If there is a single variant with large effect, like torsion dystonia, then its appearance in one group is likely due to different tradeoffs. But if IQ is driven by mutational load, populations might differ in age of reproduction and thus in mutational loads without having different tradeoffs between traits. In the long run, elevated mutational load should select for simplified design, but that could be a very long run.
Beware of identifications and tautologies.
If there is a single variant with large effect, like torsion dystonia, then its appearance in one group is likely due to different tradeoffs. But if IQ is driven by mutational load, populations might differ in age of reproduction and thus in mutational loads without having different tradeoffs between traits. In the long run, elevated mutational load should select for simplified design, but that could be a very long run.