Page "Flynn effect" Paragraph 13

Another explanation is an increased familiarity of the general population with tests and testing.

For example, children who take the very same IQ test a second time usually gain five or six points.

However, this seems to set an upper limit on the effects of test sophistication.

One problem with this explanation and other related to the schooling is, as noted above, that in the US those subsets one would expect to be affected the most show the least increases.