Dear Bernardo, thanks for the neat thread topic. Thought you might be interested in a scientific study which found that more than 80% of people get the answer to the riddle wrong, whereas only 17% correctly conclude that the picture is of the persons son.
The study suggests some sort of "immediate memory overload" may be occuring with those who solve the problem incorrectly.
If you are interested, check out the abstract at:
http://www.ncbi.nlm.nih.gov/entrez/
query.fcgi?cmd=Retrieve&db=
PubMed&list_uids=8350742&dopt=Abstract
A previous suggestion by ella12866 could also explain some cases:
"I think the confusion of the riddle might be that someone is trying to figure out who the person is that is looking at the picture rather than trying to figure out who is in the picture."
However (as noted by others in this thread), many people have also been primed with the erroneous answer by sources which they would be less inclined to question or doubt, eg parents, peers, radio and TV programs.
In my own case, at the age of about eight, it took me less than a minute to work out that my Dad's explanation must be wrong. But over the years I heard exactly the same riddle and erroneous answer (picture of self/self in mirror) repeated again and again on radio, TV etc. Like you I was perplexed as to why so many people were adamant that the only logical answer could not be the correct one, yet when asked to explain why, could not do so clearly and convincincly and often resorted to ridicule to defend their viewpoint (as well demonstrated within this very thread).
I hasten to add that I don't think this necessarily reflects poorly on the intelligence of such people, who quite possibly excel in other areas that are of equal or greater relevance to the survival of the planet (God I hope so, as they are in the majority).
I'll be surprised if this thread starts up again. But you never know.