High IQ people aren't the greatest source of further High-IQ children. The majority of High IQ people have always been generated by Middle-IQ parents. The genes for IQ are very complex and require quite a bit of mixing to generate an intelligent child--while it is true that IQ is heritable and strongly so, it is also not expressed in a single place in the Genome but rather many. Each gene associated with intelligence has long since replicated out to the entire population such that it is more common for two middle-IQ parents, carrying a dissimilar set of IQ-related genes, to combine their genetics and produce a High-IQ child.

Put statistically, IQ is actually a left-leaning chi-squared curve where the higher the IQ of the parent, the greater the probability the children will have a lower IQ. The closer the parent is to the mean, or below it, the greater the probability of generating a child with a higher IQ. The Majority of HighIQ people are smarter than their parents. The Majority of HighIQ parents have children with a slightly lower IQ.