, we make an estimate of its distance, and based on that knowledge, we make an estimate of its size.
This also makes it very clear why the Ames room is so effective: if we can’t make a reasonable estimate of the distance to a person, it’s nearly impossible to judge her size, and so we assume that we’re seeing a giant!
Granrud, C. (2009). Development of size constancy in children: A test of the metacognitive theory Attention, Perception & Psychophysics, 71 (3), 644-654 DOI: 10.3758/app.71.3.644