Kevin Langdon is writing several books and designing an inside-out clock. Karyn Huntting Peters is organizing a global problem-solving network. Alfred Simpson juggles multiple Web-programming projects in his free time. These three people might not have much in common—except for their unusually high IQs.

All three belong to exclusive high-IQ societies. Mensa International, whose members' test scores must land above the 98th percentile (or one in 50), may be the most popular, but it is just one option for the discerning test taker. The Triple Nine Society demands an IQ in the 99.9th percentile, whereas the Mega Society cuts off at the 99.9999th percentile (one in one million). The memberless Grail Society claims to accept one in 100 billion people—no one has applied so far.

Although members of IQ societies do not fit a single profile, they often join in search of a sense of belonging. “I've heard a lot of people refer to themselves as aliens until they found a group like this,” says Peters, who serves as an officer of the Prometheus Society. “But get them all together, and they become extremely talkative—they're up until five in the morning.” Langdon, who has founded or co-founded several high-IQ societies and now edits the journal of the Mega Society, also reports having joined to find people more like himself.

Others do it for the challenge. “I joined Prometheus just to see if I could,” Simpson says. He now deals with the society's membership requests. He receives about two or three a week, although only three or four new members are accepted every year. The admissions process for these societies usually hinges on a score on an approved test, some of which have names such as Mega or Titan. Typical questions include analogies, the manipulation of complex three-dimensional shapes, or word problems that must be translated into equations.

Critics question whether IQ tests measure intelligence accurately, but “genius” societies rarely claim to represent true genius: the merging of intellect, creativity and outstanding achievement. Peters notes that her high-IQ colleagues are acutely aware of their own ignorance. Simpson concurs. “None of us consider ourselves a ‘genius,’” he says. “Well, maybe there's this one guy, but he hasn't been in the society for a while now.”