From Wikipedia: In 2013, Crabtree published "Our Fragile Intellect" in Trends in Genetics, The prediction that our intellectual abilities are genetically fragile was based on the determined rate of human de novo mutations (those mutations that appear in each generation). This rate has been determined in several human populations to be about 1.20 x10-8 per nucleotide per generation with an average father's age of 29.7 years. This rate doubles every 16.5 years with the father's age and ascribes most of the new mutations to the father during the production of reproductive cells. Thus about 45 to 60 new mutations occur per generation per human genome with each new generation. The conclusion that the accumulation of these new mutations over the generations would lead to intellectual fragility was based on the estimate of the fraction of genes necessary for normal development of the nervous system, which is thought to be several thousand. The nervous system is unique in that an extraordinarily large number of genes are required for the development and function of the brain, representing perhaps 10–20% of all human genes. The simple combination of the number of genes required for normal brain development (>1000) and the fact that each human generation has 45-60 new mutations per genome led Crabtree to suggest that our intellectual abilities are particularly genetically fragile over many generations.
His thesis: Each generation, humans accumulate ~50–60 new mutations. Most are harmless. But some are slightly harmful and aren’t removed. They stack. Over ~3,000 years, humans may have accumulated multiple mutations affecting cognition. In the past, natural selection filtered out harmful traits. Today, modern society buffers survival, meaning that those mutations may persist rather than be removed.
I think that natural selection is active no less than before, and intelligent people do reproduce themselves. Today, that is unclear because of the general mixing that takes place.
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