Another possible account for the lack of illusory effect in
baboons might consider the properties of the stimuli, especially
color differences between the target and inducers. Use of different
colors to display the target and inducers reduces the magnitude of
the Ebbinghaus illusion in humans (Rabin & Adams, 1993). Although
such effects could account for an overall reduced illusory
strength in both species, it is of no help to explain why humans
continued to demonstrate the illusion with our stimuli whereas
baboons did not.