Preface: I exclude those irrational beliefs whose cause was the lack of technology, because there is nothing more to discuss if technology suffices to explain these irrational beliefs.
I ask this question because I wish to comprehend more deeply how and why intelligent people believe(d) irrational or stupid things. The most immediate example of an irrational belief is racism by the most eminent philosophers.
Evidently, some of these people must not have been 'stupid'.
Then the next insulting modifiers are probably: 'biased', 'ignorant', 'prejudiced'. But somehow, these Emotive Words or Question-Begging Epithets do not suffice to explain their irrationality.
Please also suggest books, if this question concerns a branch of philosophy on its own.