In the comments of this earlier post a father wondered to what extent one can predict adult IQ from measurements at age 5. The answer is that predictive power is fairly weak — the correlation between a score obtained at 5 and the eventual adult score is probably no more than .5 or so. However, the main limitation seems to be unreliability of any single administration of the test to a child that young. Scores averaged over several administrations are a very good predictor already at a fairly young age. The average of three scores obtained at age 5, 6 and 7 correlates about .85 with adult score. This suggests that while it is difficult to measure a child’s IQ in any single sitting, the IQ itself is relatively fixed already by age 7 or so! Of course there are the usual caveats concerning range of environments, etc. I would like to see results from larger sample sizes.