Flynn wrote a book devoted to the Flynn effect, featuring his theoretical explanation of why the intelligence of worldwide populations has apparently increased from generation to generation. The essence of his theorizing is that because of the societal impact of scientific technology, people of today are much more guided by abstract, rather than concrete, approaches to problem solving. He bases his theory in large part on gains on specific tasks, most notably Raven's matrices, Wechsler Intelligence Scale for Children (WISC) Performance subtests, and, most important, on WISC Similarities. The gains on these separate tasks over more than half a century (1947-2002) are striking. However, Flynn failed to take into account the sweeping changes in test content, administration procedures, and scoring guidelines when the 1974 WISC-R was developed from the 1949 WISC. These substantial changes challenge the meaningfulness of comparing children's performance in 1947 with their performance in 2002 on Similarities and other WISC subtests--and therefore challenge Flynn's explanation of the effect that bears his name. (Contains 2 tables and 1 note.)