The authors explored the data and concluded that the most important key to intelligence was to have intelligent parents.
There is no doubt that IQ is mostly genetic.
Here's a theory why the IQ disparity seems to be getting wider:
In the old days the son of the richest guy in town would enter the family business directly after high school, marry the prettiest girl in town and go on to have low to average IQ kids. In this example, the middling IQ of the rich son averaged with the lower IQ of the pretty girl resulted in a low to average IQ of the offspring.
Similarly, the son of the high IQ doctor in town would attend male-only college and then marry one of the low to average IQ rich daughters in town. The high IQ of the doctor and the average IQ of the rich girl resulted in average IQ of the offspring.
Now with coeducational opportunities for women. The rich son with average IQ is likely to meet a wife with a similar or higher IQ. Likewise the doctor son will likely marry another medical student also with a similar or higher IQ. As a result of this opening up of educational and professional opportunities for women, there is less IQ balancing because there is less intermarriage between high IQ and low IQ resulting in the smart getting smarter (and the dumb getting dumber) All due to the women's movement! ha, ha, ha!