By now you will know that I have often been sceptical about the view that we are becoming less intelligent. Estimating these matters is problematical. For example, can we judge where Stephen Hawkings will stand in the canon of theoretical physicists? I think it would be premature to judge, particularly when at the moment we do not have ways of testing his conjectures. Giving it 50 years seems prudent. Something may turn up. Indeed, I think Charles Murray was absolutely right, in describing human accomplishment, to stop the clock 50 years back. We are too bound up by present enthusiasms. For example, the movie we saw last week comes to mind easily, whereas only the best of those watched a decade ago stand out in memory.

However, one should not permanently discard a hypothesis simply because the early work did not support it. The potential problem was first noted by Galton in 1869. In the 1930s Raymond Cattell was pretty sure that the greater fertility of poorer and duller couples was going to bring down the population average, but was surprised to find that the data showed a contrary trend. Perhaps this was because the effects of copious fertilizer overcame a drop in the quality of the seed, but results are results, and the dysgenic hypothesis looked weak. Of course, to continue the agricultural analogy, yields could also be adversely affected by over-use of pesticides. One possible cause of less capable brains is that these sensitive organs are being poisoned by man-made toxins.

All this and more is covered in the introduction to a new paper:

What Caused over a Century of Decline in General Intelligence? Testing Predictions from the Genetic Selection and Neurotoxin Hypotheses Michael A. Woodley of Menie & Matthew A. Sarraf & Mateo Peñaherrera-Aguirre & Heitor B. F. Fernandes & David Becker

https://drive.google.com/file/d/1G85ClSUjG8CiypqXtNxofaEObSI-M2OW/view?usp=sharing

What are we to make of all this? The Woodley et al. argument is that general intelligence, the important and heritable part of mental ability, is falling; and that specific skills, the environmentally-influenced non-heritable part of mental ability, had risen over the last century and is now on a plateau.

The supportive findings are as follows: if you take the g loadings of mental tests (their saturation on the general factor of intelligence) and you link those loadings with the effect sizes of things like inbreeding depression and correlations with motor reaction times, then the strength of selection against intelligence (duller citizens having larger families) is more pronounced on g loaded abilities, but correlates negatively with the Flynn Effect (the secular rise in many, but not all mental tests).

So, it is better to track general ability rather than specific specialised skills.

If you look at the loadings for g over the decades of the last century you will see that the presumed dysgenic effect is not uniform and consistent, as would be expected from a gentle but persistent decline. The authors are aware of this, and provide counter-arguments, but it remains a puzzle, and in my view weakens the hypothesis somewhat.

Overall, we have a strong and coherent case for the dysgenic hypothesis, which critics can now respond to.