It is expected that the children of high IQ parents have lower IQs than their parents (and that children of low IQ parents have higher IQs than their parents). That is just regression to the mean. How are they separating this from that effect?
that would certainly be true if IQ were independently and identically distributed (iid), but the point here is that IQ is hereditable (not independent nor uniformly distributed) and so should show, in the long run, measurable durability in those differences across lineages. it's likely not reversion to the mean (or more to the point, the mechanism is not simply statistical in nature).
Is it? Hadn't IQs been trending upwards for the past several decades?