Another approach could be taken by going the exact opposite direction: find out what it actually is which causes differences in intelligence and use this to fin

Another approach could be taken by going the exact opposite direction: find out what it actually is which causes differences in intelligence and use this to find a way to raise the intelligence of the species _as a whole_, no matter their phenotype. After all, it is not the amount of melanin in your skin which defines intelligence. That there _is_ a genetic component to intelligence has been proven many times - search ${search_engine} for "genetic intelligence" for a summary of the current science on the subject which seems to indicate a 50% 'nature', 50% 'nurture' contribution to individual intelligence - so it does not make sense to try to put this genie back into its bottle. Someone is likely to do the research in those coming 200 years which you envision it will take for the world population to homogenise, it would be best if that someone did the research in the open and shared the results with the world instead of using it for some nefarious plan - intelligence and morality are not the same thing after all.