IQ Yo-Yo:

Test changes alter retardation diagnoses: “Since average scores on particular IQ tests rise a few points every 3 or 4 years, those tests become obsolete after a couple of decades. In order to reset the average score to 100, harder IQ tests are devised every 15 to 20 years.

Trickier tests have no practical impact on people who score within the normal IQ range of 90 to 110. But so-called renormed IQ tests create a yo-yo effect in the number of mental retardation placements in U.S. schools, a new study finds.

Rates of mental retardation among children appear to bottom out near the end of a particular test’s run, followed by a sharp rebound with the introduction of a tougher test, say Tomoe Kanaya, a graduate student at Cornell University, and her colleagues. Scores on the new test then increase over time, pulling many children from just below to just above the score of 70, which stands as the rough cutoff for mental retardation. That trend continues until the next test revision comes along.” —Science News