I have read lately that human intelligence has already peaked and that it will slowly decline over time because humans are no longer naturally selected for intelligence (Darwin award winners aside). In particular, from the first link:

Humans are losing intellectual and emotional capabilities because we no longer need intelligence to survive, a new study has claimed.

Researchers from Stanford University claim the intricate web of genes which endows us with our brain power is particularly vulnerable to mutations - and these mutations are not being selected against our modern society because we no longer need intelligence to survive.


[The lead author] argues that the combination of less selective pressure and the large number of easily affected genes is eroding our intellectual and emotional capabilities.

Although I have not read the study referred to, none of my reading from the press has indicated that the theory is based on observed phenomenon. Rather, the theorist seems to be falling back to a more general theory related to evolution, namely that a trait will "devolve" unless it factors into breeding potential (if I have characterized that correctly).

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    First of all, this was the argument for eugenics, which isn't in favor after it's been the base of Nazi ideology. The Flynn effect directly contradicts that. Humanity is getting smarter (I know it's hard to see when you have stuff like "Jersey Shore" on TV). – vartec Nov 14 '12 at 20:40
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    "we no longer need intelligence to survive" I'm not sure it's been proven that intelligent individuals were ever preferentially selected for survival. – DJClayworth Nov 14 '12 at 21:04
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    I'm not sure you need to lose the trait for the argument to work. An increase in variance would work just as well. Take the example of cooking - we now no longer need fangs to bite into our meat, but they're not gone. There might be more variability though, so the average canine gets blunter. For a trait to disappear you really need to select against it; it's not enough to stop selecting for it. At least that's my understanding of how these things work. – Ana Nov 15 '12 at 10:43
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    This is a different question than the dup in that it is looking for examples where this has been documented in other species besides humans. It is also opened up to any trait rather than focusing on human intelligence. – Chad Nov 15 '12 at 14:38
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    Related: huffingtonpost.com/2013/05/22/… – Django Reinhardt May 25 '13 at 3:37

This answer has three sections:

  1. Is human intelligence declining? No.
  2. Is the genetic foundation of human intelligence eroding? This has not been shown.
  3. Are other factors influencing the genetic component of intelligence? Probably.

Is intelligence declining?

No, human intelligence is not declining. In fact, it is increasing.

This is actually stated in the article that inspired the news stories, which is publicly available:

Our Fragile Intellect, by Gerald Crabtree Part I, Part II (PDF and "Full Text" links to right of page)

The Flynn Effect is discussed in Part II, Box I. Crabtree attributes increased scores on IQ tests to two factors: improved childhood environment, and cultural biases in the test:

these changes in IQ scores are probably linked to environmental influences including reduction in lead and other heavy metals used in gasoline and paint and the virtual elimination of hypothyroidism in children due to the widespread use of iodinated salt. These and many other advances in prenatal care and prevention of anoxia during childbirth have clear effects on our average intellectual abilities.

So even the original proponent of this idea admits that human intelligence is probably increasing. While he suggests that this increase has plateaued, he is apparently discussing wealthy, Western nations. Globally, childhood malnutrition (measured by growth stunting) has declined substantially from 1990 to 2011, from which we could predict that human intelligence is continuing to increase globally. Crabtree is discussing a process that occurs over thousands of years, and we can only speculate about the impact that medical, economic, and cultural changes will have on human intelligence over that period.

Crabtree dismisses some of this increase of IQ scores as being the result of increased levels of education, which cause the students to develop the cultural traits that produce high test scores and therefore do not reflect true intelligence. This brings up another thorny question -- how exactly can we measure intelligence, and is there even a unitary intelligence? This has been debated ad nauseum, with one classic debate being centered around Stephen J. Gould's The Mismeasure of Man. This is not a topic to delve into here; Crabtree is relying on the literature relating to X-linked Intellectual Disability, and one of his sources is available free online. This appears to focus on severe mental disabilities, so those subtleties are probably not important.

Is the genetic foundation of human intelligence eroding?

Now that I have addressed the issue of average intelligence, as such (and our risk of developing into Idiocracy), let's now examine the core issue of this theory -- is the genetic basis for intelligence eroding? Crabtree openly admits that he has no evidence that this is actually happening. In fact, the entire purpose of this essay seems to be to propose a human population genomic study that will test for patterns of relaxed selection on genes that are associated with cognitive and emotional variation (Part II, Figure 2).

Sequencing many individuals whose last common ancestors ranged from present day to 5000 years ago should produce an estimate of the rapidity of change and the level of selection operating on these genomes at various time-intervals during this 5000 year period (an interval that would span the emergence of cities for several population groups). These genomes would have sequence variations in intellectual deficiency (ID) genes but, because each generation produces only about 40 new signature mutations, these would not be enough to guide the temporal ordering. However, using the 400 million retroviral insertion sites as signatures of a specific ancient generation to estimate the age of nearby new mutations, sufficient fineness might be produced to permit dating of mutations in ID genes. Because mutations that control the evolution of specific characteristics have often been found in regulatory rather than coding regions, full genome sequences would need to be assayed. I would be very happy to learn from this test that there is no substance to my argument.

I think this hits the core points of our question. The only issue left to discuss is if Crabtrees' argument is at all reasonable. I think there is good reasons to believe that it is not reasonable, and I'm sure these are discussed by the published responses to his original essay. I see the following weaknesses:

  1. I suspect that his estimate of mutational target size is the upper bound for what is reasonable. He started by included every gene that could conceivably be said to be involved in intelligence (though he did moderate that assumption by focusing only on the fraction where the intellectual deficit has been shown to arise from loss-of-function). He also assumes that all "deleterious" mutations to these genes would decrease intelligence. Perhaps most importantly, he assumed that each gene had a major impact on intelligence, and that mutations had cumulative deleterious effects. While he did make a brief argument to justify these assumptions, his evidence is very indirect and his model conflicts with much of what we know about the typical functioning of biological systems.
  2. He seemed to assume that the only selection acting on these genes was for human-like intelligence, even after he described how these genes are found in non-human animals and these "Intellectual Deficiency" disorders often arise from the disruption of core physiological processes and include massive developmental abnormalities. He acknowledged the possibility of pleiotropy, but did not incorporate it into his calculations.
  3. He provided absolutely no evidence that the strength of selection on these traits has changed, just a bunch of hand-waving arguments about how difficult it was to be a hunter-gatherer and how easy it is to reproduce in modern society. The closest that he came to offering a plausible explanation for why selection on intelligence may be reduced was to reflect on how living in large societies must have increased selection on our immune system (which then reduces the effectiveness of selection on other traits). However, communicable disease is no longer a major selective pressure, as indicated by the CDC's charts of the cause of death. Indeed, the main causes of death for people in their reproductive prime (15-34) are accidents, homicide, and suicide -- all of which can be influenced by cognitive and emotional capabilities. If Crabtree thinks that throwing a spear is cognitively difficult, he should try driving a car.
  4. He emphasizes how ineffective natural selection can be on intellectual traits, but does not even assert that this has changed between prehistoric and modern populations.
  5. He never mentioned the immense expansion of the human population and the elimination of geographic structure in our modern population. Both of these factors increase the effectiveness of selection in a population. So even if the selection coefficient on cognitive traits has decreased, we could still expect to see the preservation of these traits, and possibly even additional evolution of these traits. Recent studies have actually measured this increase in the effectiveness of natural selection in humans.

That being said, Crabtree missed some of the obvious, traditional arguments suggesting that selection on intelligence is decreasing. These would include the reduced variation in reproductive success (associated with greater survival past reproductive age) and the lack of correlation (or even anticorrelation) between the number of children that a person has and other traits that are predictive of intelligence. I can only speculate on why he did not make those arguments.

To restate my conclusions:

  1. Human intelligence is probably still increasing
  2. There is no evidence that the genetic foundation of intelligence is eroding
  3. The essay in question dose not provide a solid reason to speculate that the genetic foundation of intelligence is eroding.

Are other factors influencing the genetic component of intelligence?

A central tenet of both Crabtree's essay and the popular science reporting on this topic is that modern (and future) humans lack the genetic foundation of intelligence that their ancestors had. Crabtree's "use it or lose it" model of mutation pressure does not capture all of the genetic processes that influence traits such as intelligence. For instance:

Inbreeding depression:

Loss-of-function mutations like those discussed by Crabtree tend to be masked by heterozygosity. Therefore, inbreeding can result in more individuals exhibiting these intellectual problems, as has been documented for populations such as Ashkenazi Jews (Canavan Disease) and North Indian Muslims. Over the past few centuries, homozygosity in the human population has probably decreased as a result of population growth, inter-continental migration, and urbanization; however, I cannot find measurements of this effect. Likewise, I cannot find any broad study of how these changes in homozygosity levels might affect the distribution of intellectual capabilities. If Crabtree is correct about how common these loss-of-function mutations are, then we might expect them to be common throughout the human population. Crabtree argued that the developmental genetics of the nervous system are such that loss-of-function intellectual deficiencies may be evident in heterozygous individuals, which would make inbreeding less important. Increasing heterozygosity has been proposed as a cause for the Flynn effect.

Assortative mating:

Charles Murray, in Coming Apart, has proposed that humans are increasingly mating with people of similar intelligence, which may greatly increase the variation of intelligence within the human population (meaning more very smart people and more very stupid people).

That brings us to the role of homogamy -- interbreeding of individuals with like characteristics. Drawing on the extensive technical literature and the CPS, sociologists Christine Schwartz and Robert Mare examined trends in "assortative marriage," as it is known in the jargon, from 1940 to 2003. They found that homogamy has increased at both ends of the educational scale -- college graduates grew more likely to marry college graduates and high school dropouts grew more likely to marry other high school dropouts. In 1960, just 3 percent of American couples both had a college degree. By 2010, that proportion stood at 25 percent.

It's not just that college graduates are likely to marry college graduates, but that graduates from elite colleges are likely to marry other graduates from elite colleges. Increased educational homogamy inevitably means increased cognitive homogamy. On average, children are neither as smart nor as dumb as their parents. They are closer to the middle. This tendency is called regression to the mean. In 2010, 87 percent of the students with 700-plus scores in Critical Reading or Mathematics had a parent with a college degree, and 57 percent had a parent with a graduate degree. Those percentages could have been predicted pretty closely just by knowing the facts about the IQs associated with different educational levels and the correlation between parental and child IQ.

The bottom line is not subject to refutation: Highly disproportionate numbers of exceptionally able children in the next generation will come from parents in the upper-middle class, and more specifically from parents who are already part of the broad elite.

Implicit eugenics:

Our society often places intelligent (or highly educated) people into low-risk occupations while relegating less intelligent (or educated) people to high risk jobs. Here I will treat educational attainment as a proxy for intelligence, which is itself a proxy for the genes that promote intelligence. The purpose is not to demonstrate that intelligence increases the odds of raising healthy children to adulthood, only to show that there are plenty of opportunities for intelligence to have an impact on the likelihood that a person has children, and that these children are likely to survive to adulthood. Sometimes this discrimination is intentional, such as when participation in higher education was grounds for deferment from the military draft. Sometimes this discrimination is the result of uncoordinated market forces, where less intelligent/educated people have fewer opportunities to earn high wages while performing a safe job. The Department of Labor provides information showing that the highest fatality rates (page 4) are found among industries that are typically not associated with high educational attainment (e.g. construction, mining, trucking, and fishing). Conversely, many people are probably turned away from these jobs exactly because they are not intelligent or emotionally stable enough to work in these dangerous conditions.

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    The Flynn effect is completely irrelevant here, as it’s not due to an actual increase in innate intelligence. The question was about innate intelligence though – natural selection would have no way of acting on anything else. – Konrad Rudolph Nov 23 '13 at 13:39
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    @KonradRudolph: It would be negligent to refuse to answer the question that was actually asked due to my own speculation about the audience's true interest, so I answered the question completely. Furthermore, interest in this topic is often motivated by fears such as those expressed by Idiocracy, where actual intelligence matters (not the just genetic component). Finally, the questions on this site make it very clear that the person asking the question often does not understand the technical terms that they are using. If they meant to ask a different question, then they should ask again. – adam.r Nov 23 '13 at 17:28
  • Oh, I actually agree with this. But the question doesn’t mention the Flynn effect (some comments do though) and your answer puts it in a prominent first place. But that’s just a minor nitpick. – Konrad Rudolph Nov 23 '13 at 17:52
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    This is what I read as the core question: "Is intelligence declining?", and the explanation part "due to lack of selection pressure" is secondary. If the phenomenon does not exist, then the explanation is irrelevant. I mentioned the Flynn effect because Crabtree discussed it, not because of the comments here. – adam.r Nov 23 '13 at 23:16
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    @BrianM.Hunt Thanks. It's an interesting question. And my first accepted answer! – adam.r Nov 26 '13 at 23:52

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