There is as yet no evidence, no hard data, no way of testing his speculation but Gerald Crabtree, a genetics Professor at Stanford University, believes that human evolution no longer selects for or favours intelligence. Our intelligence may have peaked as hunter-gatherers.
He has a point.
The intricacies of modern, “civilised”, human society where “weak” members of society are cared for by others, are such that many genetic characteristics have effectively been decoupled from survival and reproduction. “Intelligence” as one such genetic charateristic is no longer something that affects survival or reproduction. In fact from the fertility rates around the world today it is already apparent that the greater the wealth (GDP) the lower the reproduction rate. I am not sure if it can be shown explicitly but I suspect that a similar relationship may apply and that the greater the “intelligence” the lower the reproduction rate.
Crabtree has published 2 papers in Trends in Genetics
- Gerald R. Crabtree. Our fragile intellect. Part I. Trends in Genetics, 2012; DOI: 10.1016/j.tig.2012.10.002
- Gerald R. Crabtree. Our fragile intellect. Part II. Trends in Genetics, 2012; DOI: 10.1016/j.tig.2012.10.003
Human intelligence and behavior require optimal functioning of a large number of genes, which requires enormous evolutionary pressures to maintain. A provocative hypothesis … suggests that we are losing our intellectual and emotional capabilities because the intricate web of genes endowing us with our brain power is particularly susceptible to mutations and that these mutations are not being selected against in our modern society.
“The development of our intellectual abilities and the optimization of thousands of intelligence genes probably occurred in relatively non-verbal, dispersed groups of peoples before our ancestors emerged from Africa,” says the papers’ author, Dr. Gerald Crabtree, of Stanford University. In this environment, intelligence was critical for survival, and there was likely to be immense selective pressure acting on the genes required for intellectual development, leading to a peak in human intelligence.
From that point, it’s likely that we began to slowly lose ground. With the development of agriculture, came urbanization, which may have weakened the power of selection to weed out mutations leading to intellectual disabilities. Based on calculations of the frequency with which deleterious mutations appear in the human genome and the assumption that 2000 to 5000 genes are required for intellectual ability, Dr. Crabtree estimates that within 3000 years (about 120 generations) we have all sustained two or more mutations harmful to our intellectual or emotional stability. Moreover, recent findings from neuroscience suggest that genes involved in brain function are uniquely susceptible to mutations. Dr. Crabtree argues that the combination of less selective pressure and the large number of easily affected genes is eroding our intellectual and emotional capabilities.
If “intelligence” is an inherited characteristic – as it seems at least partially to be – then it is only a matter of simple arithmetic that unless the “more intelligent” reproduce at a higher rate than those of “less intelligence” then the “average intelligence” of the population will inevitably decrease.