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In my previous article, I described two studies showing Wechsler stability coefficients of 0.89 and 0.9 in young and older people tested 14 and 20 years apart respectively.

In the interest of full-disclosure, I found a study of Wechsler IQ that did not show such sky high long-term stability: 24 men and 24 women aged 39 to 44 were given the WAIS circa 1969 after having already taken it in 1956.  The correlation was 0.73.

stability

Of course the sample suffered from range restriction, with an IQ variability much smaller than the general U.S. population’s.  This probably explains why the stability was only high instead of sky high.

stability2

There was also a study as cited by Ian Deary et al. that found a 0.78 correlation between Wechsler IQ measured at age 2 and at age 15 (incredible considering IQ is thought to be unstable before age 10 and especially before age 6), though oddly, only a 0.47 correlation between Wechsler IQ at age 9 and 15.

stability3

 

So for all studies I could find, comparing Wechsler IQ in people tested at least 13 years apart, the correlations are 0.73, 0.78, 0.88, and 0.9 with a median of 0.83, which is probably an underestimate given that most studies suffer from range restriction.

If anyone knows of any other studies of long-term Wechsler IQ stability, please let me know in the comments.