1) Scores have increased the most on the problem-solving portion of intelligence tests.The reason why IQ test results are increasing is a mostly unresolved question and presents some paradoxes described by Flynn and summarized here.
2) Verbal intelligence has remained relatively flat, while non-verbal scores continue to rise.
3) Performance gains have occurred across all age groups.
4) The rise in scores exists primarily on those tests with content that does not appear to be easily learned.
The intelligence paradox: The Flynn Effect suggests that we are getting smarter relatively quickly, but it’s not obvious (and some would say flies in the face of certain evidence) that kids today are so much smarter than their parents or grandparents (except perhaps when it comes to home electronics). As Flynn writes:
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.
The mental retardation paradox: If the rate of change in IQ is extrapolated backwards, it suggests that people in 1900 had a mean IQ score somewhere between 50 and 70 judged by today’s standards. An IQ level of 75 is typically considered ‘mentally retarded.’ Flynn puts this one nicely, too: ‘Either today’s children are so bright that they should run circles around us, or their grandparents were so dull that it is surprising that they could keep a modern society ticking over.’
The identical twins paradox: Twins raised apart tend to have very similar IQ scores, typically considered strong evidence for a genetic basis for differences in IQ. The Flynn Effect instead suggests that intelligence, if it is being measured by IQ, is more malleable and subject to environmental effects. [Clearly there is no genetic evolution basis for the Flynn effect, mostly obviously because the effect occurs too fast, over a single generation.]
The most popular theories as to the cause of our collective increasing genius include:
- Increased schooling and test familiarity – This seems like an obvious cause, however it has been shown that previous generations with similar levels of education still score lower than subsequent generations.
- Generally more stimulating environments – Our world is increasingly complex, potentially increasing exposure to problem solving situations.
- Nutrition – Improved nutrition has increased human stature. It is therefore possible that improved nutrition is also responsible for increases in IQ. Recent evidence outlined in the previously linked Wired article suggests that both ends of the bell curve are showing equal increases in IQ. An increase in IQ of those at the right side of the bell curve argues against a solely nutritional cause as those with a high IQ are likely not undernourished to begin with.
- Infectious diseases - The decrease in infectious diseases experienced during development of the brain may be responsible for the Flynn Effect. Fighting off a disease and brain development are both metabolically costly and perhaps early childhood infections might come at a cost to brain development. This perhaps has the same problem as the case for a nutritional cause outlined above.
- Heterosis - The genetic component of IQ is so great that some argue that environmental causes would have a minimal effect. Essentially the idea is that the increased mixing of human populations has lead to a "hybrid vigour" effect in population IQ.
Another very interesting aspect of the Flynn Effect is that it has stopped in many developed nations. The cause is unknown.