Up front, I am specifically not interested in philosophical or ethical considerations re Eugenics and related concepts. In an effort to receive a concise answer I'll post a narrowly defined question following this preamble to set the stage.

In the movie Idiocracy there is a montage at the beginning "explaining" how the human population degrades, vis-à-vis intelligence, to an alarming (and absurd?) extent due to unplanned differential breeding resulting from several generations of the intellectually "more capable" people limiting their number of progeny, through active and passive family planning, while the bulk of humanity leaves caution to the wind. This exponential social differentiation eventually makes for a global imbalance that degrades our species' overall intellectual fitness to a debilitation level.

Is the premise of this movie merely a gag or is our species already doing something very much like this?
If yes or no, how might one measure the impact of differential procreation of this sort? Since testing results are always relative to the population as a whole and computers along with the Internet of Things would tend to obfuscate the initial degradation of mental capacity.

Based on the brief discussion in the comments - a summary of fecundity and IQ would potentially answer my question but only if it accounted for the current selection process we see going on in modern technophile societies. That is, it must account for the effect of our most educated couples generally choosing not to procreate and here I'm assuming there is in fact an effect (cf. Idiocracy for the (absurd?) degree to which the present-day trend may work itself out).

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    $\begingroup$ Welcome to Biology.SE! You might want to read about the Flynn effect $\endgroup$
    – Remi.b
    Commented Apr 17, 2015 at 2:20
  • $\begingroup$ @Remi.b - Thanks. I was aware of this phenomenon. Thus my last sentence in the question that says, in part, "Since testing results are always relative to the population as a whole". This covers both the Flynn Effect and testing bias resulting from test design in a given culture. - --- - There are many ways of measuring welfare spending against population data but, all politics aside, it seems to me that the 50-year trend (which no one disputes) may be a proxy for an answer to the question I ask above. $\endgroup$
    – user23715
    Commented Apr 17, 2015 at 14:37
  • $\begingroup$ Here is a Ted talk by Jim Flynn $\endgroup$
    – Remi.b
    Commented Apr 19, 2015 at 21:09
  • $\begingroup$ @Chris (or Matt) --- I understand the Flynn Effect. I am not asking a question about it. --- What I want to know is if there are any published studies that are looking for the Idiocracy Effect. --- It's a well known phenomenon that kennel breeding in dogs (e.g.), if poorly managed, can engender many genetic diseases and otherwise increase susceptibility to health problems in general. --- Won't the same thing happen with humans? How is this controversial? --- I understand that such a question can engender poor quality answers but that's why I posted on a moderated forum. $\endgroup$
    – user23715
    Commented Apr 20, 2015 at 18:59
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    $\begingroup$ It is not clear to me either why it has been closed. Would be good to give comments when voting for closing. Would a summary about the research on the correlation between fecundity and IQ answer your question? Eventually a discussion about statistical issues in these studies (I am assuming this is what you mean when talking about the effect of regression to the mean)? $\endgroup$
    – Remi.b
    Commented Apr 20, 2015 at 20:20

1 Answer 1


There appears to be some - note that this, of course, is highly controversial - evidence pointing at the direction that intelligence declined in the US and UK during the last century. There is a relatively new review by Michael Lynch (2016) in which he suggests that modern humans are accumulating slightly deleterious mutations with increasing rate due to relaxed purifying selection (a consequence of technology, medicine, ...), resulting in a 'future genetic load'. Let me just sum up some of the major points he makes that are relevant to assess current evolution of human intelligence:

  • Heritable mutations that lead to a reduction in fitness of 1% take about 100 generations to be eliminated by selection. Many of these mutations have a direct or indirect effect on the mutation rate (mostly increasing) and relaxed purifying selection on them might cause a positive feedback loop that causes even more mutations. Additionally, parental age increases the number of mutations passed on to the offspring. Most mutations will be slightly deleterious (keep in mind that the majority of new mutations is neutral, the majority of the remainder is deleterious and only very few mutations are beneficial. Therefore the really deleterious ones will still be selected against, but the slightly deleterious ones will not be removed by purifying selection). Accordingly, relaxed purifying selection increases the amount of slightly deleterious selection and decreases the efficiency of getting rid of them.

  • Lynch argues that the brain might be particularly sensitive to mutations as brain function depends on the finely tuned expression of thousands of genes which for him leads to the proposition that the effect of germline mutation rate on psychological disorders could be higher than expected by chance. In support, he cites Iossifov et al. (2015) who have suggested that autism spectrum disorders might be caused by de novo mutations in at least 30% of the cases and also looked at mutations interfering with IQ measurements.

  • Finally, he cites two papers that are of interest here as they deal with the change of intellectual abilities of humans over time (Crabtree (2013) and Woodley (2015)). The latter is a meta-analytic study based in the US and UK and Lynch already points out that there there are the usual issues with entangling genetic and environmental factors. However, in that study a slow decline of general intelligence of about 0.39 points per decade due to selection and about 0.84 points per decade due to mutational load is suggested. Interestingly, Woodley points out that there are two major hypothesis for this decline: (i) Since the industrial revolution people with lower general intelligence have higher average number of offspring (that might be the 'differential procreation' you have been hinting at). (ii) Mutation accumulation due to relaxed purifying selection against deleterious variants. Woodley concludes that the decline is a combination of both.

Of course, these points are strongly debatable. One of the major conceptual criticisms on Lynch's view is that slightly deleterious mutations (that Lynch claim to be accumulated as a consequence of relaxed purifying selection) are not slightly deleterious but totally neutral if they do not have a fitness effect. In terms of evolution it does not make a difference if an allele is neutral or negative selection is not acting on it because of technological interference. Another objection - more relevant to your question - is that measuring intelligence in general, or the IQ in particular, is a controversial thing to do and not at all easy to standardize which causes great difficulty in obtaining comparable data. Moreover, tracking down underlying genetics, estimating heritability of intelligence (a trait we even have difficulties to properly define and that is known to be strongly influenced by the environment), and consequently putting together a comprehensive assessment of the evolution of intelligence in humans is really tricky, set aside the ethical problems that may arise which we have not even touched yet.

A last note: In your edit you write:

[...] a summary of fecundity and IQ would potentially answer my question but only if it accounted for the current selection process we see going on in modern technophile societies.

This is problematic as to date it is really difficult, if not impossible, to detect recent selection. This is mainly due to the fact that most methods depend on detecting changes in allele frequencies over time and this needs a considerable number of generations to integrate over.

There have been some efforts to develop more accurate methods, but looking at selection in the last 100 years (which you would need to be able to when looking at the technologically modern populations) is not possible so far. The best attempt I am aware of so far is Field et al. (2016, preprint) who looked, based on genome-wide association study data, at recent selection in the last 2000 years. However, they do not mention intelligence.

Update: This paper was a pre-print. It is now published in Science: Field et al. (2016).

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    $\begingroup$ "One of the major conceptual criticisms on Lynch's view is that slightly deleterious mutations are not slightly deleterious but totally neutral if they do not have a fitness effect." -- This criticism of Lynch is not valid. In the absence of human culture the criticism would be true (a truism) but the non-natural selection imposed by human culture is precisely what I think would be interesting to measure. -- And yes, the whole concept is "loaded", a politically correct bombshell. I hesitated asking the question for just that reason. You'll note the paucity of answers my Q has garnered! $\endgroup$
    – user23715
    Commented Oct 26, 2016 at 20:26
  • $\begingroup$ @user23715: I see your point, but selection does not differ depending on the source of selective pressures. It is totally unimportant if we as humans classify it as 'artificial' or 'natural' selection. It is the same thing. This would only not be true if human culture would disappear instantly as this would take away the neutral status of some of the mutations Lynch considers as slightly deleterious. This is actually what Lynch is hinting at: Intelligence levels might drop so strongly that modern medicine cannot be maintained, exposing all these mutations. For me this is a bit far-fetched ... $\endgroup$ Commented Oct 27, 2016 at 9:07
  • $\begingroup$ Human selection, especially modern human selection, is more Lamarckian than Darwinian. -- It's possible genetic medical science will exceed our ability to harm ourselves through non-selective breeding. Maybe the next generation (or three) AI after IBM's Watson will get us what we need to keep from running off the cliff. -- I would think the rate of bad mutations would increase geometrically at some point so the cliff could seem far away and then... too late! -- Thanks for the answer. I'll let it sit for a week or ten but it will likely become the accepted answer. $\endgroup$
    – user23715
    Commented Oct 29, 2016 at 2:54
  • $\begingroup$ @user23715: As you might have seen in my answer, I also still have to let it sit as it is a very difficult topic both scientifically (intelligence is the on of the 'worst' trait one can work with) and ethically. Interesting to think about though. $\endgroup$ Commented Nov 2, 2016 at 15:43
  • $\begingroup$ You're right that the Field et al paper is the best broad attempt at an answer. -- I found How fragile is our intellect? Estimating losses in general intelligence due to both selection and mutation accumulation to be useful too as a primer on the topic with an excellent short list of citations going back to the heyday of eugenics. -- As Woodley points out, this topic is critical to human flourishing; too bad the "well was poisoned" with politics in the 1920's/30's. -- Germline repair will be a worthwhile investment. $\endgroup$
    – user23715
    Commented Nov 22, 2016 at 18:39

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