It must be simple in some way since it is so heritable. People with IQs of 90 and IQs of 140 both prosper and do fine. although there are lots of statistical differences between two such groups.

Other other hand if we take a trait like "propensity to learn language in childhood" this seems to me to be relatively invariable and fixed and so probably very complex.

Certainly one could breed for IQ and raise the population mean a lot. But what would we be doing to our children? People with 140 IQ seem to do all right but I would worry a lot about the kind of life a kid with an IQ of 220 would have.