I have come to believe that it is not so simple as IQ, although IQ plays a major part.
It is certainly not a black/white issue, either. One of the authors noted the Ashkenazi Jews were the most highly rated group, followed by East Asians, as noted in the book. The evidence certainly seems to support the hypothesis.
Of course, the next question is, is it genetics or environment? My belief is that it is both. Not a simple answer.
Anyway, as someone who has studied and read extensively on this topic for decades, I'll share my opinion later.
IQ by itself isn’t a good predictor. Combine IQ with conscientiousness (from the Big 5) and we’re talking. Analogously, SAT and high school GPA.
Regarding blacks catching up, you can trust that Flynn knows that whites have advanced, and that blacks have been catching up to a moving target.