The second idea of how to interpret the GDP-IQ relationship is based on several different results of IQ research.
As you might know, there is a general factor of intelligence, that can be extracted from any battery of cognitive tests. The so-called g-factor explains a big part of the results on any IQ-test. The essential thing is that it explains the predictive part . That means if you factor out the g-factor, IQ tests do no tell you much about educational attainment, income, criminality or performance in other cognitive domains.
As you might further know, there has been a steady rise of IQ scores, called the Flynn effect . However, the Flynn effect has not been on the g-factor. I.e. the Flynn effect has been anti-correlated with the g-loadings of different IQ tests. This explains why our grandparent’s generation does not seem to be morons, despite scoring 30 points lower on Raven’s matrices. The Flynn effect doesn’t really increase cognitive ability, rather it increases the additional factors that unfortunately do not generalize.
As the Flynn effect is still ongoing in many countries and has stopped in the most developed countries, it is obviously playing a role in the differences in national mean IQ. If one day all countries have reached the end of the Flynn effect, we would expect the differences in mean IQ to have decreased substantially.
But here comes the rub: If the differences decrease due to the Flynn effect, and the Flynn effect is not on g, and only g is predictive of performance in the real world … why would we expect the shrinking IQ gap to be accompanied by a shrinking performance gap in GDP and co?
The linear g theory says that if we could compare nations by g-factor instead of IQ, we would see a linear relationship between g and GDP. The exponential relationship observed between IQ and GDP is just an artifact of poorer countries having still a lot of Flynn left to go.
I do not endorse a strong version of the linear g theory. But given the results of IQ research cited above, the hollowness of the Flynn effect must play some role in distorting the IQ-GDP relationship.