One explanation for the Müller-Lyer illusion involves depth perception.
We are seeing depth based upon our experiences in a figure that does not have depth.
How does this happen? Look carefully at this figure.
Look at the corner that is close to you and the corner that is farther from you.
Do you see the different parts of the Müller-Lyer illusion? Let us help.
See the outlines around the two corners?
Now let us remove the rest of the image.
It is possible that we are seeing corners, one closer, and one farther.
If this is the case, our visual system makes more distant objects
like the line segment with the lines pointing inwards
look larger to compensate for the fact that otherwise it would look too small.
Since this illusion seems to depend upon seeing square structures
it seems that cultures that do not build buildings with corners are not as susceptible to this illusion.