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One explanation for the Müller-Lyer illusion involves depth perception.

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We are seeing depth based upon our experiences in a figure that does not have depth.

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How does this happen?  Look carefully at this figure.

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Look at the corner that is close to you and the corner that is farther from you.

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Do you see the different parts of the Müller-Lyer illusion?  Let us help.

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See the outlines around the two corners?

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Now let us remove the rest of the image.

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It is possible that we are seeing corners, one closer, and one farther.

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If this is the case, our visual system makes more distant objects

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like the line segment with the lines pointing inwards

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look larger to compensate for the fact that otherwise it would look too small.

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Since this illusion seems to depend upon seeing square structures

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it seems that cultures that do not build buildings with corners are not as susceptible to this illusion.