It has been a fascinating, and I must admit horrifying, three-and-a-bit years since Michael Woodley and I first discovered the first objective evidence that there has been a very substantial decline in general intelligence ('g') over the past two hundred years - the evidence was posted on this blog just a few hours after we discovered it:
Since then, Michael has taken the lead in replicating this finding in multiple other forms of data, and in a variety of paradigms; and learning more about the magnitude of change and its timescale. His industry has been astonishing!
We currently believe that general intelligence has declined by approximately two standard deviations (which is approximately 30 IQ points) since 1800 - that is, over about 8 generations
(Note added - I now regard 2 SD as overstating it, also that an SD is not of fixed size over generations. More correct would be to say intelligence has declined since 1800 by more-than-one modern SD.).
Such a decline is astonishing - at first sight. But its magnitude has been obscured by social and medical changes so that we underestimate intelligence in 1800 and over-estimate intelligence now.
On the other hand, magnitude and rapidity of decline in world class geniuses in the West (and of major innovations) does imply a decline of intelligence of at least 2 SDs - so from that perspective the rate and size of decline is pretty much as-expected.
Two hundred years ago, and for many decades afterwards, performance of the population in a wide range of tasks was substantially impaired by things like malnutrition and high rates of serious endemic infectious diseases. On any particular day, many or most people would have been ill, and their ability to do skilled activities (including examinations - or IQ tests, if they had been in existence) was significantly impaired.
Furthermore, two hundred years ago there was much less information around, and people had to think things through for themselves.
However, general intelligence is buffered against environmental change - it is hardly affected by disease, or even malnutrition - until these are of such severity as to result in death (under pre-modern conditions).
So even very sick and/or malnourished populations, who may be living in simple cultures at a subsistence level, or under conditions of multi-generational malnutrition and near starvation, may have high g - and this will become obvious in terms of high performance as soon as their environment becomes more favourable (for example the migrant Norsemen and the Chinese).
Wind-forward to today, and the general health and nutrition are much improved; and in a thousand ways it is easier for people to give a falsely high impression of their ability by deploying technology and 'parroting' the hard-won knowledge of other people. This does not represent g-driven intelligence, but a multitude of specialized, task-specific intelligences.
To caricature, in 1800, the average Man (when we was not impaired by illness) had a very deep kind of abstract reasoning and problem solving ability which was spontaneous and almost independent of education - his intelligence rose-up powerfully and unstoppably from below, rather like a geyser.
By contrast, Modern Man has a much weaker subterranean spring of intelligence and instead a 'mosaic' of separate and trained abilities, superficially 'studded' onto him by culture and education.
Modern Man has relatively very poor abstract reasoning and problem solving abilities; but can be trained to learn and can quote (or parrot) the reasons and solutions across a wide range of things - but without understanding what he is saying.
(And, indeed, without even knowing that he does not understand - since he equates 'knowing the right answers' with intelligence.)
Michael and I immediately recognized that the rate of change in intelligence that we were observing was too fast to be accounted for my natural selection favouring lower intelligence; although this does have a significant role.
We soon began to recognize that the primary mechanism was likely to be mutation accumulation due to the decline in child mortality rates from more than half to about one percent - child mortality having, through human history, served as the main (but not only) selective 'sieve' to remove the spontaneous fitness-reducing mutations which occur with every generation.
We also discovered the biological concept of 'mutational meltdown' - which sometimes leads to the extinction of a species, especially when combined with a reducing population: mutational damage accumulates so fast in a population that organisms cease to reproduce and become extinct.
Michael has gone on to confirm the plausibility of this mechanism of mutation accumulation in rapidly reducing general intelligence, and to make the first steps in quantifying it.
But our story which had begun with declining general intelligence then began to take on a much larger scope.
Because if mutation accumulation was the main mechanism for declining intelligence, then this had implications for the total fitness, indeed the viability, of the human organism.
General intelligence can be regarded as an index of reproductive potential or 'fitness', because high g depend upon a highly efficient brain, which depends on multiple genes coding for multiple and complexly-interacting brain systems. Any randomly occurring mutation has a high probability of impairing brain efficiency, so intelligence will be expected to decline incrementally with accumulating mutations.
So, declining g due to mutation accumulation only represents the tip of an iceberg of genetic damage to the fitness of an organism, or a population of organisms.
In a sense, the reduction of intelligence may be one of the lesser concerns about this world of what looks increasingly like a mutational meltdown. Because mutations will also damage what might be termed the 'basic instincts' of the population or species.
In particular, mutation accumulation will be expected to affect social and sexual instincts of the kind we used to call 'common sense' and 'human nature'.
So, common sense could be considered the normal, standard behaviours which enabled humans to function in groups, and to survive; while sexual instincts refer to the basic sexual orientation and attraction of humans; and the suite of adaptations that lead to 'pair bonding', fertile matings, raising of offspring etc.
These basic instincts used to be taken for granted; but in fact they are highly complex adaptations, and represent the product of multiple generations of natural selection. Indeed, social and sexual instincts are perhaps the most sensitive of all human traits to damage of any kind - it is change in social and sexual behaviour which is most sensitive to any form of disease or disorder affecting the brain.
This applies to genetic and chromosomal disease, which always show-up in social and sexual differences; but also to trauma. For instance the residual effect of a stroke is much more evident in terms of subtle psychological changes to social and sexual behaviour (personality) than in terms of physical function. And, even small amounts of many drugs - such as alcohol; or hormones - such as testosterone or oestrogen; will observably change, and derange, social and sexual behaviour.
In conclusion, since there has been considerable mutation accumulation over the past two hundred years - enough to cause a very large reduction in general intelligence - this must also have caused considerable damage to human social and sexual adaptations.
Therefore, both common sense and sexual instincts are impaired in modern Man.
Our basic instincts have been damaged.
Once that is realized as being necessarily entailed, then the evidence for such impairment is all around us.
The most fundamental measure is fitness, i.e. reproductive potential - and it is probably the most remarkable fact about modernity that it leads to impaired reproductive success - indeed to below-replacement fertility.
The 'demographic transition', interpreted as plain biology, is therefore strong prima facie evidence of mutation accumulation; indeed it points to incipient mutational meltdown, since the (age-adjusted) post-industrial Western population has been declining for several decades.
Mutation accumulation would also be likely to lead to the lack of common sense, the lack of basic self-preservation, the lack of what would be expected as normal and adaptive social behaviours that are so striking a feature of the West.
And, even more significantly, the lack of any concern about this lack of common sense - damage to social mechanisms has been so profound that the Western population has lost the ability to notice or feel that there is damage - that our situation is pathological.
Indeed, the obvious pathology resulting from damaged instincts is vehemently denied - and to point it out is punished. This is exactly what would be expected when the lunatics have taken-over the asylum, when disease is endemic. Disease is the new health.
Look around. We live in a profoundly weird world socio-sexual , yet there is near zero response to the fact - just a kind of bland, bewildered, vague approval that socio-sexual change means 'progress'.
The same applies to sexual instincts. What is striking is not so much the high levels of disordered sexual behaviour; but the widespread loss of the ability to notice and feel that sexual behaviour is disordered.
Past generations did not need to depend on education and were immune to propaganda when it came to sexual instincts - but modern attitudes reveal that these basic instincts have been severely damaged - so that sexual attraction, evaluations, and motivations are all - very generally - disordered.
The Western populations have suffered such extremity of damage to their evolved human nature, that they have lost even the innate sense that there was any such things as human nature to begin with.
To be in the situation of arguing about the necessity of 'common sense', or the reality of sexual instincts and other attributes of human nature, is itself strong evidence that human nature has been substantially destroyed - as would be entailed by two centuries of mutation accumulation.
And more of the same is to be expected - because it is not clear that anything substantive could be done about this problem except over a multi-generational timescale - even if there were an understanding that there is a problem, and any motivation to do anything about it; neither of which is the case.
Some extra reading and references: