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Commenter illuminaticatblog was kind enough to share with us his intelligence test scores at age 12 and at age 26. Both times he took the Wechsler intelligence scales: WISC-III at age 12; WAIS-IV at age 26.

Below is a chart comparing results at both ages. I wanted to compare apples to apples so I only included the subtests that were administered on both occasions, and used only these to calculate his verbal, performance, and full-scale IQs respectively (prorating when required). I did not adjust for the Flynn effect so scores at both ages are likely slightly inflated, but to similar degrees.

  Wechsler IQ equivalent at age 12 Wechsler IQ equivalent at age 26
VERBAL ABILITIES 125 (very bright) 133 (brilliant)*
Information (general knowledge) 130 (brilliant) 140 (very brilliant)
Similarities (verbal abstract reasoning) 110 (bright) 120 (very bright)
Arithmetic (mental math) 110 (bight) 125 (very bright)
Vocabulary (word knowledge) 125 (very bright) 120 (very bright)
Comprehension (common sense & social judgement) 130 (brilliant) 115 (bright)
NON-VERBAL VISUAL-MOTOR ABILITIES 113 (bright) 99 (average)*
Picture Completion (visual alertness) 125 (very bright) 80 (dull)
Block Design (visual organization) 115 (bright) 125 (very bright)
Digit Symbol (Rapid hand-eye coordination) 90 (average) 95 (average)
OVERALL GLOBAL INTELLECTUAL ABILITY 123 (very bright) 121 (very bright)

The first thing we notice is how remarkably consistent the overall IQ is from age 12 to 26, declining by only 2 points over those 14 years, despite the incredible amount of drama the he endured over that time.

This remarkable consistency is not surprising, as the long-term stability (over 13+ years) of Wechsler IQ is in the 0.73 to 0.9 range.

Also consistent is his verbal IQ > Performance IQ gap, though this nearly triples from 12 points at age 12 to 34 points at age 26.

At the subtest level, we see a lot less consistency than we observe with the overall score. This is not surprising because individual subtests are a lot less reliable than a composite score that combines eight different subtests (allowing error in both directions to cancel out).

Given the unreliability of individual subtests and the number of subtests, it’s statistically expected to see a few big changes and one shouldn’t over-interpret this. However the 45 point drop on Picture Completion is concerning.

Picture Completion tests one of the most important parts of intelligence because visual awareness to our environment is crucial to our ability to adapt. A close friend of mine scored low on this particular subtest despite being otherwise quite bright and I was shocked when he had driven himself to my remote winter cottage on a deflated tire.

“Did you not notice one side of the car is way lower than the other?” I asked.

No he had not. He’s extremely lucky it didn’t go flat as he was driving up there, otherwise he would have found himself stranded on an unpaved deserted forest road with no cell phone reception in the pitch blackness of a cold Canadian night.

*Because he took the WAIS-IV as an adult, which does not allow for verbal or performance IQs, I had to convert using the WAIS-III.