"Our Fragile Intellect" is a 2012 article by American biochemist

Gerald Crabtree Gerald R. Crabtree is the David Korn Professor at Stanford University and an Investigator in the Howard Hughes Medical Institute. He is known for defining the Ca2+-calcineurin-NFAT signaling pathway, pioneering the development of synthetic ligan ...

, published in the journal '' Trends in Genetics''. Crabtree's speculative and controversial thesis argues that

human intelligence Human intelligence is the Intellect, intellectual capability of humans, which is marked by complex Cognition, cognitive feats and high levels of motivation and self-awareness. Using their intelligence, humans are able to learning, learn, Concept ...

peaked sometime between 2,000 and 6,000 years ago and has been in steady decline since the advent of agriculture and increasing

urbanization Urbanization (or urbanisation in British English) is the population shift from Rural area, rural to urban areas, the corresponding decrease in the proportion of people living in rural areas, and the ways in which societies adapt to this change. ...

. 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 A hunter-gatherer or forager is a human living in a community, or according to an ancestrally derived Lifestyle, lifestyle, in which most or all food is obtained by foraging, that is, by gathering food from local naturally occurring sources, esp ...

past.Boyle, Rebecca. (2012, November 19).
Are People Getting Dumber? One Geneticist Thinks So
". ''PopSci''. Retrieved December 6, 2012. This theory is sometimes referred to as the "

Idiocracy ''Idiocracy'' is a 2006 American science fiction comedy film co-produced and directed by Mike Judge from a screenplay written by Judge and Etan Cohen based on a story written by Judge. The plot follows United States Army librarian Joe Bauers ...

hypothesis".

Thesis

Crabtree argues that advancements in modern science allow new predictions to be made about both the past and the future of humanity and we can predict "that our intellectual and emotional abilities are genetically surprisingly fragile". Recent studies of genes correlated with human intelligence on the

X chromosome The X chromosome is one of the two sex chromosomes in many organisms, including mammals, and is found in both males and females. It is a part of the XY sex-determination system and XO sex-determination system. The X chromosome was named for its u ...

indicate typical intellectual and emotional activity depends on 10% of genes. Intelligence-dependent (ID) genes appear to be widely distributed throughout the entire genome, leading to a figure of 2,000 and 5,000 genes responsible for our cognitive abilities.

Deleterious mutation In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, mitosis ...

s in these genes can impact normal intellectual and emotional functioning in humans. It is thought that in just the last 120 generations (3000 years), humans have received two or more harmful mutations to these genes, or one every 20-50 generations. Crabtree points out that he loves our society's supportive institutions and wishes that they could be extended to include more of our population. The data that support the theory that our intellectual abilities are particularly susceptible to the accumulation of mutations begins with determinations of the human intergenerational

mutation rate In genetics, the mutation rate is the frequency of new mutations in a single gene, nucleotide sequence, or organism over time. Mutation rates are not constant and are not limited to a single type of mutation; there are many different types of mu ...

. This rate has been determined in several human populations to be about 1.20 x10-8 per position per

haploid Ploidy () is the number of complete sets of chromosomes in a cell (biology), cell, and hence the number of possible alleles for Autosome, autosomal and Pseudoautosomal region, pseudoautosomal genes. Here ''sets of chromosomes'' refers to the num ...

genome A genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding genes, other functional regions of the genome such as ...

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 sperm. 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. This figure of 1.20 x10-8 mutations per nucleotide per generation predicts that about 45 to 60 new mutations will appear in each generation. These mutations might accumulate or be removed by

natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the Heredity, heritable traits characteristic of a population over generation ...

. The speculation that the nervous system and the brain would be more sensitive than other cell types and organs to the accumulation of these new mutations was based on the estimate of the fraction of genes necessary for normal development of the nervous system. The data quantifying the number of genes required for normal intellectual development comes from thousands of published studies (about 23,000 on

PubMed PubMed is an openly accessible, free database which includes primarily the MEDLINE database of references and abstracts on life sciences and biomedical topics. The United States National Library of Medicine (NLM) at the National Institute ...

from the National Library of Medicine) in which scientists have identified a mutated gene or a region of DNA associated with or causing human

intellectual disability Intellectual disability (ID), also known as general learning disability (in the United Kingdom), and formerly mental retardation (in the United States), Rosa's Law, Pub. L. 111-256124 Stat. 2643(2010).Archive is a generalized neurodevelopmental ...

. These genes may not even be expressed in the brain. For example, the

phenylalanine hydroxylase Phenylalanine hydroxylase (PAH) () is an enzyme that catalyzes the hydroxylation of the aromatic side-chain of phenylalanine to generate tyrosine. PAH is one of three members of the biopterin-dependent aromatic amino acid hydroxylases, a clas ...

gene is expressed only in the liver, yet its mutation leads to severe intellectual disability due to the accumulation of metabolites. Many of these genes operate like links on a chain rather than a robust network underlining the fragility of our intellectual abilities. For example, mutations of a single nucleotide out of the 3 billion human nucleotides in our genomes in one copy of the ARID1B gene are a common cause of intellectual disability. Estimates of the total number of genes that when mutated give rise to intellectual disability is thought to be several thousand, perhaps 10-20% of all human genes, which produces a very large target for

random mutation Evolution is the change in the heritable characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, resulting in certai ...

s. In addition, neuronal genes tend to be large and hence increase the size of the genomic target region for random mutations. The simple combination of the number and size of genes required for normal

brain development The brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It consists of nervous tissue and is typically located in the head ( cephalization), usually near organs for special sens ...

(>1000) and the fact that each new human generation has 45-60 new mutations per genome led Crabtree to suggest that our intellectual abilities are particularly genetically fragile over many generations. Seemingly the only practical implication of this theory is that perhaps men should have their children when they are young and that women should prefer younger men for mates. Several counterarguments are presented. The

Flynn effect The Flynn effect is the substantial and long-sustained increase in both fluid and crystallized intelligence test scores that were measured in many parts of the world over the 20th century, named after researcher James Flynn (academic), James Flyn ...

, for example, shows an apparent increase in IQ around the world since 1930. Crabtree attributes the rise in IQ to advancements in

environmental Environment most often refers to: __NOTOC__ * Natural environment, referring respectively to all living and non-living things occurring naturally and the physical and biological factors along with their chemical interactions that affect an organism ...

and

public health Public health is "the science and art of preventing disease, prolonging life and promoting health through the organized efforts and informed choices of society, organizations, public and private, communities and individuals". Analyzing the de ...

measures as well as improved education and other factors. The Flynn effect also shows, argues Crabtree, not an increase in intelligence, but more intelligent test taking.Ghose, Tia. (2012, November 12).
Are humans becoming less intelligent? It could very well be
". ''

LiveScience Live Science is a science news website. The publication features stories on a wide range of topics, including space, animals, health, archaeology, human behavior, and planet Earth. It also includes a reference section with links to other websites. ...

''. NBCNews.com. Retrieved December 6, 2012.

Reception

Kevin Mitchell, associate professor at the Smurfit Institute of Genetics at

Trinity College Dublin Trinity College Dublin (), officially titled The College of the Holy and Undivided Trinity of Queen Elizabeth near Dublin, and legally incorporated as Trinity College, the University of Dublin (TCD), is the sole constituent college of the Unive ...

agreed that genetic mutations could harm the development of the brain in humans and diminish intelligence; new mutations would become apparent in new generations. However, Mitchell criticizes Crabtree for failing to acknowledge the role of

. According to Mitchell, natural selection "definitely has the ability to weed out new mutations that significantly impair intellectual ability". Mitchell describes Crabtree's argument as a conceptual

fallacy A fallacy is the use of invalid or otherwise faulty reasoning in the construction of an argument that may appear to be well-reasoned if unnoticed. The term was introduced in the Western intellectual tradition by the Aristotelian '' De Sophisti ...

and says Crabtree is "thinking about things in a wrong way". Biologist

Steve Jones Steve or Steven Jones may refer to: Arts and entertainment *Steve Jones (English presenter) (born 1945), English musician, disk jockey, television presenter, and voice-over artist *Steve Jones (musician) (born 1955), English rock and roll guitar ...

, Emeritus Professor of Genetics at

University College London University College London (Trade name, branded as UCL) is a Public university, public research university in London, England. It is a Member institutions of the University of London, member institution of the Federal university, federal Uni ...

questioned the journal's decision to publish the paper, calling the study "a classic case of Arts Faculty science. Never mind the hypothesis, give me the data, and there aren't any".Connor, Steve. (2012, November 12).
Human intelligence 'peaked thousands of years ago and we've been on an intellectual and emotional decline ever since'
. ''The Independent''. Retrieved December 6, 2012. Crabtree acknowledges that the data isn't there because a slow genetic deterioration in intelligence can't be detected by comparing it to people today. 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 deficit Cognitive impairment is an inclusive term to describe any characteristic that acts as a barrier to the cognition process or different areas of cognition. Cognition, also known as cognitive function, refers to the mental processes of how a person ...

s in future generations.Flatow, Ira. (2012, November 16).
Are We Getting Dumber? Maybe, Scientist Says
. ''National Public Radio''. Science Friday.

). Retrieved December 20, 2012. Anthropologist

Robin Dunbar Robin Ian MacDonald Dunbar (born 28 June 1947) is a British biological anthropologist, evolutionary psychologist, and specialist in primate behaviour. Dunbar is professor emeritus of evolutionary psychology of the Social and Evolutionary Neuros ...

Oxford University The University of Oxford is a collegiate research university in Oxford, England. There is evidence of teaching as early as 1096, making it the oldest university in the English-speaking world and the second-oldest continuously operating u ...

argues against Crabtree's position that brain size was driven by tool use. Instead, Dunbar argues that the

social environment The social environment, social context, sociocultural context or milieu refers to the immediate physical and social setting in which people live or in which something happens or develops. It includes the culture that the individual was educated ...

drives intelligence. "In reality what has driven human and primate brain evolution is the complexity of our social world", says Dunbar. "That complex world is not going to go away. Doing things like deciding who to have as a mate or how best to rear your children will be with us forever."Collins, Nick (2012, November 12).
Civilisation is making humanity less intelligent, study claims
. ''Telegraph Online''. Retrieved December 21, 2012.

Cultural trope

Writer Andrew Brown notes that Crabtree's paper represents a familiar, reoccurring notion in both fiction and

evolutionary biology Evolutionary biology is the subfield of biology that studies the evolutionary processes such as natural selection, common descent, and speciation that produced the diversity of life on Earth. In the 1930s, the discipline of evolutionary biolo ...

. "The idea that civilized man is a degenerate and self-domesticated variation on the

wild type The wild type (WT) is the phenotype of the typical form of a species as it occurs in nature. Originally, the wild type was conceptualized as a product of the standard "normal" allele at a locus, in contrast to that produced by a non-standard, " ...

is partly a cultural trope, a result of the anxieties of

industrialized Industrialisation ( UK) or industrialization ( US) is the period of social and economic change that transforms a human group from an agrarian society into an industrial society. This involves an extensive reorganisation of an economy for the ...

life", writes Brown. The idea, Brown observes, was popular in the early 20th century fiction of

E. M. Forster Edward Morgan Forster (1 January 1879 – 7 June 1970) was an English author. He is best known for his novels, particularly '' A Room with a View'' (1908), ''Howards End'' (1910) and '' A Passage to India'' (1924). He also wrote numerous shor ...

The Machine Stops "The Machine Stops" is a science fiction short story by E. M. Forster. After initial publication in ''The Oxford and Cambridge Review'' (November 1909), the story was republished in Forster's '' The Eternal Moment and Other Stories'' in 1928. A ...

") and

Jack London John Griffith London (; January 12, 1876 – November 22, 1916), better known as Jack London, was an American novelist, journalist and activist. A pioneer of commercial fiction and American magazines, he was one of the first American authors t ...

(''

The Scarlet Plague ''The Scarlet Plague'' is a post-apocalyptic fiction novel by American writer Jack London, originally published in ''The London Magazine'' in 1912. Plot summary The story takes place in 2073, sixty years after an uncontrollable epidemic, the Re ...

''). It could also be found in the work of biologists such as

Ronald Fisher Sir Ronald Aylmer Fisher (17 February 1890 – 29 July 1962) was a British polymath who was active as a mathematician, statistician, biologist, geneticist, and academic. For his work in statistics, he has been described as "a genius who a ...

, who espoused similar concepts in ''

The Genetical Theory of Natural Selection ''The Genetical Theory of Natural Selection'' is a book by Ronald Fisher which combines Mendelian inheritance, Mendelian genetics with Charles Darwin's theory of natural selection, with Fisher being the first to argue that "Mendelism therefore va ...

'' (1930). 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 William Donald Hamilton (1 August 1936 – 7 March 2000) was a British evolutionary biologist, recognised as one of the most significant evolutionary theorists of the 20th century. Hamilton became known for his theoretical work expounding a ...

, who believed that the "life-saving efforts of modern medicine" threatened the human genome. Brown, Andrew. (2012, November 13).
Why it's unlikely we are more stupid than our hunter-gatherer ancestors
. ''The Guardian''. Retrieved December 20, 2013.

Thesis

Reception

Cultural trope

See also

References

Further reading