An alternative explanation of 
the Müller-Lyer figure is based 
on the incorrect-comparison 
theory. It holds that we cannot 
succeed fully in isolating parts 
from wholes. Despite a clear 
understanding of what parts of 
the line are to be compared, we 
cannot avoid including other 
components in our judgments. 
In the Müller-Lyer figure, we 
intend to compare only the 
shafts, but we include the 
arrowheads. The shaft plus the 
reversed arrowheads is, of 
course, longer.

    The incorrect-comparison 
theory can be restated as an 
assimilation process: The shaft 
within the reversed 
arrowheads assimilates to the 
arrowheads and thus its length 
is perceived as more like the 
overall length of the pattern. It 
is not obvious, however, what 
accounts for the presumed 
shrinking effect on the shaft 
with the normal arrowheads. 
Experiments have tested each 
type of figure separately, using 
an isolated line as a 
comparison figure. The shaft 
with the normal arrowheads 
appears only slightly shorter 
than an isolated line of equal 
length. The greater part of the 
illusion comes from the shaft 
with the reversed arrowheads. 
Therefore, an assimilation 
process is a plausible 
explanation of much of the 
Müller-Lyer effect.