The geneticist, Gerald Crabtree, has published two journal articles on this hypothesis. According to the publication PopSci, Crabtree argues that “human intelligence may have actually peaked before our ancient predecessors ever left Africa...Genetic mutations during the past several millennia are causing a decline in overall human intellectual and emotional fitness...Evolutionary pressure no longer favors intellect, so the problem is getting exponentially worse.” Crabtree argues that this process, like other evolutionary processes, take a long time to emerge.
Crabtree’s central thesis to back his claim up is, according to PopSci , that “each generation produces deleterious mutations, so down the line of human history, our intelligence is ever more impaired compared to that of our predecessors.”
He argues that you can see evidence of this process in humans sense of smell. Now, humans are largely guided by intellect when it comes to what we eat, for example. Humans have “far fewer olfactory receptors than other animals,” according to Crabtree. Dogs, on the other hand, have more olfactory receptors (the neurons responsible for smelling things).
“Once you place pressure on intellectual abilities, and take it off of olfactory abilities, the olfactory genes deteriorate,” PopSci quotes Crabtree as saying.
But other geneticists have raised questions about Crabtree’s work. In an interview with The Independent , geneticist Steve Jones said that Crabtree’s paper was like “arts faculty science...Never mind the hypothesis, give me the data, and there aren’t any. I could just as well argue that mutations have reduced our aggression, our depression and our penis length, but no journal would publish that. Why do they publish this?”