Michael Woodley of Menie spends much of his time tending his ancestral estate, pacing the linen-fold panelled rooms of the ancient house, warming his hands at the towering stone fireplace and meditating on the collapse of the aristocracy, the paucity of contemporary innovation and the lamentable and persistent downward drift of the national intellect. Now he sends me a barefoot runner with his latest manuscript, which I have read as the autumn mists creep across the Nadder valley, before penning this reply for the poor urchin to carry back to his master.
Young Woodley avers that, not only are we going to hell in a handcart, but we are doing so at a pace which he can predict with some accuracy (1.23 IQ points per decade), composed as it is of two dysgenic effects: the dull have been reproducing with greater fecundity than the bright (.39), and increasing paternal age has increased the rate of deleterious mutations (.84).
In the spirit of the age, and with an ever-present concern for your health and safety, those of you who are of nervous disposition or advancing paternal age should turn away, and listen to light classical music.
How fragile is our intellect? Estimating losses in general intelligence due to both selection and mutation accumulation. Michael A. Woodley of Menie. Personality and Individual Differences 75 (2015) 80–84
General intelligence is an adaptation to solving evolutionarily novel and domain general fitness problems, i.e. problems which occur on an irregular rather than predictable basis throughout the course of evolution and which are complex, requiring the recruitment and coordination of large numbers of specialized mechanisms in solving them (Geary, 2005; MacDonald, 2013).
it has been argued that deleterious mutations exhibiting small effects accumulate within a population – persisting within genomes for long periods of time before substantially inhibiting fitness, thus giving rise to individual differences in general intelligence (g) and other potentially mutation-sensitive traits, such as health and physical attractiveness (Miller, 2000a,b; Penke, Denissen, & Miller, 2007).
The Breeder’s equation (Fisher, 1929) is frequently employed in studies of this kind:
R ¼ S _ h2
In this equation, S constitutes the size of the selection pressure operating on IQ transformed into a phenotypic change (i.e. the degree to which the trait will change over a generation assuming no biological regression to the mean, or perfect heritability). h2 represents the additive heritability of IQ. The product of these two terms gives us the expected responsiveness to selection, or R, which in terms of IQ is scaled as a change in ‘genotypic IQ’, or the degree to which the underlying genetic potential for a certain level of IQ should decline per generation (Lynn, 2011).
Woodley does a meta-analysis of 9 studies in the US and UK (simply for cultural similarity) to estimate this dysgenic effect on heritable g. This comes to an estimated decadal heritable g decline of 0.385 points.
It may be possible to crudely estimate the impact of mutation accumulation on g. Ideal for this purpose is the study of Kong et al. (2012) in which the numbers of de novo mutations in offspring were counted and correlated with the age of their fathers. Kong et al. found a strong linear relationship between the two (r = .97) amongst a sample of 78 Icelandic parent–child trios. They estimated an average increase in the offspring’s number of de novo mutations at 2.01 per year of paternal age. At 35 years of age, i.e. one familial generation (Kong et al., 2012), fathers are producing offspring with an average of 70 de novo mutations. Using this estimate, it is possible to determine the relationship between the increase in de novo mutation and IQ-loss as a function of paternal age.
These calculations are a little more complex, because they require re-examination of previous “corrections” for birth order. Perhaps there is a book to be written about the assumptions which underlie all statistical adjustments in psychology papers, to be called “The Correction of Corrections”.
The meta-analytic aggregate estimate of the loss in heritable g due to selection (estimated in Section 2 at q = .39 points per decade) can be combined with the loss expected from mutation accumulation, which was estimated at .84 points per decade. As the latter estimate was derived using structural equations modelling, error has been controlled, therefore the loss due to mutation accumulation is symmetric in terms of reliability and validity with respect to the meta-analytic loss due to selection.
The sum of the selection and mutation accumulation losses (i.e. the overall dysgenic loss) is therefore 1.23 points of heritable g per decade, or 4.31 points per familial generation. If the 95% confidence interval for the decline estimate due to mutation accumulation (the paternal age effect; i.e. 1.53–.14 points per decade) is generalized to the sum of both estimates, this yields upper and lower bound decline values ranging from 1.92 to .53 points per decade, or 6.72 to 1.86 points per familial generation.
One potential objection to the finding that g is declining by the amount claimed here stems from the Flynn effect, which is associated with an average increase in IQ of three points per decade (Flynn, 2009). The Flynn effect is however least pronounced on the most heritable and also g loaded IQ subtests (Rushton & Jensen, 2010; te Nijenhuis & van der Flier, 2013), which indicates that secular IQ gains occur at the level of less heritable and narrow abilities, rather than on g. Selection effects and the effects of mutation load on IQ are however more pronounced on the most g loaded subtests (Peach et al., 2014; Prokosch, Yeo, & Miller, 2005; Woodley & Meisenberg, 2013). On this basis dysgenic effects and the Flynn effect could co-occur – with dysgenics reducing the level of heritable g, and various environmental improvements raising narrow abilities simultaneously, via their effects on non-g variance. This hypothesis has been termed the co-occurrence model (Woodley & Figueredo, 2013).
So, there is a prompt and visible effect caused by the liberal use of fertilizer (environmental improvements) giving us a 3 IQ point gain, and a less visible and insidious worsening of seed quality (mutation and differential fertility) losing us 1.2 IQ points. All boats rise on the rising tide of affluence, but some are leaky.
Finally, Woodley links this finding to his work showing that reaction times are slowing up, which he sees as confirmation of the same dysgenic trend.
So, in reply to young Woodley, ever conscious that grim calculations of this sort might cause dismay to sensitive souls, I have placed another log on the fire, and sent him the cheerful and uplifting words of Walter Savage Landor:
I strove with none, for none was worth my strife.
Nature I loved and, next to Nature, Art:
I warm'd both hands before the fire of life;
It sinks, and I am ready to depart.