High-IQ genes versus low-IQ genes
At a genetic level, intelligence may reduce because of a reduction in high intelligence genes in a population and/or as an accumulation of intelligence-damaging mutations in the population.
Differential fertility would lead to a decline in intelligence by a reduction in the proportion of high IQ genes in the population. This happens from a combination of the relatively less intelligent people having on average the most children, and the most intelligent people having very low fertility. Since the most intelligent people are sub-fertile, with less than two offspring per woman, the genes which have made them the most intelligent will decline in each generation - first declining as a proportion of the gene pool, and then declining in absolute prevalence.
For instance, when there is a woman with ultra-high intelligence who has zero children (which is the most usual outcome among ultra-intelligent women), then whatever it was about her genes which made her so intelligent is eliminated from the gene pool: this is the loss of ‘high-IQ genes’.
But our suggestion of mutation accumulation involves that the additional mechanism of an accumulation of what could be termed ‘low-IQ genes’. So, as well as there being a decline in intelligence from the reduced proportion of high-IQ’ gene, there is also a reduction in in the proportion of low-IQ genes: an increase in the proportion of ‘low IQ genes’ in the population.
High IQ genes have (presumably) been selected for in the past because they increased intelligence, and thereby (under ancestral - especially medieval - conditions) increased reproductive success.
But low IQ genes are not, in general, a product of natural selection: rather they are spontaneously occurring deleterious mutations, which happen with every generation due to any cause of genetic damage (e.g. electromagnetic radiation, chemical damage), or errors in replication.
These mutations will, if not eliminated, accumulate generation upon generation. Therefore when they have accumulated, the low-IQ genes were not 'selected for'; rather it was a matter of lack of selection, relaxation of natural selection. ‘Low IQ gene’ therefore usually means something like a genetic mutation that – in potentially a wide range of ways, by impairing almost any aspect of brain structure, organization or functioning; actively-damages brain processing speed and efficiency, hence reducing general intelligence.
In technical terms, the selection mechanism for eliminating these spontaneously accumulating low IQ genes is mutation-selection balance. The idea is that mutations spontaneously occur and need selectively to be eliminated. In other words, by some means, those organisms which have damaging mutations must fail to reproduce, so they will not hand-on the mutations to the next generation. The process needs to be perfect, over the long terms, otherwise the accumulation of damaging mutations will eventually prevent reproduction and damage survival to cause extinction (the term for this extinction is mutational meltdown).
The term mutation-selection balance refers to the fact that the occurrence of mutations must be balanced by the elimination of mutations: natural selection (including sexual selection – mate choice) must be powerful enough to sieve-out all the deleterious mutations. If natural selection is not strong enough to do this, then mutations will accumulate, brain function will be damaged, and intelligence will decline.
Each spontaneous mutation has about a fifty-fifty chance of damaging brain function, because the brain depends on a very high proportion of genes to develop normally and make its structural components, its proteins, enzymes, hormones, neurotransmitters and so on. Thus the brain is a large ‘mutational target’ (as Geoffrey Miller has termed it) – and will usually show up, in a quantitative fashion, the amount of mutational damage a person has. In other world, high intelligence requires ‘good genes’ – where good genes means a genome low in mutations; conversely a high mutational load will cause low intelligence.
Before the Industrial Revolution, individuals with a higher mutational load, which means a higher load of low-IQ genes (and therefore lower intelligence) had lower-than-average reproductive success due to very high (indeed, probably near total) childhood mortality rates. But since the child mortality rates fell from more than half to about one percent in most of Europe, almost all babies that are born have survived to adulthood, and most of them have reproduced. Therefore, we must assume that there have by now been several generations – in England at least eight generations - of mutation accumulation. And we must also assume that this has had a significant effect in reducing intelligence.
This produces what is truly a ‘dysgenic’ effect on intelligence, since it is not evolved, not adaptive, not a new ability – but instead a lowering of intelligence due to a pathological process; a destruction of adaptive human intelligence caused by an accumulation of damage.
And although intelligence decline is a sensitive measure of mutation accumulation – it is not the only consequence. Many other human adaptations would be destroyed by mutation accumulation – including evolved human personality types. As well as pulling down human intelligence; mutation accumulation would be expected to destroy the Endogenous personality, to impair human creativity – and would be a further nail in the coffin of genius.