The most one can safely 
conclude from the 
developmental and 
crosscultural studies currently 
available is that the depth-
processing theory is not 
unequivocally supported by 
them. With the possible 
exception of the Ponzo 
illusion, the developmental 
data and the crosscultural data 
are not consistent with one 
another in supporting this 
theory. The tendency of 
children by and large to 
perceive illusions of greater 
magnitude than adults do can 
perhaps best be explained by 
their inability to isolate the 
test lines from the inducing 
ones in which they are 
embedded. An illusory effect 
based on difficulty of this kind 
is consistent with the 
incorrect-comparison theory.

    A possible explanation of 
cross-cultural differences that 
is quite unlike one based on 
differing visual environments 
is that of innate differences in 
the structure of components of 
the nervous system. 
Differences in such factors as 
density of pigmentation of the 
lens and the central region of 
the retina, the macula, are 
even more highly correlated 
with differences in 
susceptibility to illusions than 
are differences in the physical 
environment. These 
physiological factors, if proven 
to be relevant, are rather 
different from the kinds of 
explanations of the illusions 
explored in this chapter. 
Perhaps there are other as yet 
undiscovered inherited 
differences in the structure 
and function of the perceptual 
system that might explain the 
differences in susceptibility to 
the various illusions among 
peoples of various societies in 
the world.