Cosma Shalizi reviews James R. Flynn's What is Intelligence? Beyond the Flynn Effect, in American Scientist:
On average, measured IQ has been rising at roughly 3 points per decade across the industrialized world for as far back as the data go. This means that someone who got a score of 100 on an IQ test in 1900 would get a score of only 70 for the same answers in 2000. This is the Flynn effect.
Flynn easily swats down some proposed explanations for the effect. It is too large, too widespread and too steady to be due to improved nutrition, greater familiarity with IQ tests or hybrid vigor from mixing previously isolated populations. (Nobody seems to have suggested that modern societies have natural or sexual selection for higher IQ, but the numbers wouldn’t add up in any case.) So either our ancestors of a century ago were astonishingly stupid, or IQ tests measure intelligence badly.
Flynn contends that our ancestors were no dumber than we are; rather, most of them used their minds in different ways than we do, ways to which IQ tests are more or less insensitive. That is to say, we have become increasingly skilled at the uses of intelligence that IQ tests do catch. Although he doesn’t put it this way, Flynn thinks that IQ tests are massively culturally biased, and that the culture they favor has been imposed on the populations of the developed countries (and, increasingly, the rest of the world) through cultural imperialism and social engineering.