Well, in modern times, I'm not sure that we have a comparable sample population. And even then, it depends on what traits are being selected for. Intelligence is only one trait among many, and the benefits to a trait are at least partially determined by the environmental needs.
In ancient times, however, I would submit that the branching off of the original populations for the various races of man show what happens when small isolate population breed endogamously.
Being successful as a farmer requires good health, strength, and energy (as well as intelligence). Being a successful merchant or banker requires being smart enough to not get outsmarted by people who want your stuff. Over time, stupid merchants and financiers tend to lose money and starve
So the results possibly can’t be duplicated?