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For all the talk we hear about neuroplasticity, it seems IQ, at least as measured by the Wechsler, is incredibly stable.  According to a study by Erik Lykke Mortensen and his colleagues, there was an astonishing 0.89 correlation between WISC full-scale IQ measured at age 9.5, and WAIS full-scale IQ measured at 23.5 in a sample of 26 low birth-weight kids.  That’s absolutely colossal.  To put that in perspective, when a sample of 16-year-olds (n = 80) took the WAIS and WISC within an interval of one to six weeks, the correlation was 0.88 (WAIS-R manual, pg 48).

In other words, WISC IQ measured at age 9.5 predicts a young adult’s current WAIS IQ about as well as his WISC IQ measured a few weeks ago!

Wechsler IQ appears to be much more more stable than even height!  For example, the correlation between adult height and height at age 13 was 0.7 in a sample of Copenhagen men.

And given the moderate to high correlation IQ has with everything from lifetime income to occupational status, its long-term stability is even more compelling.  A psychologist can give a 9-yea-old an hour’s worth of silly games involving cartoon pictures, jig-saw puzzles, blocks, and funny riddles, and from that predict the trajectory of his life better than his teachers and parents.  They are modern day prophets.  The cartoon drawings of black children on the WISC-R are their tarot cards.  Indeed the South Asian woman who gave me the WISC-R at age 12 even dressed like a fortune teller.


And yet they’re also scientists.  Intelligence researchers were the ones who invented correlation, factor analysis, and other techniques scientists in all fields depend on, and they invented IQ tests, one of the single most stable, predictive, and fascinating measures science has ever seen.

Of course you can dismiss the study I cited above because the sample was not large and representativeness enough, but Steve Hsu independently reported similar data, from other tests:

From fig 4.7 in Eysenck‘s Structure and Measurement of Intelligence. This is using data in which the IQ was tested *three times* over the interval listed and the results averaged. A single measurement at age 5 would probably do worse than what is listed below. Unfortunately there are only 61 kids in the study.

age range       correlation with adult score

42,48,54 months               .55
5,6,7                               .85
8,9,10                             .87
11,12,13                          .95
14,15,16                          .95

The results do suggest that g is fixed pretty early and the challenge is actually in the measuring of it as opposed to secular changes that occur as the child grows up. That is consistent with the Fagan et al. paper cited above. But it doesn’t remove the uncertainty that a parent has over the eventual IQ of their kid when he/she is only 5 years old.

Another study of 141 adults found a near perfect 0.9 correlation between WAIS full-scale IQ measured at age 50 and 70 (a 20-year interval!).

Of course none of this conclusively proves IQ is as immutable as height or as solid as a rock.  It could be that society is forcing stability on the brain by giving mental stimulation only to those who show promise young.