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Several effects are thought to contribute to the Müller-Lyer illusion. The shafts of the two arrows above are equal, although the bottom looks longer. If the figures are interpreted in three dimensions, the upper image is seen as the outside edge of a box, while the lower image is seen as the inside edge of a box. Since it is assumed that the two "solid objects" rest on the same surface, the outside edge appears closer to the viewer than the inside edge. The more "distant" line is then interpreted as larger, to correct for the difference in apparent distance.
This figure shows a variant of the illusion which does not contain depth cues, but still shows an apparent difference in the length of the lines. It is difficult to completely ignore the surroundings of the test stimulus; research has show that the illusion is stronger for children, apparently because it is more difficult for them to ignore the extraneous parts of the stimulus. Other possible explanations are confounding the length of the entire figure with the length of the test line, or relying somewhat on the surrounding white space to define the figure (a larger figure would contain more white space.)
Another possibility is that the nearest parts of the two figures are compared. The four figures above are identical, but the smaller edge is assumed to belong to a smaller object.
One way to defeat the illusion is to fixate alternate endpoints of one line a few times, then the endpoints of the other, and continue switching for a few moments. The illusion is not conscious, so neither is the correction, but repeat testing shows the strength of the illusion to be reduced. The visual system programs saccades to direct the eyes to particular points; at first, these accord with the illusory difference in lengths, but the corrections required to fixate the endpoints accurately produces feedback which reduces the apparent difference in lengths.