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A high IQ society is an organization that limits its membership to people who are within a certain high percentile of IQ test results. The oldest, largest and best-known such society is Mensa International, which was founded by Roland Berrill and Dr. Lancelot Ware in 1946. Other early societies are Intertel, founded by Ralph Haines in 1966; the International Society for Philosophical Enquiry, founded by Dr. Christopher Harding in 1974; the Triple Nine Society in 1978; the Prometheus Society, and the Mega Society.
High IQ societies typically accept a variety of IQ tests for membership eligibility, with some of the tests being tests devised by the organization founders and not validated by psychologists.
The highest reported standard score for most IQ tests is IQ 160, approximately the 99.997th percentile (leaving aside the issue of the considerable error in measurement at that level of IQ on any IQ test). IQ scores above this level are dubious as there are insufficient normative cases upon which to base a statistically justified rank-ordering. High IQ scores are less reliable than IQ scores nearer to the population median.
The entrance criteria for IQ societies vary considerably across both the kinds of tests accepted (for example, whether the tests tap primarily numerical, spatial, or verbal abilities, or whether the tests have adequate test security or not) and how high one must score in order to acquire membership.
Some societies, including widely known societies such as Mensa, accept the results of standardized tests taken elsewhere. Those are listed below by selectivity percentile (assuming the now-standard definition of IQ as a standard score with a median of 100 and a standard deviation of 15 IQ points).
norm tables that provide you with such extreme values are constructed on the basis of random extrapolation and smoothing but not on the basis of empirical data of representative samples.
[Curve-fitting] is just one of the reasons to be suspicious of reported IQ scores much higher than 160
The concerns associated with SEMs [standard errors of measurement] are actually substantially worse for scores at the extremes of the distribution, especially when scores approach the maximum possible on a test . . . when students answer most of the items correctly. In these cases, errors of measurement for scale scores will increase substantially at the extremes of the distribution. Commonly the SEM is from two to four times larger for very high scores than for scores near the mean (Lord, 1980).