Idiocracy's argument (not mine) is that if stupid people are out-breeding intelligent people, over time this will select for stupidity. Thankfully, there isn't allele for intelligence, rather it is secondary effect and expression of number of genes.

Best current explanation is that there are hundreds of genes that regulate various functions that also affect intelligence. When all of them are expressed, you get Einstein, when all but 1 expressed you just get very smart people. To get to Tea Party Fox-watching mouth-breather level you have to go fairly deep into opposite territory and it is equally unlikely to happen.

As such, our genetics strongly select for normal distribution of intelligence and selective pressures on individual alleles are unlikely to move the median.

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