Crabtree's speculative and controversial thesis argues that human intelligence peaked sometime between 2,000 and 6,000 years ago and has been in steady decline since the advent of agriculture and increasing urbanization.
Modern humans, according to Crabtree, have been losing their intellectual and emotional abilities due to accumulating gene mutations that are not being selected against as they once were in our hunter-gatherer past.
[10] In contrast to popular opinion, this figure indicates that the biological clock (in terms of accumulation of deleterious mutations with time) is ticking faster for men than for women.
[2] Biologist Steve Jones, Emeritus Professor of Genetics at University College London questioned the journal's decision to publish the paper, calling the study "a classic case of Arts Faculty science.
Instead, Crabtree argues that he is synthesizing already existing data and making a purely mathematical argument that estimates the probability of the number of new mutations that could result in cognitive deficits in future generations.
"[19] Writer Andrew Brown notes that Crabtree's paper represents a familiar, reoccurring notion in both fiction and evolutionary biology.
"The idea that civilized man is a degenerate and self-domesticated variation on the wild type is partly a cultural trope, a result of the anxieties of industrialized life", writes Brown.
The idea, Brown observes, was popular in the early 20th century fiction of E. M. Forster ("The Machine Stops") and Jack London (The Scarlet Plague).
The most important parts of Fisher's book, Brown writes, expounds on the theme that "civilization is dreadfully threatened by the way the lower classes outbreed the aristocracy."
Brown finds related sentiments expressed in the work of W. D. Hamilton, who believed that the "life-saving efforts of modern medicine" threatened the human genome.