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The population increase that humans went through in the last 2-3 thousand years has few parallels in nature, and there is plenty of evidence showing that selection does not operate in expanding populations, I can dig up refs if you want. But to put it simply, natural selection operates under the premise that some of your progeny will die (and with it presumably your "weak" genes), so in order for natural selection to operate, a certain percentage of the population has to perish. But in today's society this is not happening anymore, or in other words, both the fittest and the weakest are surviving. If both are surviving there is no selection.
As stated, this is incorrect. Natural selection works just fine in expanding populations, and does not require any deaths at all. Selection requires differential reproduction, not death, and differential reproduction can occur in an expanding population just as well as in any other population.
What is true is that
fixation of beneficial alleles is less likely in a rapidly expanding population.(*) So if alleles for high intelligence were being selected for in the expanding human population, most new mutations conferring high IQ would not reach 100% in the population -- but the population would still get smarter, on average.
(*) Fixation is less likely, but certainly not impossible. Loss of the less fit allele does not require death; it just requires failure to reproduce enough copies to maintain the allele at its existing frequency. In the real world, many humans fail to reproduce at all, and some of them have even been known to die, despite our increase in population size.
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