"Gifted Today but not Tomorrow"
http://faculty.education.uiowa.edu/docs/dlohman/gifted_today.pdfmight be of interest.
"The term gifted implies a permanent superiority. However, the majority of children who
score in the top few percentiles on ability and achievement tests in one grade do not retain their
status for more than a year or two. The tendency of those with high scores on one occasion to obtain
somewhat lower scores on a later occasion is one example of regression to the mean. We first
summarize some of the basic facts about regression to the mean. We then discuss major causes of
regression: errors of measurement, individual differences in growth, changes in the content of the
developmental score scale, and changes in the norming population across age or grade cohorts. We
then show that year-to-year regression is substantial, even for highly reliable test scores. Different
ways of combining achievement and ability test scores to reduce regression effects are illustrated.
Implications for selection policies and research on giftedness are also discussed."
The point of this is not that the children are really gaining or losing IQ over time just that the testing instruments are fairly imprecise.
