I don't know whether Rushton's work is correct or incorrect. But he's just one guy - the US army has for years being collecting IQ scores and finding the mean score for each group of people. None of you, not Sugar Shane nor OrphanCrow, can link to an IQ study that's done correctly and that shows all groups having the same mean IQ ... because they don't.
Think about it: some groups of humans left Africa, and thus split off from the people who stayed behind, tens of thousands of years ago at least.
It would be strange if mean IQs between different groups stayed the same.
While the African groups lived in tropical or subtropical climates, the wanderers into Eurasia had to survive very cold winters. There was a need to design and make winter clothes - something the Africans didn't need. It would have taken people with higher IQs to design and make decent clothing. Sure the Africans had the possibility to have high IQs to make layers of clothing but there was no 'need' because they didn't need extra clothing. They could wander around naked or just wearing a genital covering. But the Eurasians needed to make and wear clothes. Nature selected for higher IQs. There was a massive change of environment for the migrants out of Africa, and nature selected for slightly higher mean IQs. There was no change of environment for the Africans. There were different pressures on different groups of people.
"Life finds a way." - Dr Malcolm, Jurassic Park.
I've put this rather clumsily but I'm sure you get what I'm trying to say.