Everybody know this one. They only appear to be different in length. But actually, did you know that not to everyone they appeared so? There is small percentage of modern people which do not get cought by this illusion, and they actually seas those lines as they are. There aren’t many of them left on earth, but still. And the reason is not because they are diffrent species, but simply because they are buchmens and primitive tribes which are now catching up with modern culture and western style of live. So, the conclusion is that something about our environment changes our perception of reality to such extend, that now literally everybody seas this lines as different length.
There is two additional questions to this phenomena. First, what else is changed with our perception of the world along with our changes in environement. Second, why we thought for so long that these illusions (and many others) are so universal across human, what psychological assumptions are also wrong and where was the mistake we overlooked?
To make long story short, there is one very important and troubling aspect of psychological research that emerged lately and questioned years of studies. That is, overwhelmingly majority of studies are based on very tiny population of people – namely- american students. A little more was done on a little larger population – americans in all ages. But is it justify to that these “standard subjects” are as representative of the species as any other population. It turned out that it’s not, and you can read more about it here and here.