It was not on the whole, a happy experience.  I met a number of wonderful Mensans, but there were other Mensans who were brain-proud and aggressive about their IQs, who, one got the impression, would like, on being introduced, to be able to say, ’I’m Joe Doakes, and my IQ is 172,’ or, perhaps, have the figure tattooed on their forehead.  They were, as I had been in my youth, forcing their intelligence on unwilling victims.  In general, too, they felt underappreciated and undersuccessful.  As a result, they had soured on the Universe and tended to be disagreeable.

     What’s more, they were constantly jousting with each other, testing their intelligence on each other, and that sort of thing becomes wearing after awhile.

     Furthermore, I became uncomfortably aware that Mensans, however high their paper IQ might be, were likely to be as irrational as anyone else.  Many of them believed themselves to be part of a ‘superior’ group that ought to rule the world, and despised non-Mensans as inferiors.  Naturally, they tended to be right-wing conservatives, and I generally feel terribly out of sympathy with such views.

     Worse yet, there were groups among them, I found out eventually, who accepted astrology and many other pseudoscientific beliefs, and who formed ‘SIGs’ (‘special interest groups’) devoted to different varieties of intellectual trash.  Where was the credit of being associated with that sort of thing, even tangentially?

     ... I stayed on in Mensa for years, getting more and more tired of it. ... Eventually, after both Marvin and Margot [two of the New York Mensans he acknowledged as "delightful and intelligent"] had died, I did resign.

               - Isaac Asimov, I. ASIMOV: A MEMOIR