A basic lesson of sociology is that what happens within a social system is not completely determined by the individual characteristics of its members.
The reader may wonder why it took twenty years to solve our four paradoxes. The reason was that their solution lay in a comprehensive analysis of the social forces at work as society changes over time. I was distracted by three barriers to understanding, all of which downplayed the role of social evolution. Two of them singled out highly specific social trends as central: a tendency toward social mobility and mating with a wider variety of partners; and a tendency toward better nutrition. The third was a methodological mistake that made social forces seem too feeble to explain much.
Tokyo and American history
What if enhanced genes over the last century were an important cause of IQ gains over time? If so, I have exaggerated the significance of cultural evolution. I do not think anyone would propose that genes have been enhanced by eugenic reproduction, that is, reproductive patterns caused by high-IQ people having more children than low-IQ people. In America, those with more education have had fewer offspring than those with less education throughout either most or all of the twentieth century. The current data suggest that reproductive patterns, perhaps reinforced by immigration, may have cost America about one IQ point per generation (Herrnstein & Murray, 1994, ch. 15; Lynn & Van Court, 2004).