The distinction between 150 (barely recognizable as a validated score on current tests) and 180 (NOT a validated score on any test, ever) is less meaningful than many suppose:

"Put into the context of the psychometric movement as a whole, it is clear that positive extreme of the IQ distribution is not as different from other IQ levels as might have been expected. . . . While 180 IQ suggests the ability to do academic work with relative ease, it does not signify a qualitatively different organization of mind. It also does not suggest the presence of ‘genius’ in its common-sense meaning, i.e. transcendent achievement in some field. For these kinds of phenomena, IQ seems at best a crude predictor. For anything more, we will have to look to traditions other than the psychometric and to variables other than IQ."

Feldman, David (1984). A Follow-up of Subjects Scoring above 180 IQ in Terman's Genetic Studies of Genius. Exceptional Children, 50, 6, 518-523.

IQ scores, especially in childhood, vary over the course of a test-taker’s life, sometimes varying radically. Deviation IQ scoring was originally developed to make for more stability of scores over the course of childhood. Nonetheless, deviation IQs for children can also change considerably over the course of childhood (Pinneau 1961; Truch 1993, page 78; Howe 1998; Deary 2000, table 1.3). "Correlation studies of test scores provide actuarial data, applicable to group predictions. . . . Studies of individuals, on the other hand, may reveal large upward or downward shifts in test scores." (Anastasi & Urbina 1997 p. 326).

For example, young people in the famous Lewis Terman longitudinal Genetic Studies of Genius (initial n=1,444 with n=643 in main study group) when tested at high school age (n=503) were found to have dropped 9 IQ points on average in Stanford-Binet IQ. More than two dozen children dropped by 15 IQ points and six by 25 points or more. Parents of those children reported no changes in their children or even that their children were getting brighter (Shurkin 1992, pp. 89-90). Terman observed a similar drop in IQ scores in his study group upon adult IQ testing (Shurkin 1992, pp. 147-150). Samuel R. Pinneau conducted a thorough review of the Berkeley Growth Study (1928-1946; initial n=61, n after eighteen years =40). Alice Moriarty was a Ph.D. researcher at the Menninger Foundation and describes in her book (1966) a number of case studies of longitudinal observations of children's IQ. She observed several subjects whose childhood IQ varied markedly over the course of childhood, and develops hypotheses about why those IQ changes occurred. Anastasi and Urbina (1997, p. 328) point out that childhood IQ scores are poorest at predicting subsequent IQ scores when taken at preschool age. Change in scores over the course of childhood shows that there are powerful environmental effects on IQ (Anastasi & Urbina 1997, p. 327) or perhaps that IQ scores in childhood are not reliable estimates of a child’s scholastic ability.

REFERENCES

Anastasi, Anne & Urbina, Susana (1997). Psychological Testing. Upper Saddle River, NJ: Prentice Hall.

Deary, Ian J. (2000) Looking Down on Human Intelligence: From Psychometrics to the Brain. Oxford: Oxford University Press.

Howe, Michael J. A. (1998). Can IQ Change?. The Psychologist, February 1998 pages 69-72.

Moriarty, Alice E. (1966). Constancy and IQ Change: A Clinical View of Relationships between Tested IQ and Personality. Springfield, IL: Charles C. Thomas.

Pinneau, Samuel R. (1961). Changes in Intelligence Quotient Infancy to Maturity: New Insights from the Berkeley Growth Study with Implications for the Stanford-Binet Scales and Applications to Professional Practice. Boston: Houghton Mifflin.

Shurkin, Joel N. (1992). Terman's Kids: The Groundbreaking Study of How the Gifted Grow Up. Boston: Little, Brown.

Truch, Steve (1993). The WISC-III(R) Companion: A Guide to Interpretation and Educational Intervention. Austin, TX: Pro-Ed.