That is the new book by James R. Flynn. He suggests the following:

Today we have no difficulty freeing logic from concrete referents and reasoning about purely hypothetical situations. People were not always thus.

In other words, people in earlier times really were stupider when it came to abstract thought, but this was primarily for environmental reasons. These people also had more daily, practical skills, again for reasons of practice. We in contrast receive daily workouts with hypotheticals, rapidly moving images, and spatial reasoning. So Flynn is suggesting that IQ isn’t more multi-dimensional than it may seem. The Flynn Effect gains are in fact concentrated in the most spatial and abstract versions of IQ tests.

Flynn summarizes the "Dickens-Flynn" model, through which environment and IQ interact in multiplicative fashion. Smart people seek out environments which make them even smarter, and this helps reconcile the cross-sectional IQ data (adoption doesn’t change IQ so much) with the time series of increasingly higher IQ scores (environments are changing for everybody). This reconciliation was fuzzy to me, but I took Flynn to be claiming that separated identical twins will reimpose common environmental forces on themselves, thus keeping their IQs in relatively close long-run synch. I still don’t understand what kind of test (might it contrast permanent vs. temporary environmental shocks?) might falsify the Dickens-Flynn model.

Flynn also argues that the Chinese in America attained high levels of achievement before

above-average IQs.

This book doesn’t tie up all the loose ends, and it could have been written in a more organized fashion. Still it is one of the more interesting volumes of the year.

Addendum: I have long thought that the Germanic "Hausmusik" tradition was responsible for producing so many great composers in one relatively short period of time. Flynn’s book offers (unintended) hints about why it is so hard to reproduce the cultural blossomings of times past, and also why future creations will seem baffling to the old fogies.