The Flynn effect refers to a significant increase in raw IQ scores over the last 100 years or so -- equivalent of 30 points or more, in some cases. This raises a number of thorny issues. Were our ancestors idiots? Is IQ really that malleable under environmental influence (contrary to recent twin studies)? Is the principal component identified as "g" actually time dependent? See Flynn's answers below.
A couple of comments:
(i) Dramatic gains are seen only in certain areas of intelligence (see table from Sailer discussion), which are plausibly the areas in which modern life provides much more stimulation.
(ii) Average people 100 years ago were massively deprived by modern standards -- much more than we could ever reproduce in a modern twin study. US GDP per capita is 10x higher now, and average years of schooling has increased dramatically. It's not surprising that a child who only received (say) 6 years of formal schooling would be far behind someone with 12. Modern twin studies only include kids raised in a much smaller range of environments -- no twin in any recent study had less than the legally mandated US high school education. With a smaller range of environmental effects, the genetic component plays a larger role, leading to the high (.5-.7 or so) heritability result for IQ (similar to height).
(Flynn notes: In the America of 1900, adults had an average of about 7 years of schooling, a median of 6.5 years, and 25 percent had completed 4 years or less.)
(iii) The analogy with height is quite appropriate. While taller parents tend to have taller children (i.e., height is heritable), we've seen siginificant gains in average height as nutrition and diet have improved.
(iv) I think Flynn would agree that variance in adult IQ must have been much larger in the past. People like Newton or Thomas Jefferson obviously had tremendously more exposure to ideas and abstract thinking than a kid on the farm with little or no education, few books, no TV and no radio. The Flynn effect does not imply that the great geniuses of the past were necessarily inferior to those of today.
Lecture by Professor James Flynn at The Psychometrics Centre, Cambridge Assessment, University of Cambridge, 15th December 2006
Naming the paradoxes
(1) The factor analysis paradox: Factor analysis shows a first principal component called "g" or general intelligence that seems to bind performance on the various WISC subtests together. However, IQ gains over time show score gains on the WISC subtests occurring independently of one another. How can intelligence be both one and many?
(2) The intelligence paradox: If huge IQ gains are intelligence gains, why are we not stuck by the extraordinary subtlety of our children's conversation? Why do we not have to make allowances for the limitations of our parents? A difference of some 18 points in the average IQ over two generations ought to be highly visible.
(3) The MR paradox: In 1900, the average IQ scored against current norms was somewhere between 50 and 70. If IQ gains are in any sense real, we are driven to the absurd conclusion that a majority of our ancestors were mentally retarded.
(4) The identical twins paradox: Twin studies show that genes dominate individual differences in IQ and that environmental effects are feeble. IQ gains are so great as to signal the existence of environmental factors of enormous potency. How can environment be both so feeble and so potent?
The solutions in shorthand
(1) The WISC subtests measure a variety of cognitive skills that are functionally independent and responsive to changes in social priorities over time. The inter-correlations that engender "g" are binding only when comparing individuals within a static social context.
(2) Asking whether IQ gains are intelligence gains is the wrong question because it implies all or nothing cognitive progress. The 20th century has seen some cognitive skills make great gains, while others have been in the doldrums. To assess cognitive trends, we must dissect "intelligence" into solving mathematical problems, interpreting the great works of literature, finding on-the-spot solutions, assimilating the scientific world view, critical acumen, and wisdom.
(3) Our ancestors in 1900 were not mentally retarded. Their intelligence was anchored in everyday reality. We differ from them in that we can use abstractions and logic and the hypothetical to attack the formal problems that arise when science liberates thought from concrete referents. Since 1950, we have become more ingenious in going beyond previously learned rules to solve problems on the spot.
(4) At a given time, genetic differences between individuals (within a cohort) are dominant but only because they have hitched powerful environmental factors to their star. Trends over time (between cohorts) liberate environmental factors from the sway of genes and once unleashed, they can have a powerful cumulative effect.